input = DATA.read.split("\n\n")

map = {}
input.shift.split("\n").each.with_index do |row, y|
  row.chars.each.with_index do |pos, x|
    map[[y,x]] = pos unless pos == ?.
  end
end
robot = map.find { _2 == ?@ }.first

bounds = [
  (0..map.keys.map(&:first).max),
  (0..map.keys.map(&:last).max),
]
map_s = ->() {
  bounds[0].map { |y|
    bounds[1].map { |x|
      map.fetch([y,x], ?.)
    }.join
  }.join("\n")
}

Nope = Class.new(Exception)

def push(map, pos, move)
  delta = DELTAS.fetch(move)
  pospos = pos.zip(delta).map { _1 + _2 }

  case map.fetch(pospos, nil)
  when ?#
    raise Nope
  when ?O
    push(map, pospos, move)
  when nil
  else
    fail
  end

  map[pospos] = map.fetch(pos)
  map.delete(pos)

  pospos
end

DELTAS = %w[ ^ v < > ].zip([[-1,0], [1,0], [0,-1], [0, 1]]).to_h
movements = input.shift.gsub("\n", "").chars

movements.each do |move|
  begin
    robot = push(map, robot, move)
  rescue Nope
  end
end

pp map.select { _2 == ?O }.sum {|(y,x),_| 100 * y + x }

__END__
#######
#...#.#
#.....#
#..OO@#
#..O..#
#.....#
#######

<vv<<^^<<^^