Advent of Code 2024 day015.rb

day015.rb

require_relative "main"

module Day015
  INPUT = File.read("INPUT.txt")

  ROBOT = "@"
  BOX = "O"
  WALL = "#"
  EMPTY = "."

  UP = "^"
  DOWN = "v"
  LEFT = "<"
  RIGHT = ">"

  DIR_MAP = {
    UP => V[0, -1],
    DOWN => V[0, 1],
    LEFT => V[-1, 0],
    RIGHT => V[1, 0]
  }

  parts = INPUT.split("\n\n")
  grid = parts[0].to_grid
  dirs = parts[1].delete("\n").chars.map { |c| DIR_MAP[c] }

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

  r = grid.find(ROBOT)
  grid.set_value(r, EMPTY)

  dirs.each do |dir|
    potential = r + dir
    case grid.get_value(potential)
    when EMPTY
      r = potential
    when BOX
      frontier = potential
      while grid.get_value(frontier) == BOX
        frontier += dir
      end
      if grid.get_value(frontier) == EMPTY
        while frontier != potential
          grid.set_value(frontier, BOX)
          frontier -= dir
        end
        grid.set_value(potential, EMPTY)
        r = potential
      end
    else
      # Wall, do nothing
    end
  end

  answer1 = 0
  grid.each do |v, value|
    if value == BOX
      answer1 += 100 * v.y + v.x
    end
  end

  pp answer1

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

  BOX2L = "["
  BOX2R = "]"

  grid.reset
  grid = grid.as_string.gsub("#", "##").gsub("O", "[]").gsub(".", "..").gsub("@", "@.").to_grid

  r = grid.find(ROBOT)
  grid.set_value(r, EMPTY)

  dirs.each do |dir|
    potential = r + dir
    case grid.get_value(potential)
    when EMPTY
      r = potential
    when BOX2L, BOX2R
      if [DIR_MAP[LEFT], DIR_MAP[RIGHT]].include?(dir)
        frontier = potential
        while [BOX2L, BOX2R].include?(grid.get_value(frontier))
          frontier += dir * 2
        end

        if grid.get_value(frontier) == EMPTY
          while frontier != potential
            first, second = (dir == DIR_MAP[LEFT]) ? [BOX2L, BOX2R] : [BOX2R, BOX2L]
            grid.set_value(frontier, first)
            frontier -= dir
            grid.set_value(frontier, second)
            frontier -= dir
          end
          grid.set_value(potential, EMPTY)
          r = potential
        end
      else
        frontiers = [[potential]]

        stop = false
        loop do
          frontiers[-1].each do |v|
            if grid.get_value(v) == BOX2L && !frontiers[-1].include?(v.right)
              frontiers[-1].push(v.right)
            elsif grid.get_value(v) == BOX2R && !frontiers[-1].include?(v.left)
              frontiers[-1].push(v.left)
            end
          end

          nexts = frontiers[-1].map { |v| v + dir }

          stop = nexts.any? { |v| grid.get_value(v) == WALL }

          break if stop || nexts.all? { |v| grid.get_value(v) == EMPTY }

          frontiers.push(nexts.select { |v| [BOX2L, BOX2R].include?(grid.get_value(v)) })
        end

        unless stop
          frontiers.reverse_each do |frontier|
            frontier.each do |v|
              grid.set_value(v + dir, grid.get_value(v))
              grid.set_value(v, EMPTY)
            end
          end
          r = potential
        end
      end
    else
      # Wall, do nothing
    end
  end

  answer2 = 0
  grid.each do |v, value|
    if value == BOX2L
      answer2 += 100 * v.y + v.x
    end
  end

  pp answer2
end