/*
 * 第二个是书中的源代码给出了一个Collision类，其中的block方法可以实现两个物体之间的碰撞检测。
有趣的是，Collsion.block方法有两个参数，第一个是被阻碍物，第二个是障碍物
*/

package 
{
	import flash.display.MovieClip;
	import flash.geom.Point;
	
	public class Collision
	//0.2 (March 20, 2009)
	{
		public function Collision()
		{
		}
		
		//Block objects
		static public function block(objectA:MovieClip, objectB:MovieClip):void
		{
			var objectA_Halfwidth:Number = objectA.width / 2;
			var objectA_Halfheight:Number = objectA.height / 2;
			var objectB_Halfwidth:Number = objectB.width / 2;
			var objectB_Halfheight:Number = objectB.height / 2;
			var dx:Number = objectB.x - objectA.x;
			var ox:Number = objectB_Halfwidth + objectA_Halfwidth - Math.abs(dx);
			if (ox > 0)
			{
				var dy:Number = objectA.y - objectB.y;
				var oy:Number = objectB_Halfheight + objectA_Halfheight - Math.abs(dy);
				if (oy > 0)
				{
					if (ox < oy)
					{
						if (dx < 0)
						{
							//Collision on right
							oy = 0;
						}
						else
						{
							//Collision on left
							oy = 0;
							ox *= -1;
						}
					}
					else
					{
						if (dy < 0)
						{
							//Collision on Top
							ox = 0;
							oy *= -1;
						}
						else
						{
							//Collision on Bottom
							ox = 0;
						}
					}
					
					//Use the calculated x and y overlaps to 
					//Move objectA out of the collision
					
					objectA.x += ox;
					objectA.y += oy;
				}
			}
		}
		
		//General purpose method for testing Axis-based collisions. Returns true or False
		static public function test(objectA:Object,objectB:Object):Boolean
		{
			var objectA_Halfwidth=objectA.width/2;
			var objectA_Halfheight=objectA.height/2;
			var objectB_Halfwidth=objectB.width/2;
			var objectB_Halfheight=objectB.height/2;
			var dx=objectB.x-objectA.x;
			var ox=objectB_Halfwidth+objectA_Halfwidth-Math.abs(dx);
			if (0<ox)
			{
				var dy=objectA.y-objectB.y;
				var oy=objectB_Halfheight+objectA_Halfheight-Math.abs(dy);
				if (0<oy)
				{

					return true;
				}
			}
			else
			{

				return false;
			}
			return false;
		}
		
		//Collisions between the player and platform
		static public function playerAndPlatform(player:MovieClip, platform:MovieClip, bounce:Number, friction:Number):void
		{
			//This method requires the following getter and 
			//setter properties in the player object:
			//objectIsOnGround:Boolean, vx:Number, vy:Number, 
			//bounceX:Number, bounceY:Number
			
			//Decalre variables needed for the player's
			//position and dimensions
			var player_Halfwidth:Number;
			var player_Halfheight:Number;
			var player_X:Number;
			var player_Y:Number
			
			//Decalre variables needed for the physics calculations
			var bounceX:Number;
			var bounceY:Number;
			var frictionX:Number;
			var frictionY:Number;
			
			//Find out whether the player object has a collisionArea
			//subobject defined
			if(player.collisionArea != null)
			{
				//If it does, find out its width and height
				player_Halfwidth = player.collisionArea.width / 2;
			    player_Halfheight = player.collisionArea.height / 2;
				
				//Convert the collisionArea's local x,y coordinates to global coordinates
				var player_Position:Point = new Point(player.collisionArea.x, player.collisionArea.y);
            	player_X = player.localToGlobal(player_Position).x;
				player_Y = player.localToGlobal(player_Position).y;
			}
			else
			{
				//If there's no collisionArea subobject
				//Use the player's main height, width, x and y
				player_Halfwidth = player.width / 2;
			    player_Halfheight = player.height / 2;
				player_X = player.x;
				player_Y = player.y;
			}
			//Find the platform's dimensions
			var platform_Halfwidth:Number = platform.width / 2;
			var platform_Halfheight:Number = platform.height / 2;
			
			//Find the distance between the player and platfrom on the x axis
			var dx:Number = platform.x - player_X;
			
			//Find the amount of overlap on the x axis
			var ox:Number = platform_Halfwidth + player_Halfwidth - Math.abs(dx);
			
			//Check for a collision on the x axis
			if (ox > 0)
			{
				//If the objects overlap on the x axis, a collision might be occuring
				//Define the variables you need to check for a collision on the y axis
				var dy:Number = player.y - platform.y;
				var oy:Number = platform_Halfheight + player_Halfheight - Math.abs(dy);
				
				//Check for a y axis collision. We know a collision must be
				//occuring if there's a collision on both the x and y axis
				if (oy > 0)
				{
					//Yes, a collision is occuring! 
					//Now you need to find out on which side 
					//of the platform it's occuring on.
					if (ox < oy)
					{
						if (dx < 0)
						{
							//Collision on right
							oy = 0;
							dx = 1;
							dy = 0
						}
						else
						{
							//Collision on left
							oy = 0;
							ox *= -1;
							dx = -1;
							dy = 0
						}
					}
					else
					{
						if (dy < 0)
						{
							//Collision on Top
							ox = 0;
							oy *= -1;
							dx = 0;
							dy = -1;
							//set the player's isOnGround property to
							//true to enable jumping
							player.isOnGround = true;
						}
						else
						{
							//Collision on Bottom
							ox = 0;
							dx = 0;
							dy = 1;
						}
					}
					
					//Find the direction of the collision ("dot product")
					var directionOfCollision:Number = player.vx * dx + player.vy * dy;
					
					//Calculate the new direction for the bounce ("projection")
					var newDirection_X:Number = directionOfCollision * dx;
					var newDirection_Y:Number = directionOfCollision * dy;
					
					//Find the "tangent velocity":
					//the speed in the direction that the object is moving.
					//It's used for calculating additional platform friction.
					var tangent_Vx:Number = player.vx - newDirection_X;
					var tangent_Vy:Number = player.vy - newDirection_Y;
					
					//Apply collision forces if the object is moving into a collision
					if (directionOfCollision < 0)
					{
						//Calculate the friction
						frictionX = tangent_Vx * friction;
						frictionY = tangent_Vy * friction;
						
						//Calculate the amount of bounce
						bounceX = newDirection_X * bounce;
						bounceY = newDirection_Y * bounce;
					}
					else
					{
						//Prevent forces from being applied if the object is
						//moving out of a collision
						bounceX = 0;
						bounceY = 0;
						frictionX = 0;
						frictionY = 0;
					}
					//Apply platform friction
					player.vx += ox - frictionX;
					player.vy += oy - frictionY;
					
					//Move the player out of the collision
					player.x += ox;
					player.y += oy;
					
					//Bounce the player off the platform
					player.bounceX = bounceX;
					player.bounceY = bounceY;
				}
			}
		}
	}
}