Find mines!!

Actually this quiz came from 'minesweeper' of programming challenges.

It's not difficult code, so skip explanation. And I have another idea that can reduce amount of memory usage when input data is huge size (for instance, 405060 X 405060). But I don't have enough space to write it :-). Anyway below code has only one if and one case statement. It's so cool, isn't it? :-)

class Board
  def initialize(rows, cols)
    @rows, @cols = rows, cols
    @blocks = []
    rows.times { @blocks << [] }
  end
 
  def add(block)
    @blocks[block.y][block.x] = block
    block.board = self
    block
  end
 
  def get_block(x,y)
    @blocks[y][x] if inside_board?(x,y)
  end
 
  def to_s
    @blocks.map{ |line| line.map(&:to_s).join }.join("\n")
  end
 
  def friends_of(block)
    candidates = []
    (block.x-1).upto(block.x+1) do |target_x|
      (block.y-1).upto(block.y) do |target_y|
        candidates << get_block(target_x, target_y)
      end
    end
    candidates.compact
  end
 
  #sometimes, minus index of Ruby array leads extra work. :-<
  def inside_board?(x,y)
    (x >= 0 and x < @cols and y >= 0 and y < @rows)
  end
end

class Block
  attr_accessor :board, :x, :y, :around_mine_number
 
  def self.make(type, x, y)
    case type
    when '*' then MineBlock.new(x, y)
    when '.' then NormalBlock.new(x, y)
    end
  end
 
  def initialize(x, y)
    @x, @y = x, y
    @around_mine_number = 0
  end
 
  def update_with_around_blocks
    @board.friends_of(self).each do |f|
      f.update_mine_status(self)
      self.update_mine_status(f)
    end
  end
end

class MineBlock < Block
  def to_s; '*'; end
 
  def update_mine_status(other)
    other.around_mine_number += 1
  end
end

class NormalBlock < Block
  def to_s; @around_mine_number; end
 
  def update_mine_status(other); end
end

inputs = "5 3  *..  *..  ... .*. ...".split(' ')

num_of_rows, num_of_cols = inputs.slice!(0..1).map(&:to_i)
board = Board.new(num_of_rows, num_of_cols)
num_of_rows.times do |row|
  marks = inputs.slice!(0).split('')
  num_of_cols.times do |col|
    assigned_block = board.add(Block.make(marks[col], col, row))
    assigned_block.update_with_around_blocks
  end
end

puts board

PS. Egloos is not good for code posing.