Game of Life with Rspec

game_of_life.rb

require 'rspec'

class Cell
  attr_accessor :x, :y, :world

  def initialize(world, x=0, y=0)
    @world = world
    @x = x; @y = y
    @world.add(self)
  end

  def neighbors
    @world.neighbors_of(@x, @y)
  end

  def die
    @world.cells.delete(self)
  end
end

class World
  attr_accessor :cells
  def initialize
    @cells = []  
  end

  def self.load(coord_array)
    coord_array.each do |x, y|
      Cell.new(self, x, y)
    end
  end
  
  def neighbors_of(x, y)
    @cells.select {|c| c.x == x-1 || c.x == x+1 || c.x == x}.
           select {|c| c.y == y-1 || c.y == y+1 || c.y == y }.
           reject {|c| c.x == x && c.y == y}
  end

  def dead_near(x, y)
    dead = []
    (x-1..x+1).each do |x_coord|
      (y-1..y+1).each do |y_coord|
        dead << [x_coord, y_coord] if cell_at(x_coord, y_coord).nil?
      end
    end
    dead
  end

  def add(cell)
    @cells << cell if cell_at(cell.x, cell.y).nil?
  end

  def cell_at(x, y)
    @cells.select {|c| c.x == x && c.y == y}.first
  end

  def tick
    dying = []
    birthing = []
    @cells.each do |cell|
      dying << cell if cell.neighbors.count < 2
      dying << cell if cell.neighbors.count > 3
      dead_near(cell.x, cell.y).each do |dx, dy|
        birthing << [dx, dy] if neighbors_of(dx, dy).count == 3
      end
    end
    dying.each(&:die)  
    birthing.each do |x, y|
      Cell.new(self, x, y)
    end
  end
end

describe Cell do
  let(:world) { World.new }
  context "generally" do
    subject { Cell.new(world) }
    it "creates a cell " do
      subject.should be_a Cell
    end

    it "creates a cell at given coordinates" do
      c = Cell.new(world, 1, 3)
      c.x.should == 1
      c.y.should == 3
    end

    it "registers itself with a world" do
      c = Cell.new(world, 5, 4)
      world.cells.should include c
    end

    it "knows it has no neighbors" do
      subject.neighbors.count.should == 0
    end

    it "knows it has one neighbor north of it" do
      cell = Cell.new(world, 0, 1)
      subject.neighbors.count.should == 1
    end

    it "knows it has one neighbor east of it" do
      cell = Cell.new(world, 1, 0)
      subject.neighbors.count.should == 1
    end

    it "knows it has one neighbor south of it" do
      cell = Cell.new(world, 0, -1)
      subject.neighbors.count.should == 1
    end

    it "knows it has one neighbor west of it" do
      cell = Cell.new(world, -1, 0)
      subject.neighbors.count.should == 1
    end

    it "knows it has one neighbor northeast of it" do
      cell = Cell.new(world, 1, 1)
      subject.neighbors.count.should == 1
    end

    it "knows it has one neighbor northwest of it" do
      cell = Cell.new(world, -1, 1)
      subject.neighbors.count.should == 1
    end

    it "knows it has one neighbor southeast of it" do
      cell = Cell.new(world, 1, -1)
      subject.neighbors.count.should == 1
    end

    it "knows it has one neighbor southwest of it" do
      cell = Cell.new(world, -1, -1)
      subject.neighbors.count.should == 1
    end
  end

  context "dies off when ticked if fewer than 2 neighbors" do
    let(:world) { World.new }
    it "dies off when it has 0 neighbors" do
      c = Cell.new(world, 1, 1)
      world.cells.should include c
      world.tick
      world.cells.should_not include c
    end

    it "dies off when it has 1 neighbors" do
      Cell.new(world, 1, 2)

      c = Cell.new(world, 1, 1)
      world.cells.should include c
      world.tick
      world.cells.should_not include c
    end
  end

  context "lives on when ticked if 2 or 3 neighbors" do
    it "lives when it has 2 neighbors" do
      Cell.new(world, 1, 2)
      Cell.new(world, 2, 1)
      
      c = Cell.new(world, 1, 1)
      world.cells.should include c
      world.tick
      world.cells.should include c
    end

    it "lives when it has 3 neighbors" do
      Cell.new(world, 1, 2)
      Cell.new(world, 2, 1)
      Cell.new(world, -1, 2)

      c = Cell.new(world, 1, 1)
      world.cells.should include c
      world.tick
      world.cells.should include c
    end
  end

  context "dies off when ticked if more than 3 neighbors" do
    it "dies when it has 4 neighbors" do
      Cell.new(world, -1, -1)
      Cell.new(world, -1, 0)
      Cell.new(world, -1, 1)
      Cell.new(world, 0, -1)
      
      c = Cell.new(world, 0, 0)
      world.cells.should include c
      world.tick
      world.cells.should_not include c
    end

    it "dies when it has 5 neighbors" do
      Cell.new(world, -1, -1)
      Cell.new(world, -1, 0)
      Cell.new(world, -1, 1)
      Cell.new(world, 0, -1)
      Cell.new(world, 0, 1)
      
      c = Cell.new(world, 0, 0)
      world.cells.should include c
      world.tick
      world.cells.should_not include c
    end

    it "dies when it has 6 neighbors" do
      Cell.new(world, -1, 1)
      Cell.new(world, -1, 0)
      Cell.new(world, -1, 1)
      Cell.new(world, 0, -1)
      Cell.new(world, 0, 1)
      Cell.new(world, 1, -1)
      
      c = Cell.new(world, 0, 0)
      world.cells.should include c
      world.tick
      world.cells.should_not include c
    end

    it "dies when it has 7 neighbors" do
      Cell.new(world, -1, 1)
      Cell.new(world, -1, 0)
      Cell.new(world, -1, 1)
      Cell.new(world, 0, -1)
      Cell.new(world, 0, 1)
      Cell.new(world, 1, -1)
      Cell.new(world, 1, 0)
      
      c = Cell.new(world, 0, 0)
      world.cells.should include c
      world.tick
      world.cells.should_not include c
    end

    it "dies when it has 8 neighbors" do
      Cell.new(world, -1, 1)
      Cell.new(world, -1, 0)
      Cell.new(world, -1, 1)
      Cell.new(world, 0, -1)
      Cell.new(world, 0, 1)
      Cell.new(world, 1, -1)
      Cell.new(world, 1, 0)
      Cell.new(world, 1, 1)
      
      c = Cell.new(world, 0, 0)
      world.cells.should include c
      world.tick
      world.cells.should_not include c
    end
  end

  context "reproduces if exactly 3 neighbors" do
    it "does not reproduce with 2 neighbors" do
      Cell.new(world, 1, -1)
      Cell.new(world, 1, 0)

      world.tick
      world.cell_at(0, 0).should be_nil
    end

    it "does reproduce with 3 neighbors" do
      Cell.new(world, 1, -1)
      Cell.new(world, 1, 0)
      Cell.new(world, 0, 1)

      world.tick
      world.cell_at(0, 0).should_not be_nil
    end

    it "does not reproduce with 4 neighbors" do
      Cell.new(world, 0, 1)
      Cell.new(world, 0, -1)
      Cell.new(world, -1, 0)
      Cell.new(world, 1, 0)

      world.tick
      world.cell_at(0, 0).should be_nil
    end
  end
end