util.smqueue: Refactor checkpoint logic and add tests The previous checkpoint logic used recursion, and the code was more complex than needed due to maintenance of the self._next_checkpoint field. This switches to a loop, which should be a bit more efficient, partly because there are fewer calls (although it was a tail call) and because table lookups can now be cached between each execution. _call_checkpoints() is now called unconditionally for every ack, which is simpler though adds a function call, it should be more than balanced out by the other changes.

local s_char = string.char;
local s_format = string.format;
local s_gsub = string.gsub;
local s_lower = string.lower;

local char_to_hex = {};
local hex_to_char = {};

do
	local char, hex;
	for i = 0,255 do
		char, hex = s_char(i), s_format("%02x", i);
		char_to_hex[char] = hex;
		hex_to_char[hex] = char;
	end
end

local function to(s)
	return (s_gsub(s, ".", char_to_hex));
end

local function from(s)
	return (s_gsub(s_lower(s), "%X*(%x%x)%X*", hex_to_char));
end

return {
	encode = to, decode = from;
	-- COMPAT w/pre-0.12:
	to = to, from = from;
};