Advent of Code 2024 day006.rb

day006.rb

require_relative "main"

module Day006
  INPUT = File.read('INPUT.txt')

  grid = INPUT.to_grid

  NORTH = V[0, -1]
  SOUTH = V[0, 1]
  WEST = V[-1, 0]
  EAST = V[1, 0]
  BLOCK = '#'
  EMPTY = '.'

  ROTATION = {
    NORTH => EAST,
    EAST => SOUTH,
    SOUTH => WEST,
    WEST => NORTH
  }

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

  history = {}
  cursor, dir = grid.find('^'), NORTH
  loop do
    loop do
      potential = cursor + dir
      if grid.get_value(potential) != BLOCK
        history[potential] ||= [cursor, dir]
        cursor = potential
        break
      end
      dir = ROTATION[dir]
    end
    break unless grid.in_bounds?(cursor)
  end

  answer1 = history.keys.count

  pp answer1

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

  answer2 = history.keys[1...-1].sum do |v|
    grid.set_value(v, BLOCK)
    visited = Hash.new { |h, k| h[k] = 0 }
    cursor, dir = history[v]
    cycle = false
    loop do
      visited[cursor] += 1
      break if (cycle = visited[cursor] > 4)
      loop do
        potential = cursor + dir
        if grid.get_value(potential) != BLOCK
          cursor = potential
          break
        end
        dir = ROTATION[dir]
      end
      break unless grid.in_bounds?(cursor)
    end
    grid.set_value(v, EMPTY)

    cycle ? 1 : 0
  end

  pp answer2
end