adventofcode-2021/day18/part2.lua

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

function parse(line)
	local number = {}
	local depth = 0
	for char in chars(line) do
		if char == "[" then
			depth = depth + 1
		elseif char == "]" then
			depth = depth - 1
		elseif char ~= "," then
			table.insert(number, { d=depth, n=tonumber(char) })
		end
	end
	return number
end

function explode(a)
	local new = {}
	local exploded = false
	local i = 1
	while i <= #a do
		if exploded or a[i].d <= 4 then
			table.insert(new, a[i])
			i = i + 1
		else
			exploded = true
			if #new > 0 then
				new[#new].n = new[#new].n + a[i].n
			end
			table.insert(new, { d = a[i].d - 1, n = 0 })
			if #a >= i + 2 then
				table.insert(new, { d = a[i + 2].d, n = a[i + 1].n + a[i + 2].n })
			end
			i = i + 3
		end
	end
	return exploded, new
end

function split(a)
	local new = {}
	local split = false
	for i, digit in ipairs(a) do
		if split or digit.n < 10 then
			table.insert(new, digit)
		else
			split = true
			table.insert(new, { d = digit.d + 1, n = math.floor(digit.n / 2) })
			table.insert(new, { d = digit.d + 1, n = math.ceil(digit.n / 2) })
		end
	end
	return split, new
end

function reduce(a)
	local changed = true
	repeat
		changed, a = explode(a)
		if not changed then
			changed, a = split(a)
		end
	until not changed
	return a
end

function add(a, b)
	local sum = {}
	for i, digit in ipairs(a) do
		table.insert(sum, { d = digit.d + 1, n = digit.n })
	end
	for i, digit in ipairs(b) do
		table.insert(sum, { d = digit.d + 1, n = digit.n })
	end
	return reduce(sum)
end

function magnitude(number)
	local function go(d, c)
		if number[c].d > d then
			local left, right
			left, c = go(d + 1, c)
			right, c = go(d + 1, c)
			return 3 * left + 2 * right, c
		else
			return number[c].n, c + 1
		end
	end
	return go(0, 1)
end

local numbers = {}
for line in io.lines(arg[3]) do
	table.insert(numbers, parse(line))
end

local max = 0
for i, a in pairs(numbers) do
	for j, b in pairs(numbers) do
		if i ~= j then
			local m = magnitude(add(a, b))
			if m > max then max = m end
		end
	end
end

print(max)