Advent of Code 2024 day017.rb

day017.rb

require_relative "main"

module Day017
  INPUT = File.read("INPUT.txt")

  ##########
  # Part 1 #
  ##########

  parts = INPUT.split("\n\n")
  register = parts.first.split("\n").map { _1[/\d+/].to_i }
  steps = parts.last.scan(/\d+/).map(&:to_i)

  A = 0
  B = 1
  C = 2

  COMBOS = [
    -> { 0 },
    -> { 1 },
    -> { 2 },
    -> { 3 },
    -> { register[A] },
    -> { register[B] },
    -> { register[C] }
  ]

  SKIP = -1

  pointer = 0

  INSTRUCTIONS = [
    ->(o) { register[A] = register[A] / 2**COMBOS[o].call }, # adv
    ->(o) { register[B] = register[B] ^ o }, # bxl
    ->(o) { register[B] = COMBOS[o].call % 8 }, # bst
    ->(o) { register[A].zero? ? SKIP : o }, # jnz
    ->(_o) { register[B] = register[B] ^ register[C] }, # bxc
    ->(o) { COMBOS[o].call % 8 }, # out
    ->(o) { register[B] = register[A] / 2**COMBOS[o].call }, # bdv
    ->(o) { register[C] = register[A] / 2**COMBOS[o].call } # cdv
  ]

  out = []
  while pointer < steps.count
    c = steps[pointer]
    o = steps[pointer + 1]
    res = INSTRUCTIONS[c].call(o)
    if c == 5
      out.push(res)
    end
    if c == 3 && res != SKIP
      pointer = res
    else
      pointer += 2
    end
  end

  answer1 = out.join(",")

  pp answer1

  ##########
  # Part 2 #
  ##########

  # 2,4 bst B = A % 8
  # 1,1 bxl B = B ^ 1  -- XOR with 1 will subtract 1 if it is odd, add 1 if even
  # 7,5 cdv C = A / 2^B
  # 4,7 bxc B = B ^ C
  # 1,4 bxl B = B ^ 4
  # 0,3 adv A = A / 8
  # 5,5 out
  # 3,0 jnz

  # B = A % 8
  # B = B ^ 1
  # C = A / 2**B
  # B = B ^ C
  # B = B ^ 4
  # print B
  # A = A / 8

  run = ->(a, last = "") do
    b = (((a % 8) ^ 1 ^ (a / (2**(a % 8 ^ 1))) ^ 4) % 8)
    a /= 8
    var = last + b.to_s
    if !a.zero?
      run.call(a, var)
    else
      var
    end
  end

  # This was just a lot of guessing & checking...it's hard to explain what I was
  # doing, mostly exploring things in octal. I didn't come up with an algorithm that I can easily describe :(

  # ANSWER
  # Octal   05600647654025052
  # Decimal 202367025818154
  answer2 = 202367025818154

  pp answer2
end