grapple.rb

require 'rubygems' rescue nil
require 'gosu'
require 'chipmunk'

class Array
  # e.g. [1,2,3].each_link yields [1,2], [2,3]
  def each_link
    prev = first
    self[1, size].each do |item|
      yield prev, item
      prev = item
    end
  end
end

# A grappling hook
module Grapple

  SCREEN_WIDTH = 800
  SCREEN_HEIGHT = 600

  INF = 1e100

  # The number of steps to process every Gosu update The Player ship can get
  # going so fast as to "move through" a star without triggering a collision;
  # an increased number of Chipmunk step calls per update will effectively
  # avoid this issue
  SUBSTEPS = 6

  class Rope
    include CP

    attr_reader :links

    def initialize(window, space, options)
      options ||= {}
      @length = options[:length] || 100
      @window = window
      @links = []
      @length.times do |n|
        link = Body.new(1, 0.1)
        seg = Shape::Circle.new(link, 0.1, Vec2.new(0.0, 0.0))
        seg.collision_type = :rope
        seg.u = 0.5
        link.p = Vec2.new(n*5, 0)
        space.add_body(link)
        space.add_shape(seg)
        @links << link
      end

      @links.each_link do |prev, link|
        joint = Joint::Slide.new(prev, link, Vec2.new(0.0, 0.0), Vec2.new(1.0, 0.0), 0.1, 4.0)
        space.add_joint(joint)
      end
    end

    def draw
      draw_gradient
    end

    def draw_segments
      col1 = Gosu::Color.new(0xffff0000)
      col2 = Gosu::Color.new(0xffffff00)
      @links.each_link do |prev, link|
        @window.draw_line(prev.p.x, prev.p.y, col1, link.p.x, link.p.y, col2)
      end
    end

    def draw_gradient
      col = Gosu::Color.new(0xffff0000)
      i = 0
      @links.each_link do |prev, link|
        col.green = (255*i / @links.size)
        @window.draw_line(prev.p.x, prev.p.y, col, link.p.x, link.p.y, col)
        i += 1
      end
    end
  end

  class Ground
    include CP

    attr_reader :body

    def initialize(window, space)
      @window = window
      @links = []
      @body = Body.new(INF, INF)
      @p1, @p2 = Vec2.new(0, 500), Vec2.new(SCREEN_WIDTH, 440)
      @width = 20
      @seg = Shape::Segment.new(@body, @p1, @p2, @width)
      @seg.collision_type = :ground
      @seg.u = 0.99
      space.add_static_shape(@seg)
    end

    def draw
      col = Gosu::Color.new(0xff0000ff)
      @window.draw_line(@p1.x, @p1.y - @width, col, @p2.x, @p2.y - @width, col)
    end
  end

  class GameWindow < Gosu::Window
    include CP

    def initialize
      super(SCREEN_WIDTH, SCREEN_HEIGHT, false, 16)
      self.caption = "Grapple"

      @space = Space.new
      @space.damping = 0.8    
      @space.gravity = Vec2.new(0, 10.0)
      @dt = (1.0/60.0)

      @ground = Ground.new(self, @space)

      @rope = Rope.new(self, @space, :length => 15)
      @rope.links.each_with_index{|link, i| link.p = Vec2.new(300 + 100*Math.sin(i.to_f * Math::PI*2 / @rope.links.size), 200) }
      @hook = Body.new(100, 10)
      joint = Joint::Pin.new(@hook, @rope.links.last, Vec2.new(0.0, 0.0), Vec2.new(0.0, 0.0))
      @space.add_body(@hook)
      @space.add_joint(joint)
      @hook.p = Vec2.new(350, 200)

      attach = Joint::Slide.new(@ground.body, @rope.links.first, Vec2.new(300.0, 400.0), Vec2.new(0.0, 0.0), 0, 0)
      @space.add_joint(attach)

      @rope2 = Rope.new(self, @space, :length => 10)
      hanging = Body.new(INF, INF)
      joint = Joint::Pin.new(hanging, @rope2.links.first, Vec2.new(0.0, 0.0), Vec2.new(0.0, 0.0))
      @space.add_joint(joint)
      @rope2.links.each_with_index{|link, i| link.p = Vec2.new(100 - 10*i, 200) }
      hanging.p = Vec2.new(150, 0)
    end

    def update
      SUBSTEPS.times do
        # Check keyboard
        if button_down? Gosu::Button::KbSpace
          @hook.v = Vec2.new(10, -200)
        end

        #@rope.links.each{|link| link.reset_forces }

        @space.step(@dt)
      end
    end

    def draw
      @rope.draw
      @rope2.draw
      @ground.draw
    end

    def button_down(id)
      case id
      when Gosu::Button::KbEscape, char_to_button_id('q')
        close
      end
    end
  end
end

if $0 == __FILE__
  window = Grapple::GameWindow.new
  window.show
end