adventofcode-2021/day03/part2.lua

local ZERO = string.byte("0", 1)
local ONE = string.byte("1", 1)
local lines = {}
for line in io.lines(arg[3]) do
	local ones = {}
	for char = 1, #line do
		table.insert(ones, line:byte(char) == ONE)
	end
	table.insert(lines, ones)
end

function reduce(left, keep)
	local char = 1
	while #left > 1 and char <= #lines[1] do
		local ones = 0
		for k, line in pairs(left) do
			if lines[line][char] then
				ones = ones + 1
			end
		end
		local nleft = {}
		for k, line in pairs(left) do
			if keep(lines[line][char], ones, #left) then
				table.insert(nleft, line)
			end
		end
		left = nleft
		char = char + 1
	end
	return left[1]
end

function bin2dec(t)
	local v = 0
	for i = 1, #t do
		v = v * 2
		if t[i] then v = v + 1 end
	end
	return v
end

local indices = {}
for line = 1, #lines do
	table.insert(indices, line)
end

local oxygen = reduce(indices, function (bit, ones, total)
	return (not bit or ones >= total/2) and (bit or ones < total/2)
end)

local co2 = reduce(indices, function (bit, ones, total)
	return (not bit or ones < total/2) and (bit or ones >= total/2)
end)

print(bin2dec(lines[oxygen]) * bin2dec(lines[co2]))