NEIGHBORS = [
  [-1,-1], [-1,0], [-1,1],
  [ 0,-1],         [ 0,1],
  [ 1,-1], [ 1,0], [1 ,1],
]

Grid = Data.define(:rolls) do
  def size = rolls.size

  def accessible
    rolls.select {|(y,x)|
      NEIGHBORS.count {|(dy,dx)|
        rolls.include?([y+dy, x+dx])
      } < 4
    }
  end

  def removed
    return enum_for(__method__) unless block_given?

    grid = self
    loop do
      yield grid
      grid = Grid.new(grid.rolls - grid.accessible)
    end
  end
end

grid = Grid.new(
  DATA.read
    .lines(chomp: true)
    .each.with_index.inject(Set.new) do |rolls, (line, y)|
      rolls.merge(
        line.chars.each.with_index
          .select {|roll,_| roll == ?@ }
          .map { [y, _2] }
      )
    end
)

chunk = grid.removed.slice_when { _1.size == _2.size }.next
pp chunk.first.size - chunk.last.size

__END__