adventofcode-2021/day16/part2.lua

#!/usr/bin/env luajit
require("utils")

local hex = io.open(arg[3]):read("*a")
--local hex = "8A004A801A8002F478"
local hexmap = {
	["0"] = "0000", ["1"] = "0001", ["2"] = "0010", ["3"] = "0011",
	["4"] = "0100", ["5"] = "0101", ["6"] = "0110", ["7"] = "0111",
	["8"] = "1000", ["9"] = "1001", ["A"] = "1010", ["B"] = "1011",
	["C"] = "1100", ["D"] = "1101", ["E"] = "1110", ["F"] = "1111",
}

local bytes = {}
for char in chars(hex) do
	if hexmap[char] then table.insert(bytes, hexmap[char]) end
end

local bitstring = table.concat(bytes)

local function literal(bitstring)
	local v = 0
	while char(bitstring, 1) == "1" do
		v = v * 16 + bin2dec(bitstring:sub(2, 5))
		bitstring = bitstring:sub(6)
	end
	v = v * 16 + bin2dec(bitstring:sub(2, 5))
	return v, bitstring:sub(6)
end

local function countoperator(bitstring)
	local count = bin2dec(bitstring:sub(1, 11))
	bitstring = bitstring:sub(12)
	local p, packets = nil, {}
	for i = 1, count do
		p, bitstring = packet(bitstring)
		table.insert(packets, p)
	end
	return packets, bitstring
end

local function lengthoperator(bitstring)
	local length = bin2dec(bitstring:sub(1, 15))
	bitstring = bitstring:sub(16)
	local target = #bitstring - length
	local p, packets = nil, {}
	while #bitstring > target do
		p, bitstring = packet(bitstring)
		table.insert(packets, p)
	end
	return packets, bitstring
end

local function operator(bitstring)
	if char(bitstring, 1) == "1" then
		return countoperator(bitstring:sub(2))
	else
		return lengthoperator(bitstring:sub(2))
	end
end

function packet(bitstring)
	local version = bin2dec(bitstring:sub(1, 3))
	local pactype = bin2dec(bitstring:sub(4, 6))
	local content
	if pactype == 4 then
		content, bitstring = literal(bitstring:sub(7))
	else
		content, bitstring = operator(bitstring:sub(7))
	end
	return { version = version, pactype = pactype, content = content }, bitstring
end

local function intop(p, binop)
	local acc = evaluate(p.content[1])
	for i = 2, #p.content do
		acc = binop(acc, evaluate(p.content[i]))
	end
	return acc
end

local function boolop(p, binop)
	if binop(evaluate(p.content[1]), evaluate(p.content[2]))
	then return 1
	else return 0
	end
end

local typemap = {
	[0] = function(p)
		return intop(p, function(a, b) return a + b end)
	end,
	[1] = function(p)
		return intop(p, function(a, b) return a * b end)
	end,
	[2] = function(p)
		return intop(p, math.min)
	end,
	[3] = function(p)
		return intop(p, math.max)
	end,
	[4] = function(p)
		return p.content
	end,
	[5] = function(p)
		return boolop(p, function(a, b) return a > b end)
	end,
	[6] = function(p)
		return boolop(p, function(a, b) return a < b end)
	end,
	[7] = function(p)
		return boolop(p, function(a, b) return a == b end)
	end,
}

function evaluate(packet)
	return typemap[packet.pactype](packet)
end

print(evaluate(packet(bitstring)))