-- Simple template language

--

-- The new() function takes a pattern and an escape function and returns

-- a render() function.  Both are required.

--

-- The function render() takes a string template and a table of values.

-- Sequences like {name} in the template string are substituted

-- with values from the table, optionally depending on a modifier

-- symbol.

--

-- Variants are:

-- {name} is substituted for values["name"] and is escaped using the

-- second argument to new_render().  To disable the escaping, use {name!}.

-- {name.item} can be used to access table items.

-- To renter lists of items: {name# item number {idx} is {item} }

-- Or key-value pairs: {name% t[ {idx} ] = {item} }

-- To show a defaults for missing values {name? sub-template } can be used,

-- which renders a sub-template if values["name"] is false-ish.

-- {name& sub-template } does the opposite, the sub-template is rendered

-- if the selected value is anything but false or nil.

local type, tostring = type, tostring;

local pairs, ipairs = pairs, ipairs;

local s_sub, s_gsub, s_match = string.sub, string.gsub, string.match;

local t_concat = table.concat;

local function new_render(pat, escape, funcs)

	-- assert(type(pat) == "string", "bad argument #1 to 'new_render' (string expected)");

	-- assert(type(escape) == "function", "bad argument #2 to 'new_render' (function expected)");

	local function render(template, values)

		-- assert(type(template) == "string", "bad argument #1 to 'render' (string expected)");

		-- assert(type(values) == "table", "bad argument #2 to 'render' (table expected)");

		return (s_gsub(template, pat, function (block)

			block = s_sub(block, 2, -2);

			local name, raw, opt, e = s_match(block, "^([%a_][%w_.]*)(!?)(%p?)()");

			if not name then return end

			local value = values[name];

			if not value and name:find(".", 2, true) then

				value = values;

				for word in name:gmatch"[^.]+" do

					value = value[word];

					if not value then break; end

				end

			end

			if funcs then

				while opt == '|' do

					local f;

					f, raw, opt, e = s_match(block, "^([%a_][%w_.]*)(!?)(%p?)()", e);

					f = funcs[f];

					if value ~= nil and f then value = f(value); end

				end

			end

			if opt == '#' or opt == '%' then

				if type(value) ~= "table" then return ""; end

				local iter = opt == '#' and ipairs or pairs;

				local out, i, subtpl = {}, 1, s_sub(block, e);

				local subvalues = setmetatable({}, { __index = values });

				for idx, item in iter(value) do

					subvalues.idx = idx;

					subvalues.item = item;

					out[i], i = render(subtpl, subvalues), i+1;

				end

				return t_concat(out);

			elseif opt == '&' then

				if not value then return ""; end

				return render(s_sub(block, e), values);

			elseif opt == '?' and not value then

				return render(s_sub(block, e), values);

			elseif value ~= nil then

				if type(value) ~= "string" then

					value = tostring(value);

				end

				if raw ~= '!' then

					return escape(value);

				end

				return value;

			end

		end));

	end

	return render;

end

return {

	new = new_render;

};