AoC 2024

day06.rb

require 'set'
# copy a string to the clipboard
def cp(string = IRB.CurrentContext.last_value)
  IO.popen('xclip -sel clip', 'w') { |f| f << string.to_s }
  string
end

if ARGV[0] && ARGV[0][0].downcase == 't'
  @lines = "....#.....
.........#
..........
..#.......
.......#..
..........
.#..^.....
........#.
#.........
......#...".split("\n")
elsif ARGV[0] && ARGV[0][0].downcase == 'c'
  @lines = `xclip -o`.split("\n")
else
  @lines = File.readlines("input").map &:chomp
end

c = 0
x = 0
y = 0
dir = 0 # up
@dirs = [
  [0,-1], # Up
  [1,0], # Right
  [0,1], # Down
  [-1,0] # Left
]
@w = @lines.first.size
@h = @lines.size
@lines.each_with_index do |l, row|
  if (x = l.index('^'))
    y = row
    l[x] = 'x'
    break
  end
end
@start_x = x
@start_y = y
loop do
  nx = x + @dirs[dir][0]
  ny = y + @dirs[dir][1]
  break if nx < 0 || ny < 0 || nx >= @w || ny >= @h
  if @lines[ny][nx] == '#'
    # turn
    dir = (dir + 1) % 4
  else
    x = nx;
    y = ny;
    @lines[ny][nx] = 'x'
  end
end

def is_loop?(lines)
  x = @start_x
  y = @start_y
  dir = 0
  memo = []
  o_dir = nil
  loop do
    nx = x + @dirs[dir][0]
    ny = y + @dirs[dir][1]
    break if nx < 0 || ny < 0 || nx >= @w || ny >= @h
    if lines[ny][nx] == '#'
      desc = [x,y,dir]
      return true if memo.include?(desc)
      memo << desc
      # turn
      dir = (dir + 1) % 4
    elsif lines[ny][nx] == 'O' # obsticle
      return true if o_dir == dir
      o_dir ||= dir
      dir = (dir + 1) % 4
    else
      x = nx;
      y = ny;
    end
  end
  false
end

@lines.each_with_index do |l, row|
  l.chars.each_with_index do |char, x|a
    next if char != 'x'
    lines = @lines.map(&:dup)
    lines[row][x] = 'O'
    c += 1 if is_loop?(lines)
  end
end
# part 1
puts cp @lines.sum {|l| l.count('x')}
# part 2
puts cp c