24.rb

DIRS = {
  'e' => [ 1, 0 ],
  'w' => [ -1, 0 ],
  'sw' => [ -1, -1 ],
  'se' => [ 0, -1 ],
  'nw' => [ 0, 1 ],
  'ne' => [ 1, 1 ],
}.freeze

#
# part 1
#

tiles = Set.new
data.split("\n").each do |s|
  coords = [ 0, 0 ]
  until s.empty?
    dir = s[/^(?:w|e|sw|se|nw|ne)/]
    s = s[dir.length..]
    coords = coords.zip(DIRS[dir]).map(&:sum)
  end
  tiles.include?(coords) ? tiles.delete(coords) : (tiles << coords)
end
p tiles.length

#
# part 2
#

1.upto(100) do |day|
  copy = tiles.dup
  # note Range.grow helper I added after day 17
  b = tiles.to_a.transpose.map { |x| Range.new(*x.minmax).grow }
  b[0].each do |x|
    b[1].each do |y|
      adj = DIRS.values.count { |dx, dy| tiles === [ x + dx, y + dy ] }
      coord = [ x, y ]
      if tiles.include?(coord) && (adj == 0) || adj > 2
        copy.delete(coord)
      elsif !tiles.include?(coord) && adj == 2
        copy << coord
      end
    end
  end

  puts "#{day}: #{copy.length}"
  tiles = copy
end