third.rb

class Cell
  CHARACTERS = { :live => 'X', :dead => 'O' }

  attr_reader :live

  def self.live
    new(:live)
  end

  def self.dead
    new(:dead)
  end

  def initialize(live)
    @live = live
  end

  def print
    Kernel.print CHARACTERS[@live]
  end

  def evolve(neighbours)
    living_neighbours = neighbours.count { |neighbour| neighbour.live == :live }
    next_cell_lookup = {
      :live => [
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:live),
        Cell.new(:live),
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:dead),
      ],
      :dead => [
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:live),
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:dead),
        Cell.new(:dead),
      ],
    }
    next_cell_lookup[@live][living_neighbours]
  end
end

class Grid
  def initialize(rows)
    @rows = rows
    @height = rows.length
    @width = rows.first.length
  end

  def evolve
    Grid.new(
      @rows.map do |row|
        row.map do |cell|
          cell.evolve(neighbours(cell))
        end
      end
    )
  end

  def puts
    Kernel.puts "-" * @width
    @rows.each do |row|
      row.each do |cell|
        cell.print
      end
      Kernel.puts
    end
    Kernel.puts "-" * @width
  end

  def neighbours(cell)
    x, y = coordinates(cell)

    [
      @rows[north(y)][x],
      @rows[north(y)][east(x)],
      @rows[y][east(x)],
      @rows[south(y)][east(x)],
      @rows[south(y)][x],
      @rows[south(y)][west(x)],
      @rows[y][west(x)],
      @rows[north(y)][west(x)]
    ]
  end

  def coordinates(cell)
    @rows.each_with_index do |row, y|
      column = row.index(cell)
      column and return [column, y]
    end
  end

  private

  def north(y)
    (y - 1) % @height
  end

  def south(y)
    (y + 1) % @height
  end

  def east(x)
    (x + 1) % @width
  end

  def west(x)
    (x - 1) % @width
  end
end

grid = Grid.new(
  [
    [Cell.dead, Cell.dead, Cell.dead, Cell.dead, Cell.dead],
    [Cell.dead, Cell.dead, Cell.live, Cell.dead, Cell.dead],
    [Cell.dead, Cell.dead, Cell.live, Cell.dead, Cell.dead],
    [Cell.dead, Cell.dead, Cell.live, Cell.dead, Cell.dead],
    [Cell.dead, Cell.dead, Cell.dead, Cell.dead, Cell.dead]
  ]
)

grid.puts
grid.evolve.puts