if altitude < 0 then
a = -a
end
if a < -3 then
a = -3
elseif a > 80 then
a = 80
end
return mach_table[a + 4] * 0.44704 -- mph converted to m/s
end
-- END: Code required only for built-in units.
---  
local function add_style(parms, class)
-- Add selected template style to parms if not already present.
parms.templatestyles = parms.templatestyles or {}
if not parms.templatestyles[class] then
parms.templatestyles[class] = parms.:extensionTag({
name = 'templatestyles', args = { src = text_code.titles[class] }
})
end
end  
local function get_styles(parms)
-- Return string of required template styles, empty if .
if parms.templatestyles then
local t = {}
for _, v in pairs(parms.templatestyles) do
table.insert(t, v)
end
return table.concat(t)
end
return ''
end  
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, and for a symbol
-- this is only called from get_default for default_exceptions.
__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.