game_of_life.rb

require 'rspec'

class GameOfLife
  def initialize(generation)
    @generation = generation
  end

  attr_reader :generation

  def is_alive?(x, y)
    generation.include?([x, y])
  end

  def number_of_neighbors_for(x, y)
    generation.select do |dx, dy|
      (x-1..x+1).include?(dx) && (y-1..y+1).include?(dy)
    end.count - (is_alive?(x, y) ? 1 : 0)
  end

  def should_regen?(x, y)
    !generation.include?([x, y]) && number_of_neighbors_for(x, y) == 3
  end

  def regenerate_dead_neighbors(x, y)
    (-4..4).map { |m| [x+m/3, y+m%3] if should_regen?(x+m/3, y+m%3) }.compact
  end

  def should_survive?(x, y)
    [2, 3].include?(number_of_neighbors_for(x, y))
  end

  def tick
    @generation = generation.map do |x, y|
      regenerate_dead_neighbors(x, y) + (should_survive?(x, y) ? [[x, y]] : [])
    end.flatten(1).uniq.sort
  end
end



describe "Game of Life!!!" do
  let(:game) { GameOfLife.new(board) }

	context "Lonely" do
    let(:board) { [[1, 1]] }
		it "dies after one generation" do
			game.tick
			expect(game.generation).to be_empty
		end
	end

	context "Continuing" do
    let(:board) { [[0, 0], [0, 1], [1, 0], [1, 1], [2, 2], [2, 3], [3, 2], [3, 3]] } # Beacon
		it "knows how many neighbors" do
			expect(game.number_of_neighbors_for(1, 1)).to eql 4
		end

		it "2 die after one generation" do
			game.tick
			expect(game.generation).to eql [[0, 0], [0, 1], [1, 0], [2, 3], [3, 2], [3, 3]]
		end
	end

	context "Regenerate" do
    let(:board) { [[0, 0], [0, 1], [1, 0], [2, 3], [3, 2], [3, 3]] }
		it "knows how many neighbors" do
			expect(game.number_of_neighbors_for(1, 1)).to eql 3
		end

		it "regenerates after one generation with exactly 3 neighbors" do
			game.tick
			expect(game.generation).to eql [[0, 0], [0, 1], [1, 0], [1, 1], [2, 2], [2, 3], [3, 2], [3, 3]].sort
		end
	end
end