Compact but hacky and inefficient float number to string using a binary search on the precision

number_to_string.lua

-- Lookup table for format strings
local formats = {}
for i = 1, 17 do
	formats[i] = ("%%.%dg"):format(i)
end

local function _number_to_string(num, min_digits, max_digits)
	if min_digits > max_digits then
		return
	end
	local mid = (min_digits + max_digits) // 2
	local formatted = formats[mid]:format(num)
	if tonumber(formatted) == num then
		return _number_to_string(num, min_digits, mid - 1) or formatted
	end
	return _number_to_string(num, mid + 1, max_digits)
end

local function number_to_string(num)
	return assert(_number_to_string(num, 1, #formats))
end

-- Tests
for _ = 1, 1e3 do
	local x = math.random(0, 2^53 - 1) * 2^math.random(-1e3, 1e3 - 30)
	assert(tonumber(number_to_string(x)) == x)
end