game_of_life.rb

class Game
  XN = 100
  YN = 100

  def initialize
    @cells = {}
  end

  def add(x, y)
    @cells["#{x}-#{y}"] = 1
  end

  def del(x, y)
    @cells.delete("#{x}-#{y}")
  end

  def alive?(x, y)
    @cells["#{x}-#{y}"] == 1
  end

  def dead?(x, y)
    !alive?(x, y)
  end

  def neighbours(x, y)
    count = 0

    if alive?(x - 1, y - 1)
      count += 1
    end

    if alive?(x - 1, y)
      count += 1
    end

    if alive?(x - 1, y + 1)
      count += 1
    end

    if alive?(x, y - 1)
      count += 1
    end

    if alive?(x, y + 1)
      count += 1
    end

    if alive?(x + 1, y - 1)
      count += 1
    end

    if alive?(x + 1, y)
      count += 1
    end

    if alive?(x + 1, y + 1)
      count += 1
    end

    count
  end

  def run!
    for_del = []
    for_add = []

    @cells.each do |coords, _|
      x, y = coords.split('-').map(&:to_i)

      if neighbours(x, y) < 2
        for_del << [x, y]
      end

      if neighbours(x, y) > 3
        for_del << [x, y]
      end
    end

    XN.times do |x|
      YN.times do |y|
        for_add << [x, y] if dead?(x, y) && neighbours(x, y) == 3
      end
    end

    for_del.each { |x, y| del(x, y) }
    for_add.each { |x, y| add(x, y) }
  end
end

describe Game do
  let(:game) { Game.new }

  specify '#alive?' do
    game.add(0, 0)
    game.alive?(0, 0).should be_true
    game.alive?(0, 1).should be_false
  end

  specify '#del' do
    game.add(0, 0)
    game.del(0, 0)
    game.alive?(0, 0).should be_false
  end

  specify '#neighbours' do
    game.add(1, 1)
    game.add(1, 2)
    game.add(3, 3)
    game.add(3, 4)
    game.add(4, 4)

    game.neighbours(1, 1).should == 1
    game.neighbours(1, 2).should == 1
    game.neighbours(3, 3).should == 2
  end

  describe '#run!' do
    it 'cell dies when less than 2 neighbours' do
      game.add(2, 2)
      game.run!
      game.alive?(2, 2).should be_false
    end

    it 'cell lives when 2 neighbours' do
      game.add(2, 2)
      game.add(2, 3)
      game.add(3, 3)
      game.run!
      game.alive?(2, 2).should be_true
    end

    it 'cell lives when 3 neighbours' do
      game.add(2, 2)
      game.add(2, 3)
      game.add(3, 3)
      game.add(1, 1)
      game.run!
      game.alive?(2, 2).should be_true
    end

    it 'cell dies when more than 3 neighbours' do
      game.add(2, 2)
      game.add(2, 3)
      game.add(3, 3)
      game.add(1, 1)
      game.add(1, 2)
      game.run!
      game.alive?(2, 2).should be_false
    end

    it 'cell is reviewed when 3 neighbours' do
      game.add(2, 3)
      game.add(3, 3)
      game.add(1, 1)
      game.run!
      game.alive?(2, 2).should be_true
    end

    it 'cell is not reviewed when 4 neighbours' do
      game.add(2, 3)
      game.add(3, 3)
      game.add(1, 1)
      game.add(1, 2)
      game.run!
      game.alive?(2, 2).should be_false
    end
  end
end