🎄 Year 2024 🔔 Day 22

Mix.install([
  {:arrays, "~> 2.1"}
])

Setup

numbers =
  File.read!("#{__DIR__}/../../../inputs/2024/day22.txt")
  |> String.split("\n", trim: true)
  |> Enum.map(&String.to_integer/1)

Part 1

import Bitwise, only: [bxor: 2]

numbers
|> Enum.map(fn number ->
  1..2000
  |> Enum.reduce(number, fn _, number ->
    intermediate = number * 64
    number = number |> bxor(intermediate) |> Integer.mod(16_777_216)
    intermediate = div(number, 32)
    number = number |> bxor(intermediate) |> Integer.mod(16_777_216)
    intermediate = number * 2048
    _number = number |> bxor(intermediate) |> Integer.mod(16_777_216)
  end)
end)
|> Enum.sum()

Part 2

defmodule Part2 do
  def get_possible_buys(changes, buys \\ %{})

  def get_possible_buys([_, _, _], buys) do
    buys
  end

  def get_possible_buys([{a, _ae}, {b, be}, {c, ce}, {d, de} | tail], buys) do
    buys = Map.put_new(buys, {a, b, c, d}, de)
    get_possible_buys([{b, be}, {c, ce}, {d, de} | tail], buys)
  end
end

result =
  numbers
  |> Enum.map(fn number ->
    0..2000
    |> Enum.scan(number, fn i, number ->
      if i == 0 do
        number
      else
        intermediate = number * 64
        number = number |> bxor(intermediate) |> Integer.mod(16_777_216)
        intermediate = div(number, 32)
        number = number |> bxor(intermediate) |> Integer.mod(16_777_216)
        intermediate = number * 2048
        _number = number |> bxor(intermediate) |> Integer.mod(16_777_216)
      end
    end)
    |> Enum.map(&Integer.mod(&1, 10))
    |> Enum.chunk_every(2, 1, :discard)
    |> Enum.map(fn [a, b] -> {b - a, b} end)
    |> Part2.get_possible_buys()
  end)
  |> Enum.concat()
  |> Enum.group_by(&elem(&1, 0), &elem(&1, 1))
  |> Enum.map(fn {k, vs} -> {k, Enum.sum(vs)} end)
  |> Enum.max_by(fn {_k, v} -> v end)