input = ARGF.readlines(chomp: true)
  .flat_map.with_index {|row, y|
    row.chars.map.with_index {|elem, x| [[y,x], elem] }
  }.to_h

current = [[[0,0], [0,1]]]
seen = Set.new

until current.empty?
  coord, delta = current.shift
  seen << [coord, delta]

  case [elem = input[coord], delta]
  in [nil, _]
    # the beam has escaped the contraption
  in [?., _] | [?|, [_,0]] | [?-, [0,_]] # keep going
    current << [coord.zip(delta).map { _1 + _2 }, delta]
  in [?|, [0,_]] # split up and down
    current << [coord.zip([-1,0]).map { _1 + _2 }, [-1,0]]
    current << [coord.zip([1,0]).map { _1 + _2 }, [1,0]]
  in [?-, [_,0]] # split left and right
    current << [coord.zip([0,-1]).map { _1 + _2 }, [0,-1]]
    current << [coord.zip([0,1]).map { _1 + _2 }, [0,1]]
  in [?/, [0,1]] # going right, goes up
    current << [coord.zip([-1,0]).map { _1 + _2 }, [-1,0]]
  in [?/, [0,-1]] # going left, goes down
    current << [coord.zip([1,0]).map { _1 + _2 }, [1,0]]
  in [?/, [1,0]] # going down, goes left
    current << [coord.zip([0,-1]).map { _1 + _2 }, [0,-1]]
  in [?/, [-1,0]] # going up, goes right
    current << [coord.zip([0,1]).map { _1 + _2 }, [0,1]]
  in [?\\, [0,1]] # going right, goes down
    current << [coord.zip([1,0]).map { _1 + _2 }, [1,0]]
  in [?\\, [0,-1]] # going left, goes up
    current << [coord.zip([-1,0]).map { _1 + _2 }, [-1,0]]
  in [?\\, [1,0]] # going down, goes right
    current << [coord.zip([0,1]).map { _1 + _2 }, [0,1]]
  in [?\\, [-1,0]] # going up, goes left
    current << [coord.zip([0,-1]).map { _1 + _2 }, [0,-1]]
  else
    fail "unexpected element: #{elem.inspect}"
  end

  current = current.reject { seen.include?(_1) }
end

p seen.map(&:first).select { input.has_key?(_1) }.uniq.size