Practice Game Of Life

cell.rb

class Cell
  DEAD = '.'
  ALIVE = 'x'

  attr_accessor :value
  def self.create(val, position)
    if val == DEAD
      DeadCell.new(position)
    else
      LiveCell.new(position)
    end
  end

  def evolve_at(board)
    alive_neighbors = count_alive(board.neighbor_cells(@position))
    evolve(alive_neighbors)
  end

  def alive?
    value == ALIVE
  end

  def dead?
    ! alive?
  end

  private
  def count_alive(cells)
    cells.compact.count(&:alive?)
  end

  class LiveCell < Cell
    def initialize(pos)
      @position = pos
    end
    def value
      Cell::ALIVE
    end

    def evolve(alive_neighbors)
      if alive_neighbors < 2 || alive_neighbors > 3
        DeadCell.new(@position)
      else
        self
      end
    end
  end

  class DeadCell < Cell
    def initialize(pos)
      @position = pos
    end
    def value
      Cell::DEAD
    end

    def evolve(alive_neighbors)
      if alive_neighbors == 3
        LiveCell.new(@position)
      else
        self
      end
    end
  end

end

game_of_life.rb

require_relative 'cell'

class GameOfLife

  def self.start(board)
    width = board[0].size
    cells = board.flatten.each_with_index.map { |c, i| Cell.create(c, i) }
    GameOfLife.new(cells, width)
  end

  def initialize(cells, width)
    @cells = cells
    @width = width
  end

  def board
    @cells.map(&:value).each_slice(@width).to_a
  end

  def center_cell
    @cells[@cells.size / 2]
  end

  def evolve
    new_board = @cells.map { |current| current.evolve_at(self) }
    GameOfLife.new(new_board, @width)
  end

  def neighbor_cells(i)
    neighbors(i).map { |n| @cells[n] }
  end

  private
  def neighbors(x)
    # NOTE: I don't actually think this is correct
    [x - @width - 1, x - @width, x - @width + 1, x - 1, x + 1, x + @width - 1, x + @width, x + @width + 1]
  end
end

game_of_life_steps.rb

Given(/^the following setup$/) do |table|
  @game = GameOfLife.start(table.raw)
end

When(/^I evolve the board$/) do
  @game = @game.evolve
end

Then(/^the center cell should be dead$/) do
  expect(@game.center_cell).to be_dead
end

Then(/^the center cell should be alive$/) do
  expect(@game.center_cell).to be_alive
end

Then(/^I should see the following board$/) do |table|
  @expected_board = table.raw
  expect(@game.board).to eq(@expected_board)
end