Randomization of `pairs` traversal order in Lua

randpairs.lua

--[[
	Randomize `pairs` traversal order to deliberately break code that relies on it.
	Works by doing a coin toss to decide whether to "postpone" visiting the first entry
	(visiting it at the end of the traversal).
	Shuffling all keys, then iterating in that shuffled order would provide more thorough randomization; however:
	* It is less efficient if the loop is exited early; it may arbitrarily wreck performance
	  (consider a large table where `pairs` is only used to inspect very few entries.)
	* I believe this simple solution already suffices for most code
	  that incorrectly relies on a specific order of traversal.
	  Notably, this has a 50% chance of breaking code that
	  expects `pairs` to exhibit `ipairs`-like behavior.

	This does not seed the random for you,
	so if you want to achieve different outcomes across different runs,
	seed the random yourself.
]]
function pairs(tab)
	local fk, fv = next(tab)
	local sk = nil
	if math.random(2) == 1 then
		sk = fk
	end
	local f = sk ~= nil
	local done = false
	return function(t, k)
		if done then
			return nil
		end
		local rk, rv = next(t, k)
		if rk == nil then
			done = true
			if f then
				f = false
				return fk, fv
			end
			return nil
		end
		if f and rawequal(rk, fk) then
			f = false
		end
		return rk, rv
	end, tab, sk
end