Module:User:Mr. Stradivarius/sandbox5

local mArguments = require('Module:Arguments')

local validFields = {
	type = true,
	scale = true,
	region = true
}

local p = {}

local function err(msg)
	return string.format('<strong class="error">Error: %s</strong>', msg)
end

function p.main(frame)
	local args = mArguments.getArgs(frame)
	return p._main(args)
end

function p._main(args)
	local input = args[1]
	local field = args[2] or args.field
	local showCityData = args.citydata == 'yes'
	
	-- Validate input.
	if not input then
		return err('no input code specified')
	elseif not field then
		return err('no field specified')
	end
	
	-- Parse the input string.
	local substrings = mw.text.split(input, '_')
	local data = {}
	for i, substring in ipairs(substrings) do
		local key, value = mw.ustring.match(substring, '^(.-):(.*)$')
		if key and validFields[key] then
			data[key] = value
		end
	end
	
	-- Fetch the result.
	if field == 'error' then
		-- Check for missing data.
		local missingFields = {}
		for validField in pairs(validFields) do
			if not data[validField] then
				table.insert(missingFields, validField)
			end
		end
		if #missingFields > 0 then
			local msg = 'the '
				.. mw.text.listToText(missingFields)
				.. ' fields are missing'
			return err(msg)
		else
			return ''
		end
	else
		-- Return the specified field, or the blank string if it is missing.
		local result = data[field] or ''
		if field == 'type' then
			-- Check for type values like "city(20000)".
			local city, population = mw.ustring.match(result, '^(city)%((.-)%)$')
			if city then
				if showCityData then
					return population
				else
					return city
				end
			end
		end
		return result
	end
end

return p