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Module:Convert

ဝီကီးပီးဒီးယား က

Documentation for this module may be created at Module:Convert/doc

-- Convert a value from one unit of measurement to another.
-- Example: {{convert|123|lb|kg}} --> 123 pounds (56 kg)
-- See [[:en:Template:Convert/Transwiki guide]] if copying to another wiki.

local MINUS = '−'  -- Unicode U+2212 MINUS SIGN (UTF-8: e2 88 92)
local abs = math.abs
local floor = math.floor
local format = string.format
local log10 = math.log10
local ustring = mw.ustring
local ulen = ustring.len
local usub = ustring.sub

-- Configuration options to keep magic values in one location.
-- Conversion data and message text are defined in separate modules.
local config, maxsigfig
local numdot  -- must be '.' or ',' or a character which works in a regex
local numsep, numsep_remove, numsep_remove2
local data_code, all_units
local text_code
local varname        -- can be a code to use variable names that depend on value
local from_en_table  -- to translate an output string of en digits to local language
local to_en_table    -- to translate an input string of digits in local language to en
-- Use translation_table in convert/text to change the following.
local en_default           -- true uses lang=en unless convert has lang=local or local digits
local group_method = 3     -- code for how many digits are in a group
local per_word = 'per'     -- for units like "liters per kilometer"
local plural_suffix = 's'  -- only other useful value is probably '' to disable plural unit names
local omitsep              -- true to omit separator before local symbol/name

-- All units should be defined in the data module. However, to cater for quick changes
-- and experiments, any unknown unit is looked up in an extra data module, if it exists.
-- That module would be transcluded in only a small number of pages, so there should be
-- little server overhead from making changes, and changes should propagate quickly.
local extra_module  -- name of module with extra units
local extra_units   -- nil or table of extra units from extra_module

-- Some options in the invoking template can set variables used later in the module.
local currency_text  -- for a user-defined currency symbol: {{convert|12|$/ha|$=€}} (euro replaces dollar)

local function from_en(text)
  -- Input is a string representing a number in en digits with '.' decimal mark,
  -- without digit grouping (which is done just after calling this).
  -- Return the translation of the string with numdot and digits in local language.
  if numdot ~= '.' then
    text = text:gsub('%.', numdot)
  end
  if from_en_table then
    text = text:gsub('%d', from_en_table)
  end
  return text
end

local function to_en(text)
  -- Input is a string representing a number in the local language with
  -- an optional numdot decimal mark and numsep digit grouping.
  -- Return the translation of the string with '.' mark and en digits,
  -- and no separators (they have to be removed here to handle cases like
  -- numsep = '.' and numdot = ',' with input "1.234.567,8").
  if to_en_table then
    text = ustring.gsub(text, '%d', to_en_table)
  end
  if numsep_remove then
    text = text:gsub(numsep_remove, '')
  end
  if numsep_remove2 then
    text = text:gsub(numsep_remove2, '')
  end
  if numdot ~= '.' then
    text = text:gsub(numdot, '.')
  end
  return text
end

local function decimal_mark(text)
  -- Return ',' if text probably is using comma for decimal mark, or has no decimal mark.
  -- Return '.' if text probably is using dot for decimal mark.
  -- Otherwise return nothing (decimal mark not known).
  if not text:find('[.,]') then return ',' end
  text = text:gsub('^%-', ''):gsub('%+%d+/%d+$', ''):gsub('[Ee]%-?%d+$', '')
  local decimal =
    text:match('^0?([.,])%d+$') or
    text:match('%d([.,])%d?%d?$') or
    text:match('%d([.,])%d%d%d%d+$')
  if decimal then return decimal end
  if text:match('%.%d+%.') then return ',' end
  if text:match('%,%d+,') then return '.' end
end

local add_warning, with_separator  -- forward declarations
local function to_en_with_check(text, parms)
  -- Version of to_en() for a wiki using numdot = ',' and numsep = '.' to check
  -- text (an input number as a string) which might have been copied from enwiki.
  -- For example, in '1.234' the '.' could be a decimal mark or a group separator.
  -- From viwiki.
  if to_en_table then
    text = ustring.gsub(text, '%d', to_en_table)
  end
  if decimal_mark(text) == '.' then
    local original = text
    text = text:gsub(',', '')  -- for example, interpret "1,234.5" as an enwiki value
    if parms then
      add_warning(parms, 0, 'cvt_enwiki_num', original, with_separator({}, text))
    end
  else
    if numsep_remove then
      text = text:gsub(numsep_remove, '')
    end
    if numsep_remove2 then
      text = text:gsub(numsep_remove2, '')
    end
    if numdot ~= '.' then
      text = text:gsub(numdot, '.')
    end
  end
  return text
end

local function omit_separator(id)
  -- Return true if there should be no separator before id (a unit symbol or name).
  -- For zhwiki, there should be no separator if id uses local characters.
  -- The following kludge should be a sufficient test.
  if omitsep then
    if id:sub(1, 2) == '-{' then  -- for "-{...}-" content language variant
      return true
    end
    if id:byte() > 127 then
      local first = usub(id, 1, 1)
      if first ~= 'Å' and first ~= '°' and first ~= 'µ' then
        return true
      end
    end
  end
  return id:sub(1, 1) == '/'  -- no separator before units like "/ha"
end

local spell_module  -- name of module that can spell numbers
local speller       -- function from that module to handle spelling (set if needed)
local wikidata_module, wikidata_data_module  -- names of Wikidata modules
local wikidata_code, wikidata_data  -- exported tables from those modules (set if needed)

local function set_config(args)
  -- Set configuration options from template #invoke or defaults.
  config = args
  maxsigfig = config.maxsigfig or 14  -- maximum number of significant figures
  local data_module, text_module
  local sandbox = config.sandbox and ('/' .. config.sandbox) or ''
   data_module = "Module:Convert/data" .. sandbox
   text_module = "Module:Convert/text" .. sandbox
  extra_module = "Module:Convert/extra" .. sandbox
  wikidata_module = "Module:Convert/wikidata" .. sandbox
  wikidata_data_module = "Module:Convert/wikidata/data" .. sandbox
  spell_module = "Module:ConvertNumeric"
  data_code = mw.loadData(data_module)
  text_code = mw.loadData(text_module)
  all_units = data_code.all_units
  local translation = text_code.translation_table
  if translation then
    numdot = translation.numdot
    numsep = translation.numsep
    if numdot == ',' and numsep == '.' then
      if text_code.all_messages.cvt_enwiki_num then
        to_en = to_en_with_check
      end
    end
    if translation.group then
      group_method = translation.group
    end
    if translation.per_word then
      per_word = translation.per_word
    end
    if translation.plural_suffix then
      plural_suffix = translation.plural_suffix
    end
    varname = translation.varname
    from_en_table = translation.from_en
    local use_workaround = true
    if use_workaround then
      -- 2013-07-05 workaround bug by making a copy of the required table.
      -- mw.ustring.gsub fails with a table (to_en_table) as the replacement,
      -- if the table is accessed via mw.loadData.
      local source = translation.to_en
      if source then
        to_en_table = {}
        for k, v in pairs(source) do
          to_en_table[k] = v
        end
      end
    else
      to_en_table = translation.to_en
    end
    if translation.lang == 'en default' then
      en_default = true  -- for hiwiki
    end
    omitsep = translation.omitsep  -- for zhwiki
  end
  numdot = config.numdot or numdot or '.'  -- decimal mark before fractional digits
  numsep = config.numsep or numsep or ','  -- group separator for numbers
  -- numsep should be ',' or '.' or '' or ' ' or a Unicode character.
  -- numsep_remove must work in a regex to identify separators to be removed.
  if numsep ~= '' then
    numsep_remove = (numsep == '.') and '%.' or numsep
  end
  if numsep ~= ',' and numdot ~= ',' then
    numsep_remove2 = ','  -- so numbers copied from enwiki will work
  end
end

local function collection()
  -- Return a table to hold items.
  return {
    n = 0,
    add = function (self, item)
      self.n = self.n + 1
      self[self.n] = item
    end,
  }
end

local function divide(numerator, denominator)
  -- Return integers quotient, remainder resulting from dividing the two
  -- given numbers, which should be unsigned integers.
  local quotient, remainder = floor(numerator / denominator), numerator % denominator
  if not (0 <= remainder and remainder < denominator) then
    -- Floating point limits may need this, as in {{convert|160.02|Ym|ydftin}}.
    remainder = 0
  end
  return quotient, remainder
end

local function split(text, delimiter)
  -- Return a numbered table with fields from splitting text.
  -- The delimiter is used in a regex without escaping (for example, '.' would fail).
  -- Each field has any leading/trailing whitespace removed.
  local t = {}
  text = text .. delimiter  -- to get last item
  for item in text:gmatch('%s*(.-)%s*' .. delimiter) do
    table.insert(t, item)
  end
  return t
end

local function strip(text)
  -- If text is a string, return its content with no leading/trailing
  -- whitespace. Otherwise return nil (a nil argument gives a nil result).
  if type(text) == 'string' then
    return text:match("^%s*(.-)%s*$")
  end
end

local function table_len(t)
  -- Return length (<100) of a numbered table to replace #t which is
  -- documented to not work if t is accessed via mw.loadData().
  for i = 1, 100 do
    if t[i] == nil then
      return i - 1
    end
  end
end

local function wanted_category(cat)
  -- Return cat if it is wanted in current namespace, otherwise return nil.
  -- This is so tracking categories only include pages that need correction.
  local title = mw.title.getCurrentTitle()
  if title then
    local nsdefault = '0'  -- default namespace: '0' = article; '0,10' = article and template
    local namespace = title.namespace
    for _, v in ipairs(split(config.nscat or nsdefault, ',')) do
      if namespace == tonumber(v) then
        return cat
      end
    end
  end
end

local function message(parms, mcode)
  -- Return wikitext for an error message, including category if specified
  -- for the message type.
  -- mcode = numbered table specifying the message:
  --    mcode[1] = 'cvt_xxx' (string used as a key to get message info)
  --    mcode[2] = 'parm1' (string to replace '$1' if any in message)
  --    mcode[3] = 'parm2' (string to replace '$2' if any in message)
  --    mcode[4] = 'parm3' (string to replace '$3' if any in message)
  local msg
  if type(mcode) == 'table' then
    if mcode[1] == 'cvt_no_output' then
      -- Some errors should cause convert to output an empty string,
      -- for example, for an optional field in an infobox.
      return ''
    end
    msg = text_code.all_messages[mcode[1]]
  end
  parms.have_problem = true
  local function subparm(fmt, ...)
    local rep = {}
    for i, v in ipairs({...}) do
      rep['$' .. i] = v
    end
    return (fmt:gsub('$%d+', rep))
  end
  if msg then
    local parts = {}
    local regex, replace = msg.regex, msg.replace
    for i = 1, 3 do
      local limit = 40
      local s = mcode[i + 1]
      if s then
        if regex and replace then
          s = s:gsub(regex, replace)
          limit = nil  -- allow long "should be" messages
        end
        -- Escape user input so it does not break the message.
        -- To avoid tags (like {{convert|1<math>23</math>|m}}) breaking
        -- the mouseover title, any strip marker starting with char(127) is
        -- replaced with '...' (text not needing i18n).
        local append
        local pos = s:find(string.char(127), 1, true)
        if pos then
          append = '...'
          s = s:sub(1, pos - 1)
        end
        if limit and ulen(s) > limit then
          s = usub(s, 1, limit)
          append = '...'
        end
        s = mw.text.nowiki(s) .. (append or '')
      else
        s = '?'
      end
      parts['$' .. i] = s
    end
    local function ispreview()
      -- Return true if a prominent message should be shown.
      if parms.test == 'preview' or parms.test == 'nopreview' then
        -- For testing, can preview a real message or simulate a preview
        -- when running automated tests.
        return parms.test == 'preview'
      end
      local success, revid = pcall(function ()
        return (parms.frame):preprocess('{{REVISIONID}}') end)
      return success and (revid == '')
    end
    local title = string.gsub(msg[1] or 'Missing message', '$%d+', parts)
    local text = msg[2] or 'Missing message'
    local cat = wanted_category(text_code.all_categories[msg[3]]) or ''
    local anchor = msg[4] or ''
    local fmtkey = ispreview() and 'cvt_format_preview' or (msg.format or 'cvt_format')
    local fmt = text_code.all_messages[fmtkey] or 'convert: bug'
    return subparm(fmt, title:gsub('"', '&quot;'), text, cat, anchor)
  end
  return 'Convert internal error: unknown message'
end

function add_warning(parms, level, key, text1, text2)  -- for forward declaration above
  -- If enabled, add a warning that will be displayed after the convert result.
  -- To reduce output noise, only the first warning is displayed.
  if config.warnings or level < 0 then
    if level <= (tonumber(config.warnings) or 1) then
      if parms.warnings == nil then
        parms.warnings = message(parms, { key, text1, text2 })
      end
    end
  end
end

local function spell_number(parms, inout, number, numerator, denominator)
  -- Return result of spelling (number, numerator, denominator), or
  -- return nil if spelling is not available or not supported for given text.
  -- Examples (each value must be a string or nil):
  --   number  numerator  denominator  output
  --   ------  ---------  -----------  -------------------
  --   "1.23"    nil        nil        one point two three
  --    "1"      "2"        "3"        one and two thirds
  --    nil      "2"        "3"        two thirds
  if not speller then
    local function get_speller(module)
      return require(module).spell_number
    end
    local success
    success, speller = pcall(get_speller, spell_module)
    if not success or type(speller) ~= 'function' then
      add_warning(parms, 1, 'cvt_no_spell')
      return nil
    end
  end
  local case
  if parms.spell_upper == inout then
    case = true
    parms.spell_upper = nil  -- only uppercase first word in a multiple unit
  end
  local sp = not parms.opt_sp_us
  local adj = parms.opt_adjectival
  return speller(number, numerator, denominator, case, sp, adj)
end

------------------------------------------------------------------------
-- BEGIN: Code required only for built-in units.
-- LATER: If need much more code, move to another module to simplify this module.
local function speed_of_sound(altitude)
  -- This is for the Mach built-in unit of speed.
  -- Return speed of sound in metres per second at given altitude in feet.
  -- If no altitude given, use default (zero altitude = sea level).
  -- Table gives speed of sound in miles per hour at various altitudes:
  --   altitude = -17,499 to 302,499 feet
  -- mach_table[a + 4] = s where
  --   a = (altitude / 5000) rounded to nearest integer (-3 to 60)
  --   s = speed of sound (mph) at that altitude
  -- LATER: Should calculate result from an interpolation between the next
  -- lower and higher altitudes in table, rather than rounding to nearest.
  -- From: http://www.aerospaceweb.org/question/atmosphere/q0112.shtml
  local mach_table = {                                                       -- a =
    799.5, 787.0, 774.2, 761.207051,                                       -- -3 to  0
    748.0, 734.6, 721.0, 707.0, 692.8, 678.3, 663.5, 660.1, 660.1, 660.1,  --  1 to 10
    660.1, 660.1, 660.1, 662.0, 664.3, 666.5, 668.9, 671.1, 673.4, 675.6,  -- 11 to 20
    677.9, 683.7, 689.9, 696.0, 702.1, 708.1, 714.0, 719.9, 725.8, 731.6,  -- 21 to 30
    737.3, 737.7, 737.7, 736.2, 730.5, 724.6, 718.8, 712.9, 707.0, 701.1,  -- 31 to 40
    695.0, 688.9, 682.8, 676.6, 670.4, 664.1, 657.8, 652.9, 648.3, 643.7,  -- 41 to 50
    639.1, 634.4, 629.6, 624.8, 620.0, 615.2, 613.2, 613.2, 613.2, 613.5,  -- 51 to 60
  }
  altitude = altitude or 0
  local a = (altitude < 0) and -altitude or altitude
  a = floor(a / 5000 + 0.5)
  if altitude < 0 then
    a = -a
  end
  if a < -3 then
    a = -3
  elseif a > 60 then
    a = 60
  end
  return mach_table[a + 4] * 0.44704  -- mph converted to m/s
end
-- END: Code required only for built-in units.
------------------------------------------------------------------------

local function get_range(word)
  -- Return a range (string or table) corresponding to word (like "to"),
  -- or return nil if not a range word.
  local ranges = text_code.ranges
  return ranges.types[word] or ranges.types[ranges.aliases[word]]
end

local function check_mismatch(unit1, unit2)
  -- If unit1 cannot be converted to unit2, return an error message table.
  -- This allows conversion between units of the same type, and between
  -- Nm (normally torque) and ftlb (energy), as in gun-related articles.
  -- This works because Nm is the base unit (scale = 1) for both the
  -- primary type (torque), and the alternate type (energy, where Nm = J).
  -- A match occurs if the primary types are the same, or if unit1 matches
  -- the alternate type of unit2, and vice versa. That provides a whitelist
  -- of which conversions are permitted between normally incompatible types.
  if unit1.utype == unit2.utype or
    (unit1.utype == unit2.alttype and unit1.alttype == unit2.utype) then
    return nil
  end
  return { 'cvt_mismatch', unit1.utype, unit2.utype }
end

local function override_from(out_table, in_table, fields)
  -- Copy the specified fields from in_table to out_table, but do not
  -- copy nil fields (keep any corresponding field in out_table).
  for _, field in ipairs(fields) do
    if in_table[field] then
      out_table[field] = in_table[field]
    end
  end
end

local function shallow_copy(t)
  -- Return a shallow copy of table t.
  -- Do not need the features and overhead of the Scribunto mw.clone().
  local result = {}
  for k, v in pairs(t) do
    result[k] = v
  end
  return result
end

local unit_mt = {
  -- Metatable to get missing values for a unit that does not accept SI prefixes.
  -- Warning: The boolean value 'false' is returned for any missing field
  -- so __index is not called twice for the same field in a given unit.
  __index = function (self, key)
    local value
    if key == 'name1' or key == 'sym_us' then
      value = self.symbol
    elseif key == 'name2' then
      value = self.name1 .. plural_suffix
    elseif key == 'name1_us' then
      value = self.name1
      if not rawget(self, 'name2_us') then
        -- If name1_us is 'foot', do not make name2_us by appending plural_suffix.
        self.name2_us = self.name2
      end
    elseif key == 'name2_us' then
      local raw1_us = rawget(self, 'name1_us')
      if raw1_us then
        value = raw1_us .. plural_suffix
      else
        value = self.name2
      end
    elseif key == 'link' then
      value = self.name1
    else
      value = false
    end
    rawset(self, key, value)
    return value
  end
}

local function prefixed_name(unit, name, index)
  -- Return unit name with SI prefix inserted at correct position.
  -- index = 1 (name1), 2 (name2), 3 (name1_us), 4 (name2_us).
  -- The position is a byte (not character) index, so use Lua's sub().
  local pos = rawget(unit, 'prefix_position')
  if type(pos) == 'string' then
    pos = tonumber(split(pos, ',')[index])
  end
  if pos then
    return name:sub(1, pos - 1) .. unit.si_name .. name:sub(pos)
  end
  return unit.si_name .. name
end

local unit_prefixed_mt = {
  -- Metatable to get missing values for a unit that accepts SI prefixes.
  -- Before use, fields si_name, si_prefix must be defined.
  -- The unit must define _symbol, _name1 and
  -- may define _sym_us, _name1_us, _name2_us
  -- (_sym_us, _name2_us may be defined for a language using sp=us
  -- to refer to a variant unrelated to U.S. units).
  __index = function (self, key)
    local value
    if key == 'symbol' then
      value = self.si_prefix .. self._symbol
    elseif key == 'sym_us' then
      value = rawget(self, '_sym_us')
      if value then
        value = self.si_prefix .. value
      else
        value = self.symbol
      end
    elseif key == 'name1' then
      value = prefixed_name(self, self._name1, 1)
    elseif key == 'name2' then
      value = rawget(self, '_name2')
      if value then
        value = prefixed_name(self, value, 2)
      else
        value = self.name1 .. plural_suffix
      end
    elseif key == 'name1_us' then
      value = rawget(self, '_name1_us')
      if value then
        value = prefixed_name(self, value, 3)
      else
        value = self.name1
      end
    elseif key == 'name2_us' then
      value = rawget(self, '_name2_us')
      if value then
        value = prefixed_name(self, value, 4)
      elseif rawget(self, '_name1_us') then
        value = self.name1_us .. plural_suffix
      else
        value = self.name2
      end
    elseif key == 'link' then
      value = self.name1
    else
      value = false
    end
    rawset(self, key, value)
    return value
  end
}

local unit_per_mt = {
  -- Metatable to get values for a per unit of form "x/y".
  -- This is never called to determine a unit name or link because per units
  -- are handled as a special case.
  -- Similarly, the default output is handled elsewhere.
  __index = function (self, key)
    local value
    if key == 'symbol' then
      local per = self.per
      local unit1, unit2 = per[1], per[2]
      if unit1 then
        value = unit1[key] .. '/' .. unit2[key]
      else
        value = '/' .. unit2[key]
      end
    elseif key == 'sym_us' then
      value = self.symbol
    elseif key == 'scale' then
      local per = self.per
      local unit1, unit2 = per[1], per[2]
      value = (unit1 and unit1.scale or 1) * self.scalemultiplier / unit2.scale
    else
      value = false
    end
    rawset(self, key, value)
    return value
  end
}

local function make_per(unitcode, unit_table, ulookup)
  -- Return true, t where t is a per unit with unit codes expanded to unit tables,
  -- or return false, t where t is an error message table.
  local result = {
    unitcode = unitcode,
    utype = unit_table.utype,
    per = {}
  }
  override_from(result, unit_table, { 'invert', 'iscomplex', 'default', 'link', 'symbol', 'symlink' })
  result.symbol_raw = (result.symbol or false)  -- to distinguish between a defined exception and a metatable calculation
  local prefix
  for i, v in ipairs(unit_table.per) do
    if i == 1 and v == '' then
      -- First unit symbol can be empty; that gives a nil first unit table.
    elseif i == 1 and text_code.currency[v] then
      prefix = currency_text or v
    else
      local success, t = ulookup(v)
      if not success then return false, t end
      result.per[i] = t
    end
  end
  local multiplier = unit_table.multiplier
  if not result.utype then
    -- Creating an automatic per unit.
    local unit1 = result.per[1]
    local utype = (unit1 and unit1.utype or prefix or '') .. '/' .. result.per[2].utype
    local t = data_code.per_unit_fixups[utype]
    if t then
      if type(t) == 'table' then
        utype = t.utype or utype
        result.link = result.link or t.link
        multiplier = multiplier or t.multiplier
      else
        utype = t
      end
    end
    result.utype = utype
  end
  result.scalemultiplier = multiplier or 1
  result.vprefix = prefix or false  -- set to non-nil to avoid calling __index
  return true, setmetatable(result, unit_per_mt)
end

local function lookup(parms, unitcode, what, utable, fails, depth)
  -- Return true, t where t is a copy of the unit's converter table,
  -- or return false, t where t is an error message table.
  -- Parameter 'what' determines whether combination units are accepted:
  --   'no_combination'  : single unit only
  --   'any_combination' : single unit or combination or output multiple
  --   'only_multiple'   : single unit or output multiple only
  -- Parameter unitcode is a symbol (like 'g'), with an optional SI prefix (like 'kg').
  -- If, for example, 'kg' is in this table, that entry is used;
  -- otherwise the prefix ('k') is applied to the base unit ('g').
  -- If unitcode is a known combination code (and if allowed by what),
  -- a table of output multiple unit tables is included in the result.
  -- For compatibility with the old template, an underscore in a unitcode is
  -- replaced with a space so usage like {{convert|350|board_feet}} works.
  -- Wikignomes may also put two spaces or "&nbsp;" in combinations, so
  -- replace underscore, "&nbsp;", and multiple spaces with a single space.
  utable = utable or parms.unittable or all_units
  fails = fails or {}
  depth = depth and depth + 1 or 1
  if depth > 9 then
    -- There are ways to mistakenly define units which result in infinite
    -- recursion when lookup() is called. That gives a long delay and very
    -- confusing error messages, so the depth parameter is used as a guard.
    return false, { 'cvt_lookup', unitcode }
  end
  if unitcode == nil or unitcode == '' then
    return false, { 'cvt_no_unit' }
  end
  unitcode = unitcode:gsub('_', ' '):gsub('&nbsp;', ' '):gsub('  +', ' ')
  local function call_make_per(t)
    return make_per(unitcode, t,
      function (ucode) return lookup(parms, ucode, 'no_combination', utable, fails, depth) end
    )
  end
  local t = utable[unitcode]
  if t then
    if t.shouldbe then
      return false, { 'cvt_should_be', t.shouldbe }
    end
    if t.sp_us then
      parms.opt_sp_us = true
    end
    local target = t.target  -- nil, or unitcode is an alias for this target
    if target then
      local success, result = lookup(parms, target, what, utable, fails, depth)
      if not success then return false, result end
      override_from(result, t, { 'customary', 'default', 'link', 'symbol', 'symlink' })
      local multiplier = t.multiplier
      if multiplier then
        result.multiplier = tostring(multiplier)
        result.scale = result.scale * multiplier
      end
      return true, result
    end
    if t.per then
      return call_make_per(t)
    end
    local combo = t.combination  -- nil or a table of unitcodes
    if combo then
      local multiple = t.multiple
      if what == 'no_combination' or (what == 'only_multiple' and not multiple) then
        return false, { 'cvt_bad_unit', unitcode }
      end
      -- Recursively create a combination table containing the
      -- converter table of each unitcode.
      local result = { utype = t.utype, multiple = multiple, combination = {} }
      local cvt = result.combination
      for i, v in ipairs(combo) do
        local success, t = lookup(parms, v, multiple and 'no_combination' or 'only_multiple', utable, fails, depth)
        if not success then return false, t end
        cvt[i] = t
      end
      return true, result
    end
    local result = shallow_copy(t)
    result.unitcode = unitcode
    if result.prefixes then
      result.si_name = ''
      result.si_prefix = ''
      return true, setmetatable(result, unit_prefixed_mt)
    end
    return true, setmetatable(result, unit_mt)
  end
  local SIprefixes = text_code.SIprefixes
  for plen = SIprefixes[1] or 2, 1, -1 do
    -- Look for an SI prefix; should never occur with an alias.
    -- Check for longer prefix first ('dam' is decametre).
    -- SIprefixes[1] = prefix maximum #characters (as seen by mw.ustring.sub).
    local prefix = usub(unitcode, 1, plen)
    local si = SIprefixes[prefix]
    if si then
      local t = utable[usub(unitcode, plen+1)]
      if t and t.prefixes then
        local result = shallow_copy(t)
        result.unitcode = unitcode
        result.si_name = parms.opt_sp_us and si.name_us or si.name
        result.si_prefix = si.prefix or prefix
        result.scale = t.scale * 10 ^ (si.exponent * t.prefixes)
        return true, setmetatable(result, unit_prefixed_mt)
      end
    end
  end
  -- Accept any unit with an engineering notation prefix like "e6cuft"
  -- (million cubic feet), but not chained prefixes like "e3e6cuft",
  -- and not if the unit is a combination or multiple,
  -- and not if the unit has an offset or is a built-in.
  -- Only en digits are accepted.
  local has_plus = unitcode:find('+', 1, true)
  if not has_plus then
    local exponent, baseunit = unitcode:match('^e(%d+)(.*)')
    if exponent then
      local engscale = text_code.eng_scales[exponent]
      if engscale then
        local success, result = lookup(parms, baseunit, 'no_combination', utable, fails, depth)
        if success and not (result.offset or result.builtin or result.engscale) then
          result.unitcode = unitcode  -- 'e6cuft' not 'cuft'
          result.defkey = unitcode  -- key to lookup default exception
          result.engscale = engscale
          result.scale = result.scale * 10 ^ tonumber(exponent)
          return true, result
        end
      end
    end
  end
  -- Accept user-defined combinations like "acre+m2+ha" or "acre m2 ha" for output.
  -- If '+' is used, each unit code can include a space, and any error is fatal.
  -- If ' ' is used and if each space-separated word is a unit code, it is a combo,
  -- but errors are not fatal so the unit code can be looked up as an extra unit.
  local err_is_fatal
  local combo = collection()
  if has_plus then
    err_is_fatal = true
    for item in (unitcode .. '+'):gmatch('%s*(.-)%s*%+') do
      if item ~= '' then
        combo:add(item)
      end
    end
  elseif unitcode:find('%s') then
    for item in unitcode:gmatch('%S+') do
      combo:add(item)
    end
  end
  if combo.n > 1 then
    local function lookup_combo()
      if what == 'no_combination' or what == 'only_multiple' then
        return false, { 'cvt_bad_unit', unitcode }
      end
      local result = { combination = {} }
      local cvt = result.combination
      for i, v in ipairs(combo) do
        local success, t = lookup(parms, v, 'only_multiple', utable, fails, depth)
        if not success then return false, t end
        if i == 1 then
          result.utype = t.utype
        else
          local mismatch = check_mismatch(result, t)
          if mismatch then
            return false, mismatch
          end
        end
        cvt[i] = t
      end
      return true, result
    end
    local success, result = lookup_combo()
    if success or err_is_fatal then
      return success, result
    end
  end
  -- Look for x/y; split on right-most slash to get scale correct (x/y/z is x/y per z).
  local top, bottom = unitcode:match('^(.-)/([^/]+)$')
  if top and not unitcode:find('e%d') then
    -- If valid, create an automatic per unit for an "x/y" unit code.
    -- The unitcode must not include extraneous spaces.
    -- Engineering notation (apart from at start and which has been stripped before here),
    -- is not supported so do not make a per unit if find text like 'e3' in unitcode.
    local success, result = call_make_per({ per = {top, bottom} })
    if success then
      return true, result
    end
  end
  if not parms.opt_ignore_error and not get_range(unitcode) then
    -- Want the "what links here" list for the extra_module to show only cases
    -- where an extra unit is used, so do not require it if invoked from {{val}}
    -- or if looking up a range word which cannot be a unit.
    if not extra_units then
      local success, extra = pcall(function () return require(extra_module).extra_units end)
      if success and type(extra) == 'table' then
        extra_units = extra
      end
    end
    if extra_units then
      -- A unit in one data table might refer to a unit in the other table, so
      -- switch between them, relying on fails or depth to terminate loops.
      if not fails[unitcode] then
        fails[unitcode] = true
        local other = (utable == all_units) and extra_units or all_units
        local success, result = lookup(parms, unitcode, what, other, fails, depth)
        if success then
          return true, result
        end
      end
    end
  end
  if to_en_table then
    -- At fawiki it is common to translate all digits so a unit like "km2" becomes "km۲".
    local en_code = ustring.gsub(unitcode, '%d', to_en_table)
    if en_code ~= unitcode then
      return lookup(parms, en_code, what, utable, fails, depth)
    end
  end
  return false, { 'cvt_unknown', unitcode }
end

local function valid_number(num)
  -- Return true if num is a valid number.
  -- In Scribunto (different from some standard Lua), when expressed as a string,
  -- overflow or other problems are indicated with text like "inf" or "nan"
  -- which are regarded as invalid here (each contains "n").
  if type(num) == 'number' and tostring(num):find('n', 1, true) == nil then
    return true
  end
end

local function hyphenated(name, parts)
  -- Return a hyphenated form of given name (for adjectival usage).
  -- The name may be linked and the target of the link must not be changed.
  -- Hypothetical examples:
  --   [[long ton|ton]]         →  [[long ton|ton]]          (no change)
  --   [[tonne|long ton]]       →  [[tonne|long-ton]]
  --   [[metric ton|long ton]]  →  [[metric ton|long-ton]]
  --   [[long ton]]             →  [[long ton|long-ton]]
  -- Input can also have multiple links in a single name like:
  --   [[United States customary units|U.S.]] [[US gallon|gallon]]
  --   [[mile]]s per [[United States customary units|U.S.]] [[quart]]
  --   [[long ton]]s per [[short ton]]
  -- Assume that links cannot be nested (never like "[[abc[[def]]ghi]]").
  -- This uses a simple and efficient procedure that works for most cases.
  -- Some units (if used) would require more, and can later think about
  -- adding a method to handle exceptions.
  -- The procedure is to replace each space with a hyphen, but
  -- not a space after ')' [for "(pre-1954&nbsp;US) nautical mile"], and
  -- not spaces immediately before '(' or in '(...)' [for cases like
  -- "British thermal unit (ISO)" and "Calorie (International Steam Table)"].
  if name:find(' ', 1, true) then
    if parts then
      local pos
      if name:sub(1, 1) == '(' then
        pos = name:find(')', 1, true)
        if pos then
          return name:sub(1, pos+1) .. name:sub(pos+2):gsub(' ', '-')
        end
      elseif name:sub(-1) == ')' then
        pos = name:find('(', 1, true)
        if pos then
          return name:sub(1, pos-2):gsub(' ', '-') .. name:sub(pos-1)
        end
      end
      return name:gsub(' ', '-')
    end
    parts = collection()
    for before, item, after in name:gmatch('([^[]*)(%[%[[^[]*%]%])([^[]*)') do
      if item:find(' ', 1, true) then
        local prefix
        local plen = item:find('|', 1, true)
        if plen then
          prefix = item:sub(1, plen)
          item = item:sub(plen + 1, -3)
        else
          prefix = item:sub(1, -3) .. '|'
          item = item:sub(3, -3)
        end
        item = prefix .. hyphenated(item, parts) .. ']]'
      end
      parts:add(before:gsub(' ', '-') .. item .. after:gsub(' ', '-'))
    end
    if parts.n == 0 then
      -- No link like "[[...]]" was found in the original name.
      parts:add(hyphenated(name, parts))
    end
    return table.concat(parts)
  end
  return name
end

local function hyphenated_maybe(parms, want_name, sep, id, inout)
  -- Return s, f where
  --   s = id, possibly modified
  --   f = true if hyphenated
  -- Possible modifications: hyphenate; prepend '-'; append mid text.
  if id == nil or id == '' then
    return ''
  end
  local mid = (inout == (parms.opt_flip and 'out' or 'in')) and parms.mid or ''
  if want_name then
    if parms.opt_adjectival then
      return '-' .. hyphenated(id) .. mid, true
    end
    if parms.opt_add_s and id:sub(-1) ~= 's' then
      id = id .. 's'  -- for nowiki
    end
  end
  return sep .. id .. mid
end

local function use_minus(text)
  -- Return text with Unicode minus instead of '-', if present.
  if text:sub(1, 1) == '-' then
    return MINUS .. text:sub(2)
  end
  return text
end

local function digit_groups(parms, text, method)
  -- Return a numbered table of groups of digits (left-to-right, in local language).
  -- Parameter method is a number or nil:
  --   3 for 3-digit grouping (default), or
  --   2 for 3-then-2 grouping (only for digits before decimal mark).
  local len_right
  local len_left = text:find('.', 1, true)
  if len_left then
    len_right = #text - len_left
    len_left = len_left - 1
  else
    len_left = #text
  end
  local twos = method == 2 and len_left > 5
  local groups = collection()
  local run = len_left
  local n
  if run < 4 or (run == 4 and parms.opt_comma5) then
    if parms.opt_gaps then
      n = run
    else
      n = #text
    end
  elseif twos then
    n = run % 2 == 0 and 1 or 2
  else
    n = run % 3 == 0 and 3 or run % 3
  end
  while run > 0 do
    groups:add(n)
    run = run - n
    n = (twos and run > 3) and 2 or 3
  end
  if len_right then
    if groups.n == 0 then
      groups:add(0)
    end
    if parms.opt_gaps and len_right > 3 then
      local want4 = not parms.opt_gaps3  -- true gives no gap before trailing single digit
      local isfirst = true
      run = len_right
      while run > 0 do
        n = (want4 and run == 4) and 4 or (run > 3 and 3 or run)
        if isfirst then
          isfirst = false
          groups[groups.n] = groups[groups.n] + 1 + n
        else
          groups:add(n)
        end
        run = run - n
      end
    else
      groups[groups.n] = groups[groups.n] + 1 + len_right
    end
  end
  local pos = 1
  for i, length in ipairs(groups) do
    groups[i] = from_en(text:sub(pos, pos + length - 1))
    pos = pos + length
  end
  return groups
end

function with_separator(parms, text)  -- for forward declaration above
  -- Input text is a number in en digits with optional '.' decimal mark.
  -- Return an equivalent, formatted for display:
  --   with a custom decimal mark instead of '.', if wanted
  --   with thousand separators inserted, if wanted
  --   digits in local language
  -- The given text is like '123' or '123.' or '12345.6789'.
  -- The text has no sign (caller inserts that later, if necessary).
  -- When using gaps, they are inserted before and after the decimal mark.
  -- Separators are inserted only before the decimal mark.
  -- A trailing dot (as in '123.') is removed because their use appears to
  -- be accidental, and such a number should be shown as '123' or '123.0'.
  -- It is useful for convert to suppress the dot so, for example, '4000.'
  -- is a simple way of indicating that all the digits are significant.
  if text:sub(-1) == '.' then
    text = text:sub(1, -2)
  end
  if #text < 4 or parms.opt_nocomma or numsep == '' then
    return from_en(text)
  end
  local groups = digit_groups(parms, text, group_method)
  if parms.opt_gaps then
    if groups.n <= 1 then
      return groups[1] or ''
    end
    local nowrap = '<span style="white-space: nowrap">'
    local gap = '<span style="margin-left: 0.25em">'
    local close = '</span>'
    return nowrap .. groups[1] .. gap .. table.concat(groups, close .. gap, 2, groups.n) .. close .. close
  end
  return table.concat(groups, numsep)
end

-- An input value like 1.23e12 is displayed using scientific notation (1.23×10¹²).
-- That also makes the output use scientific notation, except for small values.
-- In addition, very small or very large output values use scientific notation.
-- Use format(fmtpower, significand, '10', exponent) where each argument is a string.
local fmtpower = '%s<span style="margin:0 .15em 0 .25em">×</span>%s<sup>%s</sup>'

local function with_exponent(parms, show, exponent)
  -- Return wikitext to display the implied value in scientific notation.
  -- Input uses en digits; output uses digits in local language.
  return format(fmtpower, with_separator(parms, show), from_en('10'), use_minus(from_en(tostring(exponent))))
end

local function make_sigfig(value, sigfig)
  -- Return show, exponent that are equivalent to the result of
  -- converting the number 'value' (where value >= 0) to a string,
  -- rounded to 'sigfig' significant figures.
  -- The returned items are:
  --   show: a string of digits; no sign and no dot;
  --         there is an implied dot before show.
  --   exponent: a number (an integer) to shift the implied dot.
  -- Resulting value = tonumber('.' .. show) * 10^exponent.
  -- Examples:
  --   make_sigfig(23.456, 3) returns '235', 2 (.235 * 10^2).
  --   make_sigfig(0.0023456, 3) returns '235', -2 (.235 * 10^-2).
  --   make_sigfig(0, 3) returns '000', 1 (.000 * 10^1).
  if sigfig <= 0 then
    sigfig = 1
  elseif sigfig > maxsigfig then
    sigfig = maxsigfig
  end
  if value == 0 then
    return string.rep('0', sigfig), 1
  end
  local exp, fracpart = math.modf(log10(value))
  if fracpart >= 0 then
    fracpart = fracpart - 1
    exp = exp + 1
  end
  local digits = format('%.0f', 10^(fracpart + sigfig))
  if #digits > sigfig then
    -- Overflow (for sigfig=3: like 0.9999 rounding to "1000"; need "100").
    digits = digits:sub(1, sigfig)
    exp = exp + 1
  end
  assert(#digits == sigfig, 'Bug: rounded number has wrong length')
  return digits, exp
end

-- Fraction output format.
local fracfmt = {
  { -- Like {{frac}} (fraction slash).
    -- 1/2    : sign, numerator, denominator
    -- 1+2/3  : signed_wholenumber, numerator, denominator
    '<span class="frac nowrap">%s<sup>%s</sup>&frasl;<sub>%s</sub></span>',
    '<span class="frac nowrap">%s<span class="visualhide">&nbsp;</span><sup>%s</sup>&frasl;<sub>%s</sub></span>',
  },
  { -- Like {{sfrac}} (fraction horizontal bar).
    -- 1//2   : sign, numerator, denominator (sign should probably be before the fraction, but then it can wrap, and html is already too long)
    -- 1+2//3 : signed_wholenumber, numerator, denominator
    '<span class="sfrac nowrap" style="display:inline-block; vertical-align:-0.5em; font-size:85%%; text-align:center;"><span style="display:block; line-height:1em; padding:0 0.1em;">%s%s</span><span class="visualhide">/</span><span style="display:block; line-height:1em; padding:0 0.1em; border-top:1px solid;">%s</span></span>',
    '<span class="sfrac nowrap">%s<span class="visualhide">&nbsp;</span><span style="display:inline-block; vertical-align:-0.5em; font-size:85%%; text-align:center;"><span style="display:block; line-height:1em; padding:0 0.1em;">%s</span><span class="visualhide">/</span><span style="display:block; line-height:1em; padding:0 0.1em; border-top:1px solid;">%s</span></span></span>',
  },
}

local function format_fraction(parms, inout, negative, wholestr, numstr, denstr, do_spell, style)
  -- Return wikitext for a fraction, possibly spelled.
  -- Inputs use en digits and have no sign; output uses digits in local language.
  local wikitext
  if not style then
    style = parms.opt_fraction_horizontal and 2 or 1
  end
  if wholestr == '' then
    wholestr = nil
  end
  if wholestr then
    local decorated = with_separator(parms, wholestr)
    if negative then
      decorated = MINUS .. decorated
    end
    local fmt = fracfmt[style][2]
    wikitext = format(fmt, decorated, from_en(numstr), from_en(denstr))
  else
    local sign = negative and MINUS or ''
    wikitext = format(fracfmt[style][1], sign, from_en(numstr), from_en(denstr))
  end
  if do_spell then
    if negative then
      if wholestr then
        wholestr = '-' .. wholestr
      else
        numstr = '-' .. numstr
      end
    end
    wikitext = spell_number(parms, inout, wholestr, numstr, denstr) or wikitext
  end
  return wikitext
end

local function format_number(parms, show, exponent, isnegative)
  -- Parameter show is a string or a table containing strings.
  -- Each string is a formatted number in en digits and optional '.' decimal mark.
  -- A table represents a fraction: integer, numerator, denominator;
  -- if a table is given, exponent must be nil.
  -- Return t where t is a table with fields:
  --   show = wikitext formatted to display implied value
  --          (digits in local language)
  --   is_scientific = true if show uses scientific notation
  --   clean = unformatted show (possibly adjusted and with inserted '.')
  --          (en digits)
  --   sign = '' or MINUS
  --   exponent = exponent (possibly adjusted)
  -- The clean and exponent fields can be used to calculate the
  -- rounded absolute value, if needed.
  --
  -- The value implied by the arguments is found from:
  --   exponent is nil; and
  --   show is a string of digits (no sign), with an optional dot;
  --   show = '123.4' is value 123.4, '1234' is value 1234.0;
  -- or:
  --   exponent is an integer indicating where dot should be;
  --   show is a string of digits (no sign and no dot);
  --   there is an implied dot before show;
  --   show does not start with '0';
  --   show = '1234', exponent = 3 is value 0.1234*10^3 = 123.4.
  --
  -- The formatted result:
  -- * Is for an output value and is spelled if wanted and possible.
  -- * Includes a Unicode minus if isnegative and not spelled.
  -- * Uses a custom decimal mark, if wanted.
  -- * Has digits grouped where necessary, if wanted.
  -- * Uses scientific notation if requested, or for very small or large values
  --   (which forces result to not be spelled).
  -- * Has no more than maxsigfig significant digits
  --   (same as old template and {{#expr}}).
  local xhi, xlo  -- these control when scientific notation (exponent) is used
  if parms.opt_scientific then
    xhi, xlo = 4, 2  -- default for output if input uses e-notation
  elseif parms.opt_scientific_always then
    xhi, xlo = 0, 0  -- always use scientific notation (experimental)
  else
    xhi, xlo = 10, 4  -- default
  end
  local sign = isnegative and MINUS or ''
  local maxlen = maxsigfig
  local tfrac
  if type(show) == 'table' then
    tfrac = show
    show = tfrac.wholestr
    assert(exponent == nil, 'Bug: exponent given with fraction')
  end
  if not tfrac and not exponent then
    local integer, dot, decimals = show:match('^(%d*)(%.?)(.*)')
    if integer == '0' or integer == '' then
      local zeros, figs = decimals:match('^(0*)([^0]?.*)')
      if #figs == 0 then
        if #zeros > maxlen then
          show = '0.' .. zeros:sub(1, maxlen)
        end
      elseif #zeros >= xlo then
        show = figs
        exponent = -#zeros
      elseif #figs > maxlen then
        show = '0.' .. zeros .. figs:sub(1, maxlen)
      end
    elseif #integer >= xhi then
      show = integer .. decimals
      exponent = #integer
    else
      maxlen = maxlen + #dot
      if #show > maxlen then
        show = show:sub(1, maxlen)
      end
    end
  end
  if exponent then
    local function zeros(n)
      return string.rep('0', n)
    end
    if #show > maxlen then
      show = show:sub(1, maxlen)
    end
    if exponent > xhi or exponent <= -xlo or (exponent == xhi and show ~= '1' .. zeros(xhi - 1)) then
      -- When xhi, xlo = 10, 4 (the default), scientific notation is used if the
      -- rounded value satisfies: value >= 1e9 or value < 1e-4 (1e9 = 0.1e10),
      -- except if show is '1000000000' (1e9), for example:
      -- {{convert|1000000000|m|m|sigfig=10}} → 1,000,000,000 metres (1,000,000,000 m)
      local significand
      if #show > 1 then
        significand = show:sub(1, 1) .. '.' .. show:sub(2)
      else
        significand = show
      end
      return {
        clean = '.' .. show,
        exponent = exponent,
        sign = sign,
        show = sign .. with_exponent(parms, significand, exponent-1),
        is_scientific = true,
      }
    end
    if exponent >= #show then
      show = show .. zeros(exponent - #show)  -- result has no dot
    elseif exponent <= 0 then
      show = '0.' .. zeros(-exponent) .. show
    else
      show = show:sub(1, exponent) .. '.' .. show:sub(exponent+1)
    end
  end
  local formatted_show
  if tfrac then
    show = tostring(tfrac.value)  -- to set clean in returned table
    formatted_show = format_fraction(parms, 'out', isnegative, tfrac.wholestr, tfrac.numstr, tfrac.denstr, parms.opt_spell_out)
  else
    if isnegative and show:match('^0.?0*$') then
      sign = ''  -- don't show minus if result is negative but rounds to zero
    end
    formatted_show = sign .. with_separator(parms, show)
    if parms.opt_spell_out then
      formatted_show = spell_number(parms, 'out', sign .. show) or formatted_show
    end
  end
  return {
    clean = show,
    sign = sign,
    show = formatted_show,
    is_scientific = false,  -- to avoid calling __index
  }
end

local function extract_fraction(parms, text, negative)
  -- If text represents a fraction, return
  --   value, altvalue, show, denominator
  -- where
  --   value is a number (value of the fraction in argument text)
  --   altvalue is an alternate interpretation of any fraction for the hands
  --        unit where "12.1+3/4" means 12 hands 1.75 inches
  --   show is a string (formatted text for display of an input value,
  --        and is spelled if wanted and possible)
  --   denominator is value of the denominator in the fraction
  -- Otherwise, return nil.
  -- Input uses en digits and '.' decimal mark (input has been translated).
  -- Output uses digits in local language and local decimal mark, if any.
  ------------------------------------------------------------------------
  -- Originally this function accepted x+y/z where x, y, z were any valid
  -- numbers, possibly with a sign. For example '1.23e+2+1.2/2.4' = 123.5,
  -- and '2-3/8' = 1.625. However, such usages were found to be errors or
  -- misunderstandings, so since August 2014 the following restrictions apply:
  --   x (if present) is an integer or has a single digit after decimal mark
  --   y and z are unsigned integers
  --   e-notation is not accepted
  -- The overall number can start with '+' or '-' (so '12+3/4' and '+12+3/4'
  -- and '-12-3/4' are valid).
  -- Any leading negative sign is removed by the caller, so only inputs
  -- like the following are accepted here (may have whitespace):
  --   negative = false       false        true (there was a leading '-')
  --   text     = '2/3'       '+2/3'       '2/3'
  --   text     = '1+2/3'     '+1+2/3'     '1-2/3'
  --   text     = '12.3+1/2'  '+12.3+1/2'  '12.3-1/2'
  -- Values like '12.3+1/2' are accepted, but are intended only for use
  -- with the hands unit (not worth adding code to enforce that).
  ------------------------------------------------------------------------
  local leading_plus, prefix, numstr, slashes, denstr =
    text:match('^%s*(%+?)%s*(.-)%s*(%d+)%s*(/+)%s*(%d+)%s*$')
  if not leading_plus then
    -- Accept a single U+2044 fraction slash because that may be pasted.
    leading_plus, prefix, numstr, denstr =
      text:match('^%s*(%+?)%s*(.-)%s*(%d+)%s*⁄%s*(%d+)%s*$')
    slashes = '/'
  end
  local numerator = tonumber(numstr)
  local denominator = tonumber(denstr)
  if numerator == nil or denominator == nil or (negative and leading_plus ~= '') then
    return nil
  end
  local whole, wholestr
  if prefix == '' then
    wholestr = ''
    whole = 0
  else
    -- Any prefix must be like '12+' or '12-' (whole number and fraction sign);
    -- '12.3+' and '12.3-' are also accepted (single digit after decimal point)
    -- because '12.3+1/2 hands' is valid (12 hands 3½ inches).
    local num1, num2, frac_sign = prefix:match('^(%d+)(%.?%d?)%s*([+%-])$')
    if num1 == nil then return nil end
    if num2 == '' then  -- num2 must be '' or like '.1' but not '.' or '.12'
      wholestr = num1
    else
      if #num2 ~= 2 then return nil end
      wholestr = num1 .. num2
    end
    if frac_sign ~= (negative and '-' or '+') then return nil end
    whole = tonumber(wholestr)
    if whole == nil then return nil end
  end
  local value = whole + numerator / denominator
  if not valid_number(value) then return nil end
  local altvalue = whole + numerator / (denominator * 10)
  local style = #slashes  -- kludge: 1 or 2 slashes can be used to select style
  if style > 2 then style = 2 end
  local wikitext = format_fraction(parms, 'in', negative, leading_plus .. wholestr, numstr, denstr, parms.opt_spell_in, style)
  return value, altvalue, wikitext, denominator
end

local function extract_number(parms, text, another, no_fraction)
  -- Return true, info if can extract a number from text,
  -- where info is a table with the result,
  -- or return false, t where t is an error message table.
  -- Input can use en digits or digits in local language and can
  -- have references at the end. Accepting references is intended
  -- for use in infoboxes with a field for a value passed to convert.
  -- Parameter another = true if the expected value is not the first.
  -- Before processing, the input text is cleaned:
  -- * Any thousand separators (valid or not) are removed.
  -- * Any sign (and optional following whitespace) is replaced with
  --   '-' (if negative) or '' (otherwise).
  --   That replaces Unicode minus with '-'.
  -- If successful, the returned info table contains named fields:
  --   value    = a valid number
  --   altvalue = a valid number, usually same as value but different
  --              if fraction used (for hands unit)
  --   singular = true if value is 1 or -1 (to use singular form of units)
  --   clean    = cleaned text with any separators and sign removed
  --              (en digits and '.' decimal mark)
  --   show     = text formatted for output, possibly with ref strip markers
  --              (digits in local language and custom decimal mark)
  -- The resulting show:
  -- * Is for an input value and is spelled if wanted and possible.
  -- * Has a rounded value, if wanted.
  -- * Has digits grouped where necessary, if wanted.
  -- * If negative, a Unicode minus is used; otherwise the sign is
  --   '+' (if the input text used '+'), or is '' (if no sign in input).
  text = strip(text or '')
  local reference
  local pos = text:find('\127', 1, true)
  if pos then
    local before = text:sub(1, pos - 1)
    local remainder = text:sub(pos)
    local refs = {}
    while #remainder > 0 do
      local ref, spaces
      ref, spaces, remainder = remainder:match('^(\127[^\127]*UNIQ[^\127]*%-ref[^\127]*\127)(%s*)(.*)')
      if ref then
        table.insert(refs, ref)
      else
        refs = {}
        break
      end
    end
    if #refs > 0 then
      text = strip(before)
      reference = table.concat(refs)
    end
  end
  local clean = to_en(text, parms)
  if clean == '' then
    return false, { another and 'cvt_no_num2' or 'cvt_no_num' }
  end
  local isnegative, propersign = false, ''  -- most common case
  local singular, show, denominator
  local value = tonumber(clean)
  local altvalue
  if value then
    local sign = clean:sub(1, 1)
    if sign == '+' or sign == '-' then
      propersign = (sign == '+') and '+' or MINUS
      clean = clean:sub(2)
    end
    if value < 0 then
      isnegative = true
      value = -value
    end
  else
    local valstr
    for _, prefix in ipairs({ '-', MINUS, '&minus;' }) do
      -- Including '-' means inputs like '- 2' (with space) are accepted as -2.
      -- It also sets isnegative in case input is a fraction like '-2-3/4'.
      local plen = #prefix
      if clean:sub(1, plen) == prefix then
        valstr = clean:sub(plen + 1)
        break
      end
    end
    if valstr then
      isnegative = true
      propersign = MINUS
      clean = valstr
      value = tonumber(clean)
    end
    if value == nil then
      if not no_fraction then
        value, altvalue, show, denominator = extract_fraction(parms, clean, isnegative)
      end
      if value == nil then
        return false, { 'cvt_bad_num', text }
      end
      if value <= 1 then
        singular = true  -- for example, "½ mile" or "one half mile" (singular unit)
      end
    end
  end
  if not valid_number(value) then  -- for example, "1e310" may overflow
    return false, { 'cvt_invalid_num' }
  end
  if show == nil then
    -- clean is a non-empty string with no spaces, and does not represent a fraction,
    -- and value = tonumber(clean) is a number >= 0.
    -- If the input uses e-notation, show will be displayed using a power of ten, but
    -- we use the number as given so it might not be normalized scientific notation.
    -- The input value is spelled if specified so any e-notation is ignored;
    -- that allows input like 2e6 to be spelled as "two million" which works
    -- because the spell module converts '2e6' to '2000000' before spelling.
    local function rounded(value, default, exponent)
      local precision = parms.opt_ri
      if precision then
        local fmt = '%.' .. format('%d', precision) .. 'f'
        local result = fmt:format(tonumber(value) + 2e-14)  -- fudge for some common cases of bad rounding
        if not exponent then
          singular = (tonumber(result) == 1)
        end
        return result
      end
      return default
    end
    singular = (value == 1)
    local scientific
    local significand, exponent = clean:match('^([%d.]+)[Ee]([+%-]?%d+)')
    if significand then
      show = with_exponent(parms, rounded(significand, significand, exponent), exponent)
      scientific = true
    else
      show = with_separator(parms, rounded(value, clean))
    end
    show = propersign .. show
    if parms.opt_spell_in then
      show = spell_number(parms, 'in', propersign .. rounded(value, clean)) or show
      scientific = false
    end
    if scientific then
      parms.opt_scientific = true
    end
  end
  if isnegative and (value ~= 0) then
    value = -value
    altvalue = -(altvalue or value)
  end
  return true, {
    value = value,
    altvalue = altvalue or value,
    singular = singular,
    clean = clean,
    show = show .. (reference or ''),
    denominator = denominator,
  }
end

local function get_number(text)
  -- Return v, f where:
  --   v = nil (text is not a number)
  -- or
  --   v = value of text (text is a number)
  --   f = true if value is an integer
  -- Input can use en digits or digits in local language,
  -- but no separators, no Unicode minus, and no fraction.
  if text then
    local number = tonumber(to_en(text))
    if number then
      local _, fracpart = math.modf(number)
      return number, (fracpart == 0)
    end
  end
end

local function gcd(a, b)
  -- Return the greatest common denominator for the given values,
  -- which are known to be positive integers.
  if a > b then
    a, b = b, a
  end
  if a <= 0 then
    return b
  end
  local r = b % a
  if r <= 0 then
    return a
  end
  if r == 1 then
    return 1
  end
  return gcd(r, a)
end

local function fraction_table(value, denominator)
  -- Return value as a string or a table:
  -- * If result is a string, there is no fraction, and the result
  --   is value formatted as a string of en digits.
  -- * If result is a table, it represents a fraction with named fields:
  --   wholestr, numstr, denstr (strings of en digits for integer, numerator, denominator).
  -- The result is rounded to the nearest multiple of (1/denominator).
  -- If the multiple is zero, no fraction is included.
  -- No fraction is included if value is very large as the fraction would
  -- be unhelpful, particularly if scientific notation is required.
  -- Input value is a non-negative number.
  -- Input denominator is a positive integer for the desired fraction.
  if value <= 0 then
    return '0'
  end
  if denominator <= 0 or value > 1e8 then
    return format('%.2f', value)
  end
  local integer, decimals = math.modf(value)
  local numerator = floor((decimals * denominator) +
    0.5 + 2e-14)  -- add fudge for some common cases of bad rounding
  if numerator >= denominator then
    integer = integer + 1
    numerator = 0
  end
  local wholestr = tostring(integer)
  if numerator > 0 then
    local div = gcd(numerator, denominator)
    if div > 1 then
      numerator = numerator / div
      denominator = denominator / div
    end
    return {
      wholestr = (integer > 0) and wholestr or '',
      numstr = tostring(numerator),
      denstr = tostring(denominator),
      value = value,
    }
  end
  return wholestr
end

local function preunits(count, preunit1, preunit2)
  -- If count is 1:
  --     ignore preunit2
  --     return p1
  -- else:
  --     preunit1 is used for preunit2 if the latter is empty
  --     return p1, p2
  -- where:
  --     p1 is text to insert before the input unit
  --     p2 is text to insert before the output unit
  --     p1 or p2 may be nil to mean "no preunit"
  -- Using '+ ' gives output like "5+ feet" (no preceding space).
  local function withspace(text, i)
    -- Insert space at beginning if i == 1, or at end if i == -1.
    -- However, no space is inserted if there is a space or '&nbsp;'
    -- or '-' at that position ('-' is for adjectival text).
    local current = text:sub(i, i)
    if current == ' ' or current == '-' then
      return text
    end
    if i == 1 then
      current = text:sub(1, 6)
    else
      current = text:sub(-6, -1)
    end
    if current == '&nbsp;' then
      return text
    end
    if i == 1 then
      return ' ' .. text
    end
    return text .. ' '
  end
  preunit1 = preunit1 or ''
  local trim1 = strip(preunit1)
  if count == 1 then
    if trim1 == '' then
      return nil
    end
    return withspace(withspace(preunit1, 1), -1)
  end
  preunit2 = preunit2 or ''
  local trim2 = strip(preunit2)
  if trim1 == '' and trim2 == '' then
    return nil, nil
  end
  if trim1 ~= '+' then
    preunit1 = withspace(preunit1, 1)
  end
  if trim2 == '&#32;' then  -- trick to make preunit2 empty
    preunit2 = nil
  elseif trim2 == '' then
    preunit2 = preunit1
  elseif trim2 ~= '+' then
    preunit2 = withspace(preunit2, 1)
  end
  return preunit1, preunit2
end

local function range_text(range, want_name, parms, before, after, inout)
  -- Return before .. rtext .. after
  -- where rtext is the text that separates two values in a range.
  local rtext, adj_text, exception
  if type(range) == 'table' then
    -- Table must specify range text for ('off' and 'on') or ('input' and 'output'),
    -- and may specify range text for 'adj=on',
    -- and may specify exception = true.
    rtext = range[want_name and 'off' or 'on'] or
        range[((inout == 'in') == (parms.opt_flip == true)) and 'output' or 'input']
    adj_text = range['adj']
    exception = range['exception']
  else
    rtext = range
  end
  if parms.opt_adjectival then
    if want_name or (exception and parms.abbr_org == 'on') then
      rtext = adj_text or rtext:gsub(' ', '-'):gsub('&nbsp;', '-')
    end
  end
  if rtext == '–' and after:sub(1, #MINUS) == MINUS then
    rtext = '&nbsp;– '
  end
  return before .. rtext .. after
end

local function get_composite(parms, iparm, in_unit_table)
  -- Look for a composite input unit. For example, {{convert|1|yd|2|ft|3|in}}
  -- would result in a call to this function with
  --   iparm = 3 (parms[iparm] = "2", just after the first unit)
  --   in_unit_table = (unit table for "yd"; contains value 1 for number of yards)
  -- Return true, iparm, unit where
  --   iparm = index just after the composite units (7 in above example)
  --   unit = composite unit table holding all input units,
  -- or return true if no composite unit is present in parms,
  -- or return false, t where t is an error message table.
  local default, subinfo
  local composite_units, count = { in_unit_table }, 1
  local fixups = {}
  local total = in_unit_table.valinfo[1].value
  local subunit = in_unit_table
  while subunit.subdivs do  -- subdivs is nil or a table of allowed subdivisions
    local subcode = strip(parms[iparm+1])
    local subdiv = subunit.subdivs[subcode] or subunit.subdivs[(all_units[subcode] or {}).target]
    if not subdiv then
      break
    end
    local success
    success, subunit = lookup(parms, subcode, 'no_combination')
    if not success then return false, subunit end  -- should never occur
    success, subinfo = extract_number(parms, parms[iparm])
    if not success then return false, subinfo end
    iparm = iparm + 2
    subunit.inout = 'in'
    subunit.valinfo = { subinfo }
    -- Recalculate total as a number of subdivisions.
    -- subdiv[1] = number of subdivisions per previous unit (integer > 1).
    total = total * subdiv[1] + subinfo.value
    if not default then  -- set by the first subdiv with a default defined
      default = subdiv.default
    end
    count = count + 1
    composite_units[count] = subunit
    if subdiv.unit or subdiv.name then
      fixups[count] = { unit = subdiv.unit, name = subdiv.name, valinfo = subunit.valinfo }
    end
  end
  if count == 1 then
    return true  -- no error and no composite unit
  end
  for i, fixup in pairs(fixups) do
    local unit = fixup.unit
    local name = fixup.name
    if not unit or (count > 2 and name) then
      composite_units[i].fixed_name = name
    else
      local success, alternate = lookup(parms, unit, 'no_combination')
      if not success then return false, alternate end  -- should never occur
      alternate.inout = 'in'
      alternate.valinfo = fixup.valinfo
      composite_units[i] = alternate
    end
  end
  return true, iparm, {
    utype = in_unit_table.utype,
    scale = subunit.scale,  -- scale of last (least significant) unit
    valinfo = { { value = total, clean = subinfo.clean, denominator = subinfo.denominator } },
    composite = composite_units,
    default = default or in_unit_table.default
  }
end

local function translate_parms(parms, kv_pairs)
  -- Update fields in parms by translating each key:value in kv_pairs to terms
  -- used by this module (may involve translating from local language to English).
  -- Also, checks are performed which may display warnings, if enabled.
  -- Return true if successful or return false, t where t is an error message table.
  currency_text = nil  -- local testing can hold module in memory; must clear globals
  if kv_pairs.adj and kv_pairs.sing then
    -- For enwiki (before translation), warn if attempt to use adj and sing
    -- as the latter is a deprecated alias for the former.
    if kv_pairs.adj ~= kv_pairs.sing and kv_pairs.sing ~= '' then
      add_warning(parms, 1, 'cvt_unknown_option', 'sing=' .. kv_pairs.sing)
    end
    kv_pairs.sing = nil
  end
  for loc_name, loc_value in pairs(kv_pairs) do
    local en_name = text_code.en_option_name[loc_name]
    if en_name then
      local en_value
      if en_name == '$' or en_name == 'frac' or en_name == 'sigfig' then
        if loc_value == '' then
          add_warning(parms, 2, 'cvt_empty_option', loc_name)
        elseif en_name == '$' then
          -- Value should be a single character like "€" for the euro currency symbol, but anything is accepted.
          currency_text = (loc_value == 'euro') and '€' or loc_value
        else
          local minimum
          local number, is_integer = get_number(loc_value)
          if en_name == 'frac' then
            minimum = 2
            if number and number < 0 then
              parms.opt_fraction_horizontal = true
              number = -number
            end
          else
            minimum = 1
          end
          if number and is_integer and number >= minimum then
            en_value = number
          else
            add_warning(parms, 1, (en_name == 'frac' and 'cvt_bad_frac' or 'cvt_bad_sigfig'), loc_value)
          end
        end
      elseif en_name == 'stylein' or en_name == 'styleout' or en_name == 'qual' or en_name == 'qid' or en_name == 'input' then
        en_value = loc_value ~= '' and loc_value or nil  -- accept non-empty user text with no validation
        if en_name == 'input' then
          -- May have something like {{convert|input=}} (empty input) if source is an infobox
          -- with optional fields. In that case, want to output nothing rather than an error.
          parms.input_text = loc_value  -- keep input because parms.input is nil if loc_value == ''
        end
      else
        en_value = text_code.en_option_value[en_name][loc_value]
        if en_value and en_value:sub(-1) == '?' then
          en_value = en_value:sub(1, -2)
          add_warning(parms, -1, 'cvt_deprecated', loc_name .. '=' .. loc_value)
        end
        if en_value == nil then
          if loc_value == '' then
            add_warning(parms, 2, 'cvt_empty_option', loc_name)
          else
            add_warning(parms, 1, 'cvt_unknown_option', loc_name .. '=' .. loc_value)
          end
        elseif en_value == '' then
          en_value = nil  -- an ignored option like adj=off
        elseif type(en_value) == 'string' and en_value:sub(1, 4) == 'opt_' then
          for _, v in ipairs(split(en_value, ',')) do
            local lhs, rhs = v:match('^(.-)=(.+)$')
            if rhs then
              parms[lhs] = tonumber(rhs) or rhs
            else
              parms[v] = true
            end
          end
          en_value = nil
        end
      end
      parms[en_name] = en_value
    else
      add_warning(parms, 1, 'cvt_unknown_option', loc_name .. '=' .. loc_value)
    end
  end
  local abbr_entered = parms.abbr
  local cfg_abbr = config.abbr
  if cfg_abbr then
    -- Don't warn if invalid because every convert would show that warning.
    if cfg_abbr == 'on always' then
      parms.abbr = 'on'
    elseif cfg_abbr == 'off always' then
      parms.abbr = 'off'
    elseif parms.abbr == nil then
      if cfg_abbr == 'on default' then
        parms.abbr = 'on'
      elseif cfg_abbr == 'off default' then
        parms.abbr = 'off'
      end
    end
  end
  if parms.abbr then
    if parms.abbr == 'unit' then
      parms.abbr = 'on'
      parms.number_word = true
    end
    parms.abbr_org = parms.abbr  -- original abbr, before any flip
  elseif parms.opt_hand_hh then
    parms.abbr_org = 'on'
    parms.abbr = 'on'
  else
    parms.abbr = 'out'  -- default is to abbreviate output only (use symbol, not name)
  end
  if parms.opt_order_out then
    -- Disable options that do not work in a useful way with order=out.
    parms.opt_flip = nil  -- override adj=flip
    parms.opt_spell_in = nil
    parms.opt_spell_out = nil
    parms.opt_spell_upper = nil
  end
  if parms.opt_spell_out and not abbr_entered then
    parms.abbr = 'off'  -- should show unit name when spelling the output value
  end
  if parms.opt_flip then
    local function swap_in_out(option)
      local value = parms[option]
      if value == 'in' then
        parms[option] = 'out'
      elseif value == 'out' then
        parms[option] = 'in'
      end
    end
    swap_in_out('abbr')
    swap_in_out('lk')
    if parms.opt_spell_in and not parms.opt_spell_out then
      -- For simplicity, and because it does not appear to be needed,
      -- user cannot set an option to spell the output only.
      parms.opt_spell_in = nil
      parms.opt_spell_out = true
    end
  end
  if parms.opt_spell_upper then
    parms.spell_upper = parms.opt_flip and 'out' or 'in'
  end
  if parms.opt_table or parms.opt_tablecen then
    if abbr_entered == nil and parms.lk == nil then
      parms.opt_values = true
    end
    parms.table_align = parms.opt_table and 'right' or 'center'
  end
  if parms.table_align or parms.opt_sortable_on then
    parms.need_table_or_sort = true
  end
  local disp_joins = text_code.disp_joins
  local default_joins = disp_joins['b']
  parms.join_between = default_joins[3] or '; '
  local disp = parms.disp
  if disp == nil then  -- special case for the most common setting
    parms.joins = default_joins
  elseif disp == 'x' then
    -- Later, parms.joins is set from the input parameters.
  else
    -- Old template does this.
    local abbr = parms.abbr
    if disp == 'slash' then
      if abbr_entered == nil then
        disp = 'slash-nbsp'
      elseif abbr == 'in' or abbr == 'out' then
        disp = 'slash-sp'
      else
        disp = 'slash-nosp'
      end
    elseif disp == 'sqbr' then
      if abbr == 'on' then
        disp = 'sqbr-nbsp'
      else
        disp = 'sqbr-sp'
      end
    end
    parms.joins = disp_joins[disp] or default_joins
    parms.join_between = parms.joins[3] or parms.join_between
    parms.wantname = parms.joins.wantname
  end
  if (en_default and not parms.opt_lang_local and (parms[1] or ''):find('%d')) or parms.opt_lang_en then
    from_en_table = nil
  end
  if en_default and from_en_table then
    -- For hiwiki: localized symbol/name is defined with the US symbol/name field,
    -- and is used if output uses localized numbers.
    parms.opt_sp_us = true
  end
  return true
end

local function get_values(parms)
  -- If successful, update parms and return true, v, i where
  --   v = table of input values
  --   i = index to next entry in parms after those processed here
  -- or return false, t where t is an error message table.
  local valinfo = collection()  -- numbered table of input values
  local range = collection()  -- numbered table of range items (having, for example, 2 range items requires 3 input values)
  local had_nocomma  -- true if removed "nocomma" kludge from second parameter (like "tonocomma")
  local parm2 = strip(parms[2])
  if parm2 and parm2:sub(-7, -1) == 'nocomma' then
    parms[2] = strip(parm2:sub(1, -8))
    parms.opt_nocomma = true
    had_nocomma = true
  end
  local function extractor(i)
    -- If the parameter is not a value, try unpacking it as a range ("1-23" for "1 to 23").
    -- However, "-1-2/3" is a negative fraction (-1⅔), so it must be extracted first.
    -- Do not unpack a parameter if it is like "3-1/2" which is sometimes incorrectly
    -- used instead of "3+1/2" (and which should not be interpreted as "3 to ½").
    -- Unpacked items are inserted into the parms table.
    -- The tail recursion allows combinations like "1x2 to 3x4".
    local valstr = strip(parms[i])  -- trim so any '-' as a negative sign will be at start
    local success, result = extract_number(parms, valstr, i > 1)
    if not success and valstr and i < 20 then  -- check i to limit abuse
      local lhs, sep, rhs = valstr:match('^(%S+)%s+(%S+)%s+(%S.*)')
      if lhs and not (sep == '-' and rhs:match('/')) then
        if sep:find('%d') then
          return success, result  -- to reject {{convert|1 234 567|m}} with a decent message (en only)
        end
        parms[i] = rhs
        table.insert(parms, i, sep)
        table.insert(parms, i, lhs)
        return extractor(i)
      end
      if not valstr:match('%-.*/') then
        for _, sep in ipairs(text_code.ranges.words) do
          local start, stop = valstr:find(sep, 2, true)  -- start at 2 to skip any negative sign for range '-'
          if start then
            parms[i] = valstr:sub(stop + 1)
            table.insert(parms, i, sep)
            table.insert(parms, i, valstr:sub(1, start - 1))
            return extractor(i)
          end
        end
      end
    end
    return success, result
  end
  local i = 1
  local is_change
  while true do
    local success, info = extractor(i)  -- need to set parms.opt_nocomma before calling this
    if not success then return false, info end
    i = i + 1
    if is_change then
      info.is_change = true  -- value is after "±" and so is a change (significant for range like {{convert|5|±|5|°C}})
      is_change = nil
    end
    valinfo:add(info)
    local range_item = get_range(strip(parms[i]))
    if not range_item then
      break
    end
    i = i + 1
    range:add(range_item)
    if type(range_item) == 'table' then
      -- For range "x", if append unit to some values, append it to all.
      parms.in_range_x = parms.in_range_x or range_item.in_range_x
      parms.out_range_x = parms.out_range_x or range_item.out_range_x
      parms.abbr_range_x = parms.abbr_range_x or range_item.abbr_range_x
      is_change = range_item.is_range_change
    end
  end
  if range.n > 0 then
    if range.n > 30 then  -- limit abuse, although 4 is a more likely upper limit
      return false, { 'cvt_invalid_num' }  -- misleading message but it will do
    end
    parms.range = range
  elseif had_nocomma then
    return false, { 'cvt_unknown', parm2 }
  end
  return true, valinfo, i
end

local function simple_get_values(parms)
  -- If input is like "{{convert|valid_value|valid_unit|...}}",
  -- return true, i, in_unit, in_unit_table
  -- i = index in parms of what follows valid_unit, if anything.
  -- The valid_value is not negative and does not use a fraction, and
  -- no options requiring further processing of the input are used.
  -- Otherwise, return nothing or return false, parm1 for caller to interpret.
  -- Testing shows this function is successful for 96% of converts in articles,
  -- and that on average it speeds up converts by 8%.
  if parms.opt_ri or parms.opt_spell_in then return end
  local clean = to_en(strip(parms[1] or ''), parms)
  if #clean > 10 or not clean:match('^[0-9.]+$') then
    return false, clean
  end
  local value = tonumber(clean)
  if not value then return end
  local info = {
    value = value,
    altvalue = value,
    singular = (value == 1),
    clean = clean,
    show = with_separator(parms, clean),
  }
  local in_unit = strip(parms[2])
  local success, in_unit_table = lookup(parms, in_unit, 'no_combination')
  if not success then return end
  in_unit_table.valinfo = { info }
  return true, 3, in_unit, in_unit_table
end

local function wikidata_call(parms, operation, ...)
  -- Return true, s where s is the result of a Wikidata operation,
  -- or return false, t where t is an error message table.
  local function worker(...)
    wikidata_code = wikidata_code or require(wikidata_module)
    wikidata_data = wikidata_data or mw.loadData(wikidata_data_module)
    return wikidata_code[operation](wikidata_data, ...)
  end
  local success, status, result = pcall(worker, ...)
  if success then
    return status, result
  end
  if parms.opt_sortable_debug then
    -- Use debug=yes to crash if an error while accessing Wikidata.
    error('Error accessing Wikidata: ' .. status, 0)
  end
  return false, { 'cvt_wd_fail' }
end

local function get_parms(parms, args)
  -- If successful, update parms and return true, unit where
  --   parms is a table of all arguments passed to the template
  --        converted to named arguments, and
  --   unit is the input unit table;
  -- or return false, t where t is an error message table.
  -- For special processing (not a convert), can also return
  -- true, wikitext where wikitext is the final result.
  -- The returned input unit table may be for a fake unit using the specified
  -- unit code as the symbol and name, and with bad_mcode = message code table.
  -- MediaWiki removes leading and trailing whitespace from the values of
  -- named arguments. However, the values of numbered arguments include any
  -- whitespace entered in the template, and whitespace is used by some
  -- parameters (example: the numbered parameters associated with "disp=x").
  local kv_pairs = {}  -- table of input key:value pairs where key is a name; needed because cannot iterate parms and add new fields to it
  for k, v in pairs(args) do
    if type(k) == 'number' or k == 'test' then  -- parameter "test" is reserved for testing and is not translated
      parms[k] = v
    else
      kv_pairs[k] = v
    end
  end
  if parms.test == 'wikidata' then
    local ulookup = function (ucode)
      -- Use empty table for parms so it does not accumulate results when used repeatedly.
      return lookup({}, ucode, 'no_combination')
    end
    return wikidata_call(parms, '_listunits', ulookup)
  end
  local success, msg = translate_parms(parms, kv_pairs)
  if not success then return false, msg end
  if parms.input then
    success, msg = wikidata_call(parms, '_adjustparameters', parms, 1)
    if not success then return false, msg end
  end
  local success, i, in_unit, in_unit_table = simple_get_values(parms)
  if not success then
    if type(i) == 'string' and i:match('^NNN+$') then
      -- Some infoboxes have examples like {{convert|NNN|m}} (3 or more "N").
      -- Output an empty string for these.
      return false, { 'cvt_no_output' }
    end
    local valinfo
    success, valinfo, i = get_values(parms)
    if not success then return false, valinfo end
    in_unit = strip(parms[i])
    i = i + 1
    success, in_unit_table = lookup(parms, in_unit, 'no_combination')
    if not success then
      in_unit = in_unit or ''
      if parms.opt_ignore_error then  -- display given unit code with no error (for use with {{val}})
        in_unit_table = ''  -- suppress error message and prevent processing of output unit
      end
      in_unit_table = setmetatable({
        symbol = in_unit, name2 = in_unit, utype = in_unit,
        scale = 1, default = '', defkey = '', linkey = '',
        bad_mcode = in_unit_table }, unit_mt)
    end
    in_unit_table.valinfo = valinfo
  end
  if parms.test == 'msg' then
    -- Am testing the messages produced when no output unit is specified, and
    -- the input unit has a missing or invalid default.
    -- Set two units for testing that.
    -- LATER: Remove this code.
    if in_unit == 'chain' then
      in_unit_table.default = nil  -- no default
    elseif in_unit == 'rd' then
      in_unit_table.default  = "ft!X!m"  -- an invalid expression
    end
  end
  in_unit_table.inout = 'in'  -- this is an input unit
  if not parms.range then
    local success, inext, composite_unit = get_composite(parms, i, in_unit_table)
    if not success then return false, inext end
    if composite_unit then
      in_unit_table = composite_unit
      i = inext
    end
  end
  if in_unit_table.builtin == 'mach' then
    -- As with old template, a number following Mach as the input unit is the altitude,
    -- and there is no way to specify an altitude for the output unit.
    -- Could put more code in this function to get any output unit and check for
    -- an altitude following that unit.
    local success, info = extract_number(parms, parms[i], false, true)
    if success then
      i = i + 1
      in_unit_table.altitude = info.value
    end
  end
  local word = strip(parms[i])
  i = i + 1
  local precision, is_bad_precision
  local function set_precision(text)
    local number, is_integer = get_number(text)
    if number then
      if is_integer then
        precision = number
      else
        precision = text
        is_bad_precision = true
      end
      return true  -- text was used for precision, good or bad
    end
  end
  if word and not set_precision(word) then
    parms.out_unit = parms.out_unit or word
    if set_precision(strip(parms[i])) then
      i = i + 1
    end
  end
  if parms.opt_adj_mid then
    word = parms[i]
    i = i + 1
    if word then  -- mid-text words
      if word:sub(1, 1) == '-' then
        parms.mid = word
      else
        parms.mid = ' ' .. word
      end
    end
  end
  if parms.opt_one_preunit then
    parms[parms.opt_flip and 'preunit2' or 'preunit1'] = preunits(1, parms[i])
    i = i + 1
  end
  if parms.disp == 'x' then
    -- Following is reasonably compatible with the old template.
    local first = parms[i] or ''
    local second = parms[i+1] or ''
    i = i + 2
    if strip(first) == '' then  -- user can enter '&#32;' rather than ' ' to avoid the default
      first = ' [&nbsp;' .. first
      second = '&nbsp;]' .. second
    end
    parms.joins = { first, second }
  elseif parms.opt_two_preunits then
    local p1, p2 = preunits(2, parms[i], parms[i+1])
    i = i + 2
    if parms.preunit1 then
      -- To simplify documentation, allow unlikely use of adj=pre with disp=preunit
      -- (however, an output unit must be specified with adj=pre and with disp=preunit).
      parms.preunit1 = parms.preunit1 .. p1
      parms.preunit2 = p2
    else
      parms.preunit1, parms.preunit2 = p1, p2
    end
  end
  if precision == nil then
    if set_precision(strip(parms[i])) then
      i = i + 1
    end
  end
  if is_bad_precision then
    add_warning(parms, 1, 'cvt_bad_prec', precision)
  else
    parms.precision = precision
  end
  return true, in_unit_table
end

local function record_default_precision(parms, out_current, precision)
  -- If necessary, adjust parameters and return a possibly adjusted precision.
  -- When converting a range of values where a default precision is required,
  -- that default is calculated for each value because the result sometimes
  -- depends on the precise input and output values. This function may cause
  -- the entire convert process to be repeated in order to ensure that the
  -- same default precision is used for each individual convert.
  -- If that were not done, a range like 1000 to 1000.4 may give poor results
  -- because the first output could be heavily rounded, while the second is not.
  -- For range 1000.4 to 1000, this function can give the second convert the
  -- same default precision that was used for the first.
  if not parms.opt_round_each then
    local maxdef = out_current.max_default_precision
    if maxdef then
      if maxdef < precision then
        parms.do_convert_again = true
        out_current.max_default_precision = precision
      else
        precision = out_current.max_default_precision
      end
    else
      out_current.max_default_precision = precision
    end
  end
  return precision
end

local function default_precision(parms, invalue, inclean, denominator, outvalue, in_current, out_current, extra)
  -- Return a default value for precision (an integer like 2, 0, -2).
  -- If denominator is not nil, it is the value of the denominator in inclean.
  -- Code follows procedures used in old template.
  local fudge = 1e-14  -- {{Order of magnitude}} adds this, so we do too
  local prec, minprec, adjust
  local subunit_ignore_trailing_zero
  local subunit_more_precision  -- kludge for "in" used in input like "|2|ft|6|in"
  local composite = in_current.composite
  if composite then
    subunit_ignore_trailing_zero = true  -- input "|2|st|10|lb" has precision 0, not -1
    if composite[#composite].exception == 'subunit_more_precision' then
      subunit_more_precision = true  -- do not use standard precision with input like "|2|ft|6|in"
    end
  end
  if denominator and denominator > 0 then
    prec = math.max(log10(denominator), 1)
  else
    -- Count digits after decimal mark, handling cases like '12.345e6'.
    local exponent
    local integer, dot, decimals, expstr = inclean:match('^(%d*)(%.?)(%d*)(.*)')
    local e = expstr:sub(1, 1)
    if e == 'e' or e == 'E' then
      exponent = tonumber(expstr:sub(2))
    end
    if dot == '' then
      prec = subunit_ignore_trailing_zero and 0 or -integer:match('0*$'):len()
    else
      prec = #decimals
    end
    if exponent then
      -- So '1230' and '1.23e3' both give prec = -1, and '0.00123' and '1.23e-3' give 5.
      prec = prec - exponent
    end
  end
  if in_current.istemperature and out_current.istemperature then
    -- Converting between common temperatures (°C, °F, °R, K); not keVT.
    -- Kelvin value can be almost zero, or small but negative due to precision problems.
    -- Also, an input value like -300 C (below absolute zero) gives negative kelvins.
    -- Calculate minimum precision from absolute value.
    adjust = 0
    local kelvin = abs((invalue - in_current.offset) * in_current.scale)
    if kelvin < 1e-8 then  -- assume nonzero due to input or calculation precision problem
      minprec = 2
    else
      minprec = 2 - floor(log10(kelvin) + fudge)  -- 3 sigfigs in kelvin
    end
  else
    if invalue == 0 or outvalue <= 0 then
      -- We are never called with a negative outvalue, but it might be zero.
      -- This is special-cased to avoid calculation exceptions.
      return record_default_precision(parms, out_current, 0)
    end
    if out_current.exception == 'integer_more_precision' and floor(invalue) == invalue then
      -- With certain output units that sometimes give poor results
      -- with default rounding, use more precision when the input
      -- value is equal to an integer. An example of a poor result
      -- is when input 50 gives a smaller output than input 49.5.
      -- Experiment shows this helps, but it does not eliminate all
      -- surprises because it is not clear whether "50" should be
      -- interpreted as "from 45 to 55" or "from 49.5 to 50.5".
      adjust = -log10(in_current.scale)
    elseif subunit_more_precision then
      -- Conversion like "{{convert|6|ft|1|in|cm}}" (where subunit is "in")
      -- has a non-standard adjust value, to give more output precision.
      adjust = log10(out_current.scale) + 2
    else
      adjust = log10(abs(invalue / outvalue))
    end
    adjust = adjust + log10(2)
    -- Ensure that the output has at least two significant figures.
    minprec = 1 - floor(log10(outvalue) + fudge)
  end
  if extra then
    adjust = extra.adjust or adjust
    minprec = extra.minprec or minprec
  end
  return record_default_precision(parms, out_current, math.max(floor(prec + adjust), minprec))
end

local function convert(parms, invalue, info, in_current, out_current)
  -- Convert given input value from one unit to another.
  -- Return output_value (a number) if a simple convert, or
  -- return f, t where
  --   f = true, t = table of information with results, or
  --   f = false, t = error message table.
  local inscale = in_current.scale
  local outscale = out_current.scale
  if not in_current.iscomplex and not out_current.iscomplex then
    return invalue * (inscale / outscale)  -- minimize overhead for most common case
  end
  if in_current.invert or out_current.invert then
    -- Inverted units, such as inverse length, inverse time, or
    -- fuel efficiency. Built-in units do not have invert set.
    if (in_current.invert or 1) * (out_current.invert or 1) < 0 then
      return 1 / (invalue * inscale * outscale)
    end
    return invalue * (inscale / outscale)
  elseif in_current.offset then
    -- Temperature (there are no built-ins for this type of unit).
    if info.is_change then
      return invalue * (inscale / outscale)
    end
    return (invalue - in_current.offset) * (inscale / outscale) + out_current.offset
  else
    -- Built-in unit.
    local in_builtin = in_current.builtin
    local out_builtin = out_current.builtin
    if in_builtin and out_builtin then
      if in_builtin == out_builtin then
        return invalue
      end
      -- There are no cases (yet) where need to convert from one
      -- built-in unit to another, so this should never occur.
      return false, { 'cvt_bug_convert' }
    end
    if in_builtin == 'mach' or out_builtin == 'mach' then
      local adjust
      if in_builtin == 'mach' then
        inscale = speed_of_sound(in_current.altitude)
        adjust = outscale / 0.1
      else
        outscale = speed_of_sound(out_current.altitude)
        adjust = 0.1 / inscale
      end
      return true, {
        outvalue = invalue * (inscale / outscale),
        adjust = log10(adjust) + log10(2),
      }
    elseif in_builtin == 'hand' then
      -- 1 hand = 4 inches; 1.2 hands = 6 inches.
      -- Decimals of a hand are only defined for the first digit, and
      -- the first fractional digit should be a number of inches (1, 2 or 3).
      -- However, this code interprets the entire fractional part as the number
      -- of inches / 10 (so 1.75 inches would be 0.175 hands).
      -- A value like 12.3 hands is exactly 12*4 + 3 inches; base default precision on that.
      local integer, fracpart = math.modf(invalue)
      local inch_value = 4 * integer + 10 * fracpart  -- equivalent number of inches
      local factor = inscale / outscale
      if factor == 4 then
        -- Am converting to inches: show exact result, and use "inches" not "in" by default.
        if parms.abbr_org == nil then
          out_current.usename = true
        end
        local show = format('%g', abs(inch_value))  -- show and clean are unsigned
        if not show:find('e', 1, true) then
          return true, {
            invalue = inch_value,
            outvalue = inch_value,
            clean = show,
            show = show,
          }
        end
      end
      local outvalue = (integer + 2.5 * fracpart) * factor
      local fracstr = info.clean:match('%.(.*)') or ''
      local fmt
      if fracstr == '' then
        fmt = '%.0f'
      else
        fmt = '%.' .. format('%d', #fracstr - 1) .. 'f'
      end
      return true, {
        invalue = inch_value,
        clean = format(fmt, inch_value),
        outvalue = outvalue,
        minprec = 0,
      }
    end
  end
  return false, { 'cvt_bug_convert' }  -- should never occur
end

local function user_style(parms, i)
  -- Return text for a user-specified style for a table cell, or '' if none,
  -- given i = 1 (input style) or 2 (output style).
  local style = parms[(i == 1) and 'stylein' or 'styleout']
  if style then
    style = style:gsub('"', '')
    if style ~= '' then
      if style:sub(-1) ~= ';' then
        style = style .. ';'
      end
      return style
    end
  end
  return ''
end

local function make_table_or_sort(parms, invalue, info, in_current, scaled_top)
  -- Set options to handle output for a table or a sort key, or both.
  -- The text sort key is based on the value resulting from converting
  -- the input to a fake base unit with scale = 1, and other properties
  -- required for a conversion derived from the input unit.
  -- For other modules, return the sort key in a hidden span element, and
  -- the scaled value used to generate the sort key.
  -- If scaled_top is set, it is the scaled value of the numerator of a per unit
  -- to be combined with this unit (the denominator) to make the sort key.
  -- Scaling only works with units that convert with a factor (not temperature).
  local sortkey, scaled_value
  if parms.opt_sortable_on then
    local base = {  -- a fake unit with enough fields for a valid convert
      scale = 1,
      invert = in_current.invert and 1,
      iscomplex = in_current.iscomplex,
      offset = in_current.offset and 0,
    }
    local outvalue, extra = convert(parms, invalue, info, in_current, base)
    if extra then
      outvalue = extra.outvalue
    end
    if in_current.istemperature then
      -- Have converted to kelvin; assume numbers close to zero have a
      -- rounding error and should be zero.
      if abs(outvalue) < 1e-12 then
        outvalue = 0
      end
    end
    if scaled_top and outvalue ~= 0 then
      outvalue = scaled_top / outvalue
    end
    scaled_value = outvalue
    if not valid_number(outvalue) then
      if outvalue < 0 then
        sortkey = '1000000000000000000'
      else
        sortkey = '9000000000000000000'
      end
    elseif outvalue == 0 then
      sortkey = '5000000000000000000'
    else
      local mag = floor(log10(abs(outvalue)) + 1e-14)
      local prefix
      if outvalue > 0 then
        prefix = 7000 + mag
      else
        prefix = 2999 - mag
        outvalue = outvalue + 10^(mag+1)
      end
      sortkey = format('%d', prefix) .. format('%015.0f', floor(outvalue * 10^(14-mag)))
    end
  end
  local sortspan
  if sortkey and (parms.opt_sortable_debug or not parms.table_align) then
    sortspan = parms.opt_sortable_debug and
      '<span style="border:1px solid;display:inline;" class="sortkey">' .. sortkey .. '♠</span>' or
      '<span style="display:none" class="sortkey">' .. sortkey .. '♠</span>'
    parms.join_before = sortspan
  end
  if parms.table_align then
    local style = 'style="text-align:' .. parms.table_align .. ';'
    local sort = sortkey and ' data-sort-value="' .. sortkey .. '"' or ''
    local joins = {}
    for i = 1, 2 do
      joins[i] = (i == 1 and '' or '\n|') .. style .. user_style(parms, i) .. '"' .. sort .. '|'
    end
    parms.table_joins = joins
  end
  return sortspan, scaled_value
end

local cvt_to_hand

local function cvtround(parms, info, in_current, out_current)
  -- Return true, t where t is a table with the conversion results; fields:
  --   show = rounded, formatted string with the result of converting value in info,
  --      using the rounding specified in parms.
  --   singular = true if result (after rounding and ignoring any negative sign)
  --      is "1", or like "1.00", or is a fraction with value < 1;
  --   (and more fields shown below, and a calculated 'absvalue' field).
  -- or return false, t where t is an error message table.
  -- Input info.clean uses en digits (it has been translated, if necessary).
  -- Output show uses en or non-en digits as appropriate, or can be spelled.
  if out_current.builtin == 'hand' then
    return cvt_to_hand(parms, info, in_current, out_current)
  end
  local invalue = in_current.builtin == 'hand' and info.altvalue or info.value
  local outvalue, extra = convert(parms, invalue, info, in_current, out_current)
  if parms.need_table_or_sort then
    parms.need_table_or_sort = nil  -- process using first input value only
    make_table_or_sort(parms, invalue, info, in_current)
  end
  if extra then
    if not outvalue then return false, extra end
    invalue = extra.invalue or invalue
    outvalue = extra.outvalue
  end
  if not valid_number(outvalue) then
    return false, { 'cvt_invalid_num' }
  end
  local isnegative
  if outvalue < 0 then
    isnegative = true
    outvalue = -outvalue
  end
  local precision, show, exponent
  local denominator = out_current.frac
  if denominator then
    show = fraction_table(outvalue, denominator)
  else
    precision = parms.precision
    if not precision then
      if parms.sigfig then
        show, exponent = make_sigfig(outvalue, parms.sigfig)
      elseif parms.opt_round then
        local n = parms.opt_round
        if n == 0.5 then
          local integer, fracpart = math.modf(floor(2 * outvalue + 0.5) / 2)
          if fracpart == 0 then
            show = format('%.0f', integer)
          else
            show = format('%.1f', integer + fracpart)
          end
        else
          show = format('%.0f', floor((outvalue / n) + 0.5) * n)
        end
      else
        local inclean = info.clean
        if extra then
          inclean = extra.clean or inclean
          show = extra.show
        end
        if not show then
          precision = default_precision(parms, invalue, inclean, info.denominator, outvalue, in_current, out_current, extra)
        end
      end
    end
  end
  if precision then
    if precision >= 0 then
      local fudge
      if precision <= 8 then
        -- Add a fudge to handle common cases of bad rounding due to inability
        -- to precisely represent some values. This makes the following work:
        -- {{convert|-100.1|C|K}} and {{convert|5555000|um|m|2}}.
        -- Old template uses #expr round, which invokes PHP round().
        -- LATER: Investigate how PHP round() works.
        fudge = 2e-14
      else
        fudge = 0
      end
      local fmt = '%.' .. format('%d', precision) .. 'f'
      local success
      success, show = pcall(format, fmt, outvalue + fudge)
      if not success then
        return false, { 'cvt_big_prec', tostring(precision) }
      end
    else
      precision = -precision  -- #digits to zero (in addition to any digits after dot)
      local shift = 10 ^ precision
      show = format('%.0f', outvalue/shift)
      if show ~= '0' then
        exponent = #show + precision
      end
    end
  end
  local t = format_number(parms, show, exponent, isnegative)
  if type(show) == 'string' then
    -- Set singular using match because on some systems 0.99999999999999999 is 1.0.
    if exponent then
      t.singular = (exponent == 1 and show:match('^10*$'))
    else
      t.singular = (show == '1' or show:match('^1%.0*$'))
    end
  else
    t.fraction_table = show
    t.singular = (outvalue <= 1)  -- cannot have 'fraction == 1', but if it were possible it would be singular
  end
  t.raw_absvalue = outvalue  -- absolute value before rounding
  return true, setmetatable(t, {
    __index = function (self, key)
      if key == 'absvalue' then
        -- Calculate absolute value after rounding, if needed.
        local clean, exponent = rawget(self, 'clean'), rawget(self, 'exponent')
        local value = tonumber(clean)  -- absolute value (any negative sign has been ignored)
        if exponent then
          value = value * 10^exponent
        end
        rawset(self, key, value)
        return value
      end
    end })
end

function cvt_to_hand(parms, info, in_current, out_current)
  -- Convert input to hands, inches.
  -- Return true, t where t is a table with the conversion results;
  -- or return false, t where t is an error message table.
  if parms.abbr_org == nil then
    out_current.usename = true  -- default is to show name not symbol
  end
  local precision = parms.precision
  local frac = out_current.frac
  if not frac and precision and precision > 1 then
    frac = (precision == 2) and 2 or 4
  end
  local out_next = out_current.out_next
  if out_next then
    -- Use magic knowledge to determine whether the next unit is inches without requiring i18n.
    -- The following ensures that when the output combination "hand in" is used, the inches
    -- value is rounded to match the hands value. Also, displaying say "61½" instead of 61.5
    -- is better as 61.5 implies the value is not 61.4.
    if out_next.exception == 'subunit_more_precision' then
      out_next.frac = frac
    end
  end
  -- Convert to inches; calculate hands from that.
  local dummy_unit_table = { scale = out_current.scale / 4, frac = frac }
  local success, outinfo = cvtround(parms, info, in_current, dummy_unit_table)
  if not success then return false, outinfo end
  local tfrac = outinfo.fraction_table
  local inches = outinfo.raw_absvalue
  if tfrac then
    inches = floor(inches)  -- integer part only; fraction added later
  else
    inches = floor(inches + 0.5)  -- a hands measurement never shows decimals of an inch
  end
  local hands, inches = divide(inches, 4)
  outinfo.absvalue = hands + inches/4  -- supposed to be the absolute rounded value, but this is close enough
  local inchstr = tostring(inches)  -- '0', '1', '2' or '3'
  if precision and precision <= 0 then  -- using negative or 0 for precision rounds to nearest hand
    hands = floor(outinfo.raw_absvalue/4 + 0.5)
    inchstr = ''
  elseif tfrac then
    -- Always show an integer before fraction (like "15.0½") because "15½" means 15-and-a-half hands.
    inchstr = numdot .. format_fraction(parms, 'out', false, inchstr, tfrac.numstr, tfrac.denstr)
  else
    inchstr = numdot .. from_en(inchstr)
  end
  outinfo.show = outinfo.sign .. with_separator(parms, format('%.0f', hands)) .. inchstr
  return true, outinfo
end

local function evaluate_condition(value, condition)
  -- Return true or false from applying a conditional expression to value,
  -- or throw an error if invalid.
  -- A very limited set of expressions is supported:
  --    v < 9
  --    v * 9 < 9
  -- where
  --    'v' is replaced with value
  --    9 is any number (as defined by Lua tonumber)
  --      only en digits are accepted
  --    '<' can also be '<=' or '>' or '>='
  -- In addition, the following form is supported:
  --    LHS and RHS
  -- where
  --    LHS, RHS = any of above expressions.
  local function compare(value, text)
    local arithop, factor, compop, limit = text:match('^%s*v%s*([*]?)(.-)([<>]=?)(.*)$')
    if arithop == nil then
      error('Invalid default expression', 0)
    elseif arithop == '*' then
      factor = tonumber(factor)
      if factor == nil then
        error('Invalid default expression', 0)
      end
      value = value * factor
    end
    limit = tonumber(limit)
    if limit == nil then
      error('Invalid default expression', 0)
    end
    if compop == '<' then
      return value < limit
    elseif compop == '<=' then
      return value <= limit
    elseif compop == '>' then
      return value > limit
    elseif compop == '>=' then
      return value >= limit
    end
    error('Invalid default expression', 0)  -- should not occur
  end
  local lhs, rhs = condition:match('^(.-%W)and(%W.*)')
  if lhs == nil then
    return compare(value, condition)
  end
  return compare(value, lhs) and compare(value, rhs)
end

local function get_default(value, unit_table)
  -- Return true, s where s = name of unit's default output unit,
  -- or return false, t where t is an error message table.
  -- Some units have a default that depends on the input value
  -- (the first value if a range of values is used).
  -- If '!' is in the default, the first bang-delimited field is an
  -- expression that uses 'v' to represent the input value.
  -- Example: 'v < 120 ! small ! big ! suffix' (suffix is optional)
  -- evaluates 'v < 120' as a boolean with result
  -- 'smallsuffix' if (value < 120), or 'bigsuffix' otherwise.
  -- Input must use en digits and '.' decimal mark.
  local default = data_code.default_exceptions[unit_table.defkey or unit_table.symbol] or unit_table.default
  if not default then
    local per = unit_table.per
    if per then
      local function a_default(v, u)
        local success, ucode = get_default(v, u)
        if not success then
          return '?'  -- an unlikely error has occurred; will cause lookup of default to fail
        end
        -- Attempt to use only the first unit if a combination or output multiple.
        -- This is not bulletproof but should work for most cases.
        -- Where it does not work, the convert will need to specify the wanted output unit.
        local t = all_units[ucode]
        if t then
          local combo = t.combination
          if combo then
            -- For a multiple like ftin, the "first" unit (ft) is last in the combination.
            local i = t.multiple and table_len(combo) or 1
            ucode = combo[i]
          end
        else
          -- Try for an automatically generated combination.
          local item = ucode:match('^(.-)%+') or ucode:match('^(%S+)%s')
          if all_units[item] then
            return item
          end
        end
        return ucode
      end
      local unit1, unit2 = per[1], per[2]
      local def1 = (unit1 and a_default(value, unit1) or unit_table.vprefix or '')
      local def2 = a_default(1, unit2)  -- 1 because per unit of denominator
      return true, def1 .. '/' .. def2
    end
    return false, { 'cvt_no_default', unit_table.symbol }
  end
  if default:find('!', 1, true) == nil then
    return true, default
  end
  local t = split(default, '!')
  if #t == 3 or #t == 4 then
    local success, result = pcall(evaluate_condition, value, t[1])
    if success then
      default = result and t[2] or t[3]
      if #t == 4 then
        default = default .. t[4]
      end
      return true, default
    end
  end
  return false, { 'cvt_bad_default', unit_table.symbol }
end

local linked_pages  -- to record linked pages so will not link to the same page more than once

local function unlink(unit_table)
  -- Forget that the given unit has previously been linked (if it has).
  -- That is needed when processing a range of inputs or outputs when an id
  -- for the first range value may have been evaluated, but only an id for
  -- the last value is displayed, and that id may need to be linked.
  linked_pages[unit_table.unitcode or unit_table] = nil
end

local function make_link(link, id, unit_table)
  -- Return wikilink "[[link|id]]", possibly abbreviated as in examples:
  --   [[mile]]  --> [[mile]]
  --   [[mile]]s --> [[mile]]s
  -- However, just id is returned if:
  -- * no link given (so caller does not need to check if a link was defined); or
  -- * link has previously been used during the current convert (to avoid overlinking).
  local link_key
  if unit_table then
    link_key = unit_table.unitcode or unit_table
  else
    link_key = link
  end
  if not link or link == '' or linked_pages[link_key] then
    return id
  end
  linked_pages[link_key] = true
  -- Following only works for language en, but it should be safe on other wikis,
  -- and overhead of doing it generally does not seem worthwhile.
  local l = link:sub(1, 1):lower() .. link:sub(2)
  if link == id or l == id then
    return '[[' .. id .. ']]'
  elseif link .. 's' == id or l .. 's' == id then
    return '[[' .. id:sub(1, -2) .. ']]s'
  else
    return '[[' .. link .. '|' .. id .. ']]'
  end
end

local function variable_name(clean, unit_table)
  -- For slwiki, a unit name depends on the value.
  -- Parameter clean is the unsigned rounded value in en digits, as a string.
  -- Value             Source    Example for "m"
  -- integer 1:        name1     meter  (also is the name of the unit)
  -- integer 2:        var{1}    metra
  -- integer 3 and 4:  var{2}    metri
  -- integer else:     var{3}    metrov (0 and 5 or more)
  -- real/fraction:    var{4}    metra
  -- var{i} means the i'th field in unit_table.varname if it exists and has
  -- an i'th field, otherwise name2.
  -- Fields are separated with "!" and are not empty.
  -- A field for a unit using an SI prefix has the prefix name inserted,
  -- replacing '#' if found, or before the field otherwise.
  local vname
  if clean == '1' then
    vname = unit_table.name1
  elseif unit_table.varname then
    local i
    if clean == '2' then
      i = 1
    elseif clean == '3' or clean == '4' then
      i = 2
    elseif clean:find('.', 1, true) then
      i = 4
    else
      i = 3
    end
    if i > 1 and varname == 'pl' then
      i = i - 1
    end
    vname = split(unit_table.varname, '!')[i]
  end
  if vname then
    local si_name = rawget(unit_table, 'si_name') or ''
    local pos = vname:find('#', 1, true)
    if pos then
      vname = vname:sub(1, pos - 1) .. si_name .. vname:sub(pos + 1)
    else
      vname = si_name .. vname
    end
    return vname
  end
  return unit_table.name2
end

local function linked_id(parms, unit_table, key_id, want_link, clean)
  -- Return final unit id (symbol or name), optionally with a wikilink,
  -- and update unit_table.sep if required.
  -- key_id is one of: 'symbol', 'sym_us', 'name1', 'name1_us', 'name2', 'name2_us'.
  local abbr_on = (key_id == 'symbol' or key_id == 'sym_us')
  if abbr_on and want_link then
    local symlink = rawget(unit_table, 'symlink')
    if symlink then
      return symlink  -- for exceptions that have the linked symbol built-in
    end
  end
  local multiplier = rawget(unit_table, 'multiplier')
  local per = unit_table.per
  if per then
    local unit1 = per[1]  -- top unit_table, or nil
    local unit2 = per[2]  -- bottom unit_table
    if abbr_on then
      if not unit1 then
        unit_table.sep = ''  -- no separator in "$2/acre"
      end
      if not want_link then
        local symbol = unit_table.symbol_raw
        if symbol then
          return symbol  -- for exceptions that have the symbol built-in
        end
      end
    end
    local key_id2  -- unit2 is always singular
    if key_id == 'name2' then
      key_id2 = 'name1'
    elseif key_id == 'name2_us' then
      key_id2 = 'name1_us'
    else
      key_id2 = key_id
    end
    local result
    if abbr_on then
      result = '/'
    elseif omitsep then
      result = per_word
    elseif unit1 then
      result = ' ' .. per_word .. ' '
    else
      result = per_word .. ' '
    end
    if want_link and unit_table.link then
      if abbr_on or not varname then
        result = (unit1 and linked_id(parms, unit1, key_id, false, clean) or '') .. result .. linked_id(parms, unit2, key_id2, false, '1')
      else
        result = (unit1 and variable_name(clean, unit1) or '') .. result .. variable_name('1', unit2)
      end
      if omit_separator(result) then
        unit_table.sep = ''
      end
      return make_link(unit_table.link, result, unit_table)
    end
    if unit1 then
      result = linked_id(parms, unit1, key_id, want_link, clean) .. result
      if unit1.sep then
        unit_table.sep = unit1.sep
      end
    elseif omitsep then
      unit_table.sep = ''
    end
    return result .. linked_id(parms, unit2, key_id2, want_link, '1')
  end
  if multiplier then
    -- A multiplier (like "100" in "100km") forces the unit to be plural.
    multiplier = from_en(multiplier)
    if not omitsep then
      multiplier = multiplier .. (abbr_on and '&nbsp;' or ' ')
    end
    if not abbr_on then
      if key_id == 'name1' then
        key_id = 'name2'
      elseif key_id == 'name1_us' then
        key_id = 'name2_us'
      end
    end
  else
    multiplier = ''
  end
  local id = unit_table.fixed_name or ((varname and not abbr_on) and variable_name(clean, unit_table) or unit_table[key_id])
  if omit_separator(id) then
    unit_table.sep = ''
  end
  if want_link then
    local link = data_code.link_exceptions[unit_table.linkey or unit_table.symbol] or unit_table.link
    if link then
      local before = ''
      local i = unit_table.customary
      if i == 1 and parms.opt_sp_us then
        i = 2  -- show "U.S." not "US"
      end
      if i == 3 and abbr_on then
        i = 4  -- abbreviate "imperial" to "imp"
      end
      local customary = text_code.customary_units[i]
      if customary then
        -- LATER: This works for language en only, but it's esoteric so ignore for now.
        local pertext
        if id:sub(1, 1) == '/' then
          -- Want unit "/USgal" to display as "/U.S. gal", not "U.S. /gal".
          pertext = '/'
          id = id:sub(2)
        elseif id:sub(1, 4) == 'per ' then
          -- Similarly want "per U.S. gallon", not "U.S. per gallon" (but in practice this is unlikely to be used).
          pertext = 'per '
          id = id:sub(5)
        else
          pertext = ''
        end
        -- Omit any "US"/"U.S."/"imp"/"imperial" from start of id since that will be inserted.
        local removes = (i < 3) and { 'US&nbsp;', 'US ', 'U.S.&nbsp;', 'U.S. ' } or { 'imp&nbsp;', 'imp ', 'imperial ' }
        for _, prefix in ipairs(removes) do
          local plen = #prefix
          if id:sub(1, plen) == prefix then
            id = id:sub(plen + 1)
            break
          end
        end
        before = pertext .. make_link(customary.link, customary[1]) .. ' '
      end
      id = before .. make_link(link, id, unit_table)
    end
  end
  return multiplier .. id
end

local function make_id(parms, which, unit_table)
  -- Return id, f where
  --   id = unit name or symbol, possibly modified
  --   f = true if id is a name, or false if id is a symbol
  -- using the value for index 'which', and for 'in' or 'out' (unit_table.inout).
  -- Result is '' if no symbol/name is to be used.
  -- In addition, set unit_table.sep = ' ' or '&nbsp;' or ''
  -- (the separator that caller will normally insert before the id).
  if parms.opt_values then
    unit_table.sep = ''
    return ''
  end
  local inout = unit_table.inout
  local info = unit_table.valinfo[which]
  local abbr_org = parms.abbr_org
  local adjectival = parms.opt_adjectival
  local lk = parms.lk
  local want_link = (lk == 'on' or lk == inout)
  local usename = unit_table.usename
  local singular = info.singular
  local want_name
  if usename then
    want_name = true
  else
    if abbr_org == nil then
      if parms.wantname then
        want_name = true
      end
      if unit_table.usesymbol then
        want_name = false
      end
    end
    if want_name == nil then
      local abbr = parms.abbr
      if abbr == 'on' or abbr == inout or (abbr == 'mos' and inout == 'out') then
        want_name = false
      else
        want_name = true
      end
    end
  end
  local key
  if want_name then
    if lk == nil and unit_table.builtin == 'hand' then
      want_link = true
    end
    if parms.opt_use_nbsp then
      unit_table.sep = '&nbsp;'
    else
      unit_table.sep = ' '
    end
    if parms.opt_singular then
      local value
      if inout == 'in' then
        value = info.value
      else
        value = info.absvalue
      end
      if value then  -- some unusual units do not always set value field
        value = abs(value)
        singular = (0 < value and value < 1.0001)
      end
    end
    if unit_table.engscale then
      -- engscale: so "|1|e3kg" gives "1 thousand kilograms" (plural)
      singular = false
    end
    key = (adjectival or singular) and 'name1' or 'name2'
    if parms.opt_sp_us then
      key = key .. '_us'
    end
  else
    if unit_table.builtin == 'hand' then
      if parms.opt_hand_hh then
        unit_table.symbol = 'hh'  -- LATER: might want i18n applied to this
      end
    end
    unit_table.sep = '&nbsp;'
    key = parms.opt_sp_us and 'sym_us' or 'symbol'
  end
  return linked_id(parms, unit_table, key, want_link, info.clean), want_name
end

local function decorate_value(parms, unit_table, which, number_word)
  -- If needed, update unit_table so values will be shown with extra information.
  -- For consistency with the old template (but different from fmtpower),
  -- the style to display powers of 10 includes "display:none" to allow some
  -- browsers to copy, for example, "10³" as "10^3", rather than as "103".
  local info
  local engscale = unit_table.engscale
  local prefix = unit_table.vprefix
  if engscale or prefix then
    info = unit_table.valinfo[which]
    if info.decorated then
      return  -- do not redecorate if repeating convert
    end
    info.decorated = true
    if engscale then
      local inout = unit_table.inout
      local abbr = parms.abbr
      if (abbr == 'on' or abbr == inout) and not parms.number_word then
        info.show = info.show ..
        '<span style="margin-left:0.2em">×<span style="margin-left:0.1em">' ..
        from_en('10') ..
        '</span></span><s style="display:none">^</s><sup>' ..
        from_en(tostring(engscale.exponent)) .. '</sup>'
      elseif number_word then
        local number_id
        local lk = parms.lk
        if lk == 'on' or lk == inout then
          number_id = make_link(engscale.link, engscale[1])
        else
          number_id = engscale[1]
        end
        -- WP:NUMERAL recommends "&nbsp;" in values like "12 million".
        info.show = info.show .. (parms.opt_adjectival and '-' or '&nbsp;') .. number_id
      end
    end
    if prefix then
      info.show = prefix .. info.show
    end
  end
end

local function process_input(parms, in_current)
  -- Processing required once per conversion.
  -- Return block of text to represent input (value/unit).
  if parms.opt_output_only or parms.opt_output_number_only or parms.opt_output_unit_only then
    parms.joins = { '', '' }
    return ''
  end
  local first_unit
  local composite = in_current.composite  -- nil or table of units
  if composite then
    first_unit = composite[1]
  else
    first_unit = in_current
  end
  local id1, want_name = make_id(parms, 1, first_unit)
  local sep = first_unit.sep  -- separator between value and unit, set by make_id
  local preunit = parms.preunit1
  if preunit then
    sep = ''  -- any separator is included in preunit
  else
    preunit = ''
  end
  if parms.opt_input_unit_only then
    parms.joins = { '', '' }
    if composite then
      local parts = { id1 }
      for i, unit in ipairs(composite) do
        if i > 1 then
          table.insert(parts, (make_id(parms, 1, unit)))
        end
      end
      id1 = table.concat(parts, ' ')
    end
    if want_name and parms.opt_adjectival then
      return preunit .. hyphenated(id1)
    end
    return  preunit .. id1
  end
  if parms.opt_also_symbol and not composite then
    local join1 = parms.joins[1]
    if join1 == ' (' or join1 == ' [' then
      parms.joins = { ' [' .. first_unit[parms.opt_sp_us and 'sym_us' or 'symbol'] .. ']' .. join1 , parms.joins[2] }
    end
  end
  if in_current.builtin == 'mach' and first_unit.sep ~= '' then  -- '' means omitsep with non-enwiki name
    local prefix = id1 .. '&nbsp;'
    local range = parms.range
    local valinfo = first_unit.valinfo
    local result = prefix .. valinfo[1].show
    if range then
      -- For simplicity and because more not needed, handle one range item only.
      local prefix2 = make_id(parms, 2, first_unit) .. '&nbsp;'
      result = range_text(range[1], want_name, parms, result, prefix2 .. valinfo[2].show, 'in')
    end
    return preunit .. result
  end
  if composite then
    -- Simplify: assume there is no range, and no decoration.
    local mid = (not parms.opt_flip) and parms.mid or ''
    local sep1 = '&nbsp;'
    local sep2 = ' '
    if parms.opt_adjectival and want_name then
      sep1 = '-'
      sep2 = '-'
    end
    if omitsep and sep == '' then
      -- Testing the id of the most significant unit should be sufficient.
      sep1 = ''
      sep2 = ''
    end
    local parts = { first_unit.valinfo[1].show .. sep1 .. id1 }
    for i, unit in ipairs(composite) do
      if i > 1 then
        table.insert(parts, unit.valinfo[1].show .. sep1 .. (make_id(parms, 1, unit)))
      end
    end
    return table.concat(parts, sep2) .. mid
  end
  local add_unit = (parms.abbr == 'mos') or
    parms[parms.opt_flip and 'out_range_x' or 'in_range_x'] or
    (not want_name and parms.abbr_range_x)
  local range = parms.range
  if range and not add_unit then
    unlink(first_unit)
  end
  local id = range and make_id(parms, range.n + 1, first_unit) or id1
  local extra, was_hyphenated = hyphenated_maybe(parms, want_name, sep, id, 'in')
  if was_hyphenated then
    add_unit = false
  end
  local result
  local valinfo = first_unit.valinfo
  if range then
    for i = 0, range.n do
      local number_word
      if i == range.n then
        add_unit = false
        number_word = true
      end
      decorate_value(parms, first_unit, i+1, number_word)
      local show = valinfo[i+1].show
      if add_unit then
        show = show .. first_unit.sep .. (i == 0 and id1 or make_id(parms, i+1, first_unit))
      end
      if i == 0 then
        result = show
      else
        result = range_text(range[i], want_name, parms, result, show, 'in')
      end
    end
  else
    decorate_value(parms, first_unit, 1, true)
    result = valinfo[1].show
  end
  return result .. preunit .. extra
end

local function process_one_output(parms, out_current)
  -- Processing required for each output unit.
  -- Return block of text to represent output (value/unit).
  local inout = out_current.inout  -- normally 'out' but can be 'in' for order=out
  local id1, want_name = make_id(parms, 1, out_current)
  local sep = out_current.sep  -- set by make_id
  local preunit = parms.preunit2
  if preunit then
    sep = ''  -- any separator is included in preunit
  else
    preunit = ''
  end
  if parms.opt_output_unit_only then
    if want_name and parms.opt_adjectival then
      return preunit .. hyphenated(id1)
    end
    return preunit .. id1
  end
  if out_current.builtin == 'mach' and out_current.sep ~= '' then  -- '' means omitsep with non-enwiki name
    local prefix = id1 .. '&nbsp;'
    local range = parms.range
    local valinfo = out_current.valinfo
    local result = prefix .. valinfo[1].show
    if range then
      -- For simplicity and because more not needed, handle one range item only.
      result = range_text(range[1], want_name, parms, result, prefix .. valinfo[2].show, inout)
    end
    return preunit .. result
  end
  local add_unit = (parms[parms.opt_flip and 'in_range_x' or 'out_range_x'] or
    (not want_name and parms.abbr_range_x)) and
    not parms.opt_output_number_only
  local range = parms.range
  if range and not add_unit then
    unlink(out_current)
  end
  local id = range and make_id(parms, range.n + 1, out_current) or id1
  local extra, was_hyphenated = hyphenated_maybe(parms, want_name, sep, id, inout)
  if was_hyphenated then
    add_unit = false
  end
  local result
  local valinfo = out_current.valinfo
  if range then
    for i = 0, range.n do
      local number_word
      if i == range.n then
        add_unit = false
        number_word = true
      end
      decorate_value(parms, out_current, i+1, number_word)
      local show = valinfo[i+1].show
      if add_unit then
        show = show .. out_current.sep .. (i == 0 and id1 or make_id(parms, i+1, out_current))
      end
      if i == 0 then
        result = show
      else
        result = range_text(range[i], want_name, parms, result, show, inout)
      end
    end
  else
    decorate_value(parms, out_current, 1, true)
    result = valinfo[1].show
  end
  if parms.opt_output_number_only then
    return result
  end
  return result .. preunit .. extra
end

local function make_output_single(parms, in_unit_table, out_unit_table)
  -- Return true, item where item = wikitext of the conversion result
  -- for a single output (which is not a combination or a multiple);
  -- or return false, t where t is an error message table.
  if parms.opt_order_out and in_unit_table.unitcode == out_unit_table.unitcode then
    out_unit_table.valinfo = in_unit_table.valinfo
  else
    out_unit_table.valinfo = collection()
    for _, v in ipairs(in_unit_table.valinfo) do
      local success, info = cvtround(parms, v, in_unit_table, out_unit_table)
      if not success then return false, info end
      out_unit_table.valinfo:add(info)
    end
  end
  return true, process_one_output(parms, out_unit_table)
end

local function make_output_multiple(parms, in_unit_table, out_unit_table)
  -- Return true, item where item = wikitext of the conversion result
  -- for an output which is a multiple (like 'ftin');
  -- or return false, t where t is an error message table.
  local inout = out_unit_table.inout  -- normally 'out' but can be 'in' for order=out
  local multiple = out_unit_table.multiple  -- table of scaling factors (will not be nil)
  local combos = out_unit_table.combination  -- table of unit tables (will not be nil)
  local abbr = parms.abbr
  local abbr_org = parms.abbr_org
  local disp = parms.disp
  local want_name = (abbr_org == nil and (disp == 'or' or disp == 'slash')) or
            not (abbr == 'on' or abbr == inout or abbr == 'mos')
  local want_link = (parms.lk == 'on' or parms.lk == inout)
  local mid = parms.opt_flip and parms.mid or ''
  local sep1 = '&nbsp;'
  local sep2 = ' '
  if parms.opt_adjectival and want_name then
    sep1 = '-'
    sep2 = '-'
  end
  local do_spell = parms.opt_spell_out
  parms.opt_spell_out = nil  -- so the call to cvtround does not spell the value
  local function make_result(info, isfirst)
    local fmt, outvalue, sign
    local results = {}
    for i = 1, #combos do
      local tfrac, thisvalue, strforce
      local out_current = combos[i]
      out_current.inout = inout
      local scale = multiple[i]
      if i == 1 then  -- least significant unit ('in' from 'ftin')
        local decimals
        out_current.frac = out_unit_table.frac
        local success, outinfo = cvtround(parms, info, in_unit_table, out_current)
        if not success then return false, outinfo end
        if isfirst then
          out_unit_table.valinfo = { outinfo }  -- in case output value of first least significant unit is needed
        end
        sign = outinfo.sign
        tfrac = outinfo.fraction_table
        if outinfo.is_scientific then
          strforce = outinfo.show
          decimals = ''
        elseif tfrac then
          decimals = ''
        else
          local show = outinfo.show  -- number as a string in local language
          local p1, p2 = show:find(numdot, 1, true)
          decimals = p1 and show:sub(p2 + 1) or ''  -- text after numdot, if any
        end
        fmt = '%.' .. ulen(decimals) .. 'f'  -- to reproduce precision
        if decimals == '' then
          if tfrac then
            outvalue = floor(outinfo.raw_absvalue)  -- integer part only; fraction added later
          else
            outvalue = floor(outinfo.raw_absvalue + 0.5)  -- keep all integer digits of least significant unit
          end
        else
          outvalue = outinfo.absvalue
        end
      end
      if scale then
        outvalue, thisvalue = divide(outvalue, scale)
      else
        thisvalue = outvalue
      end
      local id
      if want_name then
        if varname then
          local clean
          if strforce or tfrac then
            clean = '.1'  -- dummy value to force name for floating point
          else
            clean = format(fmt, thisvalue)
          end
          id = variable_name(clean, out_current)
        else
          local key = 'name2'
          if parms.opt_adjectival then
            key = 'name1'
          elseif tfrac then
            if thisvalue == 0 then
              key = 'name1'
            end
          elseif parms.opt_singular then
            if 0 < thisvalue and thisvalue < 1.0001 then
              key = 'name1'
            end
          else
            if thisvalue == 1 then
              key = 'name1'
            end
          end
          id = out_current[key]
        end
      else
        id = out_current['symbol']
      end
      if i == 1 and omit_separator(id) then
        -- Testing the id of the least significant unit should be sufficient.
        sep1 = ''
        sep2 = ''
      end
      if want_link then
        local link = out_current.link
        if link then
          id = make_link(link, id, out_current)
        end
      end
      local strval
      local spell_inout = (i == #combos or outvalue == 0) and inout or ''  -- trick so the last value processed (first displayed) has uppercase, if requested
      if strforce and outvalue == 0 then
        sign = ''  -- any sign is in strforce
        strval = strforce  -- show small values in scientific notation; will only use least significant unit
      elseif tfrac then
        local wholestr = (thisvalue > 0) and tostring(thisvalue) or nil
        strval = format_fraction(parms, spell_inout, false, wholestr, tfrac.numstr, tfrac.denstr, do_spell)
      else
        strval = (thisvalue == 0) and from_en('0') or with_separator(parms, format(fmt, thisvalue))
        if do_spell then
          strval = spell_number(parms, spell_inout, strval) or strval
        end
      end
      table.insert(results, strval .. sep1 .. id)
      if outvalue == 0 then
        break
      end
      fmt = '%.0f'  -- only least significant unit can have a non-integral value
    end
    local reversed, count = {}, #results
    for i = 1, count do
      reversed[i] = results[count + 1 - i]
    end
    return true, sign .. table.concat(reversed, sep2)
  end
  local valinfo = in_unit_table.valinfo
  local success, result = make_result(valinfo[1], true)
  if not success then return false, result end
  local range = parms.range
  if range then
    for i = 1, range.n do
      local success, result2 = make_result(valinfo[i+1])
      if not success then return false, result2 end
      result = range_text(range[i], want_name, parms, result, result2, inout)
    end
  end
  return true, result .. mid
end

local function process(parms, in_unit_table, out_unit_table)
  -- Return true, s, outunit where s = final wikitext result,
  -- or return false, t where t is an error message table.
  linked_pages = {}
  local success, bad_output
  local bad_input_mcode = in_unit_table.bad_mcode  -- nil if input unit is a valid convert unit
  local out_unit = parms.out_unit
  if out_unit == nil or out_unit == '' or type(out_unit) == 'function' then
    if bad_input_mcode or parms.opt_input_unit_only then
      bad_output = ''
    else
      local getdef = type(out_unit) == 'function' and out_unit or get_default
      success, out_unit = getdef(in_unit_table.valinfo[1].value, in_unit_table)
      parms.out_unit = out_unit
      if not success then
        bad_output = out_unit
      end
    end
  end
  if not bad_output and not out_unit_table then
    success, out_unit_table = lookup(parms, out_unit, 'any_combination')
    if success then
      local mismatch = check_mismatch(in_unit_table, out_unit_table)
      if mismatch then
        bad_output = mismatch
      end
    else
      bad_output = out_unit_table
    end
  end
  local lhs, rhs
  local flipped = parms.opt_flip and not bad_input_mcode
  if bad_output then
    rhs = (bad_output == '') and '' or message(parms, bad_output)
  elseif parms.opt_input_unit_only then
    rhs = ''
  else
    local combos  -- nil (for 'ft' or 'ftin'), or table of unit tables (for 'm ft')
    if not out_unit_table.multiple then  -- nil/false ('ft' or 'm ft'), or table of factors ('ftin')
      combos = out_unit_table.combination
    end
    local frac = parms.frac  -- nil or denominator of fraction for output values
    if frac then
      -- Apply fraction to the unit (if only one), or to non-SI units (if a combination),
      -- except that if a precision is also specified, the fraction only applies to
      -- the hand unit; that allows the following result:
      -- {{convert|156|cm|in hand|1|frac=2}} → 156 centimetres (61.4 in; 15.1½ hands)
      -- However, the following is handled elsewhere as a special case:
      -- {{convert|156|cm|hand in|1|frac=2}} → 156 centimetres (15.1½ hands; 61½ in)
      if combos then
        local precision = parms.precision
        for _, unit in ipairs(combos) do
          if unit.builtin == 'hand' or (not precision and not unit.prefixes) then
            unit.frac = frac
          end
        end
      else
        out_unit_table.frac = frac
      end
    end
    local outputs = {}
    local imax = combos and #combos or 1  -- 1 (single unit) or number of unit tables
    if imax == 1 then
      parms.opt_order_out = nil  -- only useful with an output combination
    end
    if not flipped and not parms.opt_order_out then
      -- Process left side first so any duplicate links (from lk=on) are suppressed
      -- on right. Example: {{convert|28|e9pc|e9ly|abbr=off|lk=on}}
      lhs = process_input(parms, in_unit_table)
    end
    for i = 1, imax do
      local success, item
      local out_current = combos and combos[i] or out_unit_table
      out_current.inout = 'out'
      if i == 1 then
        if imax > 1 and out_current.builtin == 'hand' then
          out_current.out_next = combos[2]  -- built-in hand can influence next unit in a combination
        end
        if parms.opt_order_out then
          out_current.inout = 'in'
        end
      end
      if out_current.multiple then
        success, item = make_output_multiple(parms, in_unit_table, out_current)
      else
        success, item = make_output_single(parms, in_unit_table, out_current)
      end
      if not success then return false, item end
      outputs[i] = item
    end
    if parms.opt_order_out then
      lhs = outputs[1]
      table.remove(outputs, 1)
    end
    local sep = parms.table_joins and parms.table_joins[2] or parms.join_between
    rhs = table.concat(outputs, sep)
  end
  if flipped or not lhs then
    local input = process_input(parms, in_unit_table)
    if flipped then
      lhs = rhs
      rhs = input
    else
      lhs = input
    end
  end
  if parms.join_before then
    lhs = parms.join_before .. lhs
  end
  local wikitext
  if bad_input_mcode then
    if bad_input_mcode == '' then
      wikitext = lhs
    else
      wikitext = lhs .. message(parms, bad_input_mcode)
    end
  elseif parms.table_joins then
    wikitext = parms.table_joins[1] .. lhs .. parms.table_joins[2] .. rhs
  else
    wikitext = lhs .. parms.joins[1] .. rhs .. parms.joins[2]
  end
  if parms.warnings and not bad_input_mcode then
    wikitext = wikitext .. parms.warnings
  end
  return true, wikitext, out_unit_table
end

local function main_convert(frame)
  -- Do convert, and if needed, do it again with higher default precision.
  local parms = { frame = frame }  -- will hold template arguments, after translation
  set_config(frame.args)
  local success, result = get_parms(parms, frame:getParent().args)
  if success then
    if type(result) ~= 'table' then
      return tostring(result)
    end
    local in_unit_table = result
    local out_unit_table
    for _ = 1, 2 do  -- use counter so cannot get stuck repeating convert
      success, result, out_unit_table = process(parms, in_unit_table, out_unit_table)
      if success and parms.do_convert_again then
        parms.do_convert_again = false
      else
        break
      end
    end
  end
  -- If input=x gives a problem, the result should be just the user input
  -- (if x is a property like P123 it has been replaced with '').
  -- An unknown input unit would display the input and an error message
  -- with success == true at this point.
  -- Also, can have success == false with a message to output an empty string.
  if success then
    if parms.have_problem and parms.input_text then
      return parms.input_text
    end
    return result
  end
  return parms.input_text and parms.input_text or message(parms, result)
end

local function _unit(unitcode, options)
  -- Helper function for Module:Val to look up a unit.
  -- Parameter unitcode must be a string to identify the wanted unit.
  -- Parameter options must be nil or a table with optional fields:
  --   value = number (for sort key; default value is 1)
  --   scaled_top = nil for a normal unit, or a number for a unit which is
  --                the denominator of a per unit (for sort key)
  --   si = { 'symbol', 'link' }
  --                (a table with two strings) to make an SI unit
  --                that will be used for the look up
  --   link = true if result should be [[linked]]
  --   sort = 'on' or 'debug' if result should include a sort key in a
  --                span element ('debug' makes the key visible)
  --   name = true for the name of the unit instead of the symbol
  --   us = true for the US spelling of the unit, if any
  -- Return nil if unitcode is not a non-empty string.
  -- Otherwise return a table with fields:
  --   text = requested symbol or name of unit, optionally linked
  --   scaled_value = input value adjusted by unit scale; used for sort key
  --   sortspan = span element with sort key like that provided by {{ntsh}},
  --     calculated from the result of converting value
  --     to a base unit with scale 1.
  --   unknown = true if the unitcode was not known
  unitcode = strip(unitcode)
  if unitcode == nil or unitcode == '' then
    return nil
  end
  set_config({})
  linked_pages = {}
  options = options or {}
  local parms = {
    abbr = options.name and 'off' or 'on',
    lk = options.link and 'on' or nil,
    opt_sp_us = options.us and true or nil,
    opt_ignore_error = true,  -- do not add pages using this function to 'what links here' for Module:Convert/extra
    opt_sortable_on = options.sort == 'on' or options.sort == 'debug',
    opt_sortable_debug = options.sort == 'debug',
  }
  if options.si then
    -- Make a dummy table of units (just one unit) for lookup to use.
    -- This makes lookup recognize any SI prefix in the unitcode.
    local symbol = options.si[1] or '?'
    parms.unittable = { [symbol] = {
      _name1 = symbol,
      _name2 = symbol,
      _symbol = symbol,
      utype = symbol,
      scale = symbol == 'g' and 0.001 or 1,
      prefixes = 1,
      default = symbol,
      link = options.si[2],
    }}
  end
  local success, unit_table = lookup(parms, unitcode, 'no_combination')
  if not success then
    unit_table = setmetatable({
      symbol = unitcode, name2 = unitcode, utype = unitcode,
      scale = 1, default = '', defkey = '', linkey = '' }, unit_mt)
  end
  local value = tonumber(options.value) or 1
  local clean = tostring(abs(value))
  local info = {
    value = value,
    altvalue = value,
    singular = (clean == '1'),
    clean = clean,
    show = clean,
  }
  unit_table.inout = 'in'
  unit_table.valinfo = { info }
  local sortspan, scaled_value
  if options.sort then
    sortspan, scaled_value = make_table_or_sort(parms, value, info, unit_table, options.scaled_top)
  end
  return {
    text = make_id(parms, 1, unit_table),
    sortspan = sortspan,
    scaled_value = scaled_value,
    unknown = not success and true or nil,
  }
end

return { convert = main_convert, _unit = _unit }