snake.rb

class GameItem
  attr_accessor :x, :y

  def initialize(app, x, y, colour = '#fff')
    @app = app
    @x = x
    @y = y
    @app.stroke colour
    @rect = app.rect x * cell_size, y * cell_size, cell_size - 1, cell_size - 1
  end

  def move_to(x, y)
    @x = x
    @y = y
    move_rect
  end

  def move_by(x, y)
    @x += x
    @y += y
    move_rect
  end

  def cell_size
    10
  end

  private

    def move_rect
      @rect.move @x * cell_size, @y * cell_size
    end
end

class Snake
  attr_accessor :segments

  class Segment < GameItem
  end

  def initialize(app, x, y)
    @app = app
    change_direction :left
    @segments = []
    @x = x
    @y = y

    add_segment
  end

  def length
    @segments.size
  end

  def add_segment
    segment = nil
    if @segments.empty?
      segment = Segment.new @app, @x, @y, '#ff0000'
    else
      segment = Segment.new @app, @segments.last.x, @segments.last.y
      segment.move_by @direction_x * -1, @direction_y * -1
    end

    # Make sure the segment is added to the back
    @segments << segment
  end

  def move
    @segments.reverse.each_with_index do |segment, i|
      if i + 1 == length
        segment.move_by @direction_x, @direction_y
      else
        next_segment = @segments.reverse[i + 1]
        segment.move_to next_segment.x, next_segment.y
      end
    end

    @x = @segments.first.x
    @y = @segments.first.y
  end

  def change_direction(direction)
    @direction = direction
    case direction
      when :up
        @direction_x = 0
        @direction_y = -1
      when :down
        @direction_x = 0
        @direction_y = 1
      when :left
        @direction_x = -1
        @direction_y = 0
      when :right
        @direction_x = 1
        @direction_y = 0
     end
  end
end

Shoes.app do
  background '#000'

  snake = Snake.new self, 25, 25

  cell_size = 10
  delay = 1

  stroke '#fff'

  20.times do
    snake.add_segment
  end

  animate 5 do
    keypress do |key|
      case key
        when :up, :down, :left, :right
          snake.change_direction key
      end
    end
    snake.move
  end
end