--

-- A string.format wrapper that gracefully handles invalid arguments

--

local tostring = tostring;

local select = select;

local assert = assert;

local unpack = unpack;

local type = type;

local function format(format, ...)

	local args, args_length = { ... }, select('#', ...);

	-- format specifier spec:

	-- 1. Start: '%%'

	-- 2. Flags: '[%-%+ #0]'

	-- 3. Width: '%d?%d?'

	-- 4. Precision: '%.?%d?%d?'

	-- 5. Option: '[cdiouxXaAeEfgGqs%%]'

	--

	-- The options c, d, E, e, f, g, G, i, o, u, X, and x all expect a number as argument, whereas q and s expect a string.

	-- This function does not accept string values containing embedded zeros, except as arguments to the q option.

	-- a and A are only in Lua 5.2+

	-- process each format specifier

	local i = 0;

	format = format:gsub("%%[^cdiouxXaAeEfgGqs%%]*[cdiouxXaAeEfgGqs%%]", function(spec)

		if spec ~= "%%" then

			i = i + 1;

			local arg = args[i];

			if arg == nil then -- special handling for nil

				arg = "<nil>"

				args[i] = "<nil>";

			end

			local option = spec:sub(-1);

			if option == "q" or option == "s" then -- arg should be string

				args[i] = tostring(arg);

			elseif type(arg) ~= "number" then -- arg isn't number as expected?

				args[i] = tostring(arg);

				spec = "[%s]";

			end

		end

		return spec;

	end);

	-- process extra args

	while i < args_length do

		i = i + 1;

		local arg = args[i];

		if arg == nil then

			args[i] = "<nil>";

		else

			args[i] = tostring(arg);

		end

		format = format .. " [%s]"

	end

	return format:format(unpack(args));

end

local function test()

	assert(format("%s", "hello") == "hello");

	assert(format("%s") == "<nil>");

	assert(format("%s", true) == "true");

	assert(format("%d", true) == "[true]");

	assert(format("%%", true) == "% [true]");

end

return {

	format = format;

	test = test;

};