/*
 * jQuery JavaScript Plugin jquery.particlePhysics.js 0.9.1
 * http://bugsoftware.co.uk/jQuery/
 *
 * Copyright (c) 2010 Ritchie Comley
 * Dual licensed under the MIT and GPL licenses.
 *
 * Date: 2010-02-09 (Tue, 09 February 2010)
 * Revision: 36
 *
 * Dependencies:
 * jQuery 1.4 (jquery.com)
 * 
 */

(function(jQuery){ 

	jQuery.particlePhysics = {};

	var gravityVector = {x:0,y:0,z:-1};
	
	jQuery.particlePhysics.setGravityVector = function(x, y, z)
		{
			gravityVector.x = x;
			gravityVector.y = y;
			gravityVector.z = z;
		};
	
	jQuery.particlePhysics.calculateForces = function(particles, phys, springs)
		{
		   var i, p1, p2, len,dx,dy,dz;;
		   var down = gravityVector;
		   var zero = {x:0,y:0,z:0};
		   var f = {};

			for(i=0; i < particles.length; i++)
			{
				particles[i].f = zero;

				if (particles[i].fixed)
				   continue;
				   
				/* Gravitation */
				particles[i].f.x += phys.gravitational * particles[i].m * down.x;
				particles[i].f.y += phys.gravitational * particles[i].m * down.y;
				particles[i].f.z += phys.gravitational * particles[i].m * down.z;

				/* Viscous drag */
				particles[i].f.x -= phys.viscousdrag * particles[i].v.x;
				particles[i].f.y -= phys.viscousdrag * particles[i].v.y;
				particles[i].f.z -= phys.viscousdrag * particles[i].v.z;
			}

			/* Handle the spring interaction */
			for(i = 0; i < springs.length; i++)
			{
				p1 = springs[i].from;
				p2 = springs[i].to;
				dx = particles[p1].p.x - particles[p2].p.x;
				dy = particles[p1].p.y - particles[p2].p.y;
				dz = particles[p1].p.z - particles[p2].p.z;
				
				// get the length of the spring
				len = Math.sqrt((dx*dx) + (dy*dy) + (dz*dz));
				
				f.x  = springs[i].springconstant  * (len - springs[i].restlength);
				f.x += springs[i].dampingconstant * (particles[p1].v.x - particles[p2].v.x) * dx / len;
				f.x *= -(dx / len);
				f.y  = springs[i].springconstant  * (len - springs[i].restlength);
				f.y += springs[i].dampingconstant * (particles[p1].v.y - particles[p2].v.y) * dy / len;
				f.y *= -(dy / len);
				f.z  = springs[i].springconstant  * (len - springs[i].restlength);
				f.z += springs[i].dampingconstant * (particles[p1].v.z - particles[p2].v.z) * dz / len;
				f.z *= -(dz / len);
				
				if (!particles[p1].fixed) {
					particles[p1].f.x += f.x;
					particles[p1].f.y += f.y;
					particles[p1].f.z += f.z;
				}
				
				if (!particles[p2].fixed) {
					particles[p2].f.x -= f.x;
					particles[p2].f.y -= f.y;
					particles[p2].f.z -= f.z;
				}
			}
			
			return particles;
		};

	/*
	   Perform one step of the solver
	*/
	jQuery.particlePhysics.updateParticles = function(
			particles,
			phys,
			springs,
			dt,
			method)
		{
			var i;
			var ptmp = [];
			var deriv = [];

			particles = jQuery.particlePhysics.calculateForces(particles,phys,springs);
			
			deriv = jQuery.particlePhysics.calculateDerivatives(particles);
			
			switch (method)
			{
				case 0:										// Euler
				
				for (i = 0; i < particles.length; i++)
				{
					particles[i].p.x += deriv[i].dpdt.x * dt;
					particles[i].p.y += deriv[i].dpdt.y * dt;
					particles[i].p.z += deriv[i].dpdt.z * dt;
					particles[i].v.x += deriv[i].dvdt.x * dt;
					particles[i].v.y += deriv[i].dvdt.y * dt;
					particles[i].v.z += deriv[i].dvdt.z * dt;
				}
				break;
				
				case 1:										// Midpoint 
				
				ptmp = [];
				
				for (i = 0; i < particles.length; i++)
				{
					ptmp[i] = particles[i];
					
					ptmp[i].p.x += deriv[i].dpdt.x * dt / 2;
					ptmp[i].p.y += deriv[i].dpdt.y * dt / 2;
					ptmp[i].p.z += deriv[i].dpdt.z * dt / 2;
					ptmp[i].p.x += deriv[i].dvdt.x * dt / 2;
					ptmp[i].p.y += deriv[i].dvdt.y * dt / 2;
					ptmp[i].p.z += deriv[i].dvdt.z * dt / 2;
				}
				
				ptmp = jQuery.particlePhysics.calculateForces(ptmp, phys, springs);
				deriv = jQuery.particlePhysics.calculateDerivatives(ptmp);
				
				for (i = 0; i < particles.length; i++)
				{
					particles[i].p.x += deriv[i].dpdt.x * dt;
					particles[i].p.y += deriv[i].dpdt.y * dt;
					particles[i].p.z += deriv[i].dpdt.z * dt;
					particles[i].v.x += deriv[i].dvdt.x * dt;
					particles[i].v.y += deriv[i].dvdt.y * dt;
					particles[i].v.z += deriv[i].dvdt.z * dt;
				}
				

				break;
			}

			return particles;
		};

	/*
	   Calculate the derivatives
	   dp/dt = v
	   dv/dt = f / m
	*/
	jQuery.particlePhysics.calculateDerivatives = function (particles)
		{
			var i;
			var derivatives = [];
			
			for(i = 0; i < particles.length; i++)
			{
				derivatives[i] = {};
				derivatives[i].dpdt = {};
				derivatives[i].dvdt = {};
				
				derivatives[i].dpdt.x = particles[i].v.x;
				derivatives[i].dpdt.y = particles[i].v.y;
				derivatives[i].dpdt.z = particles[i].v.z;
				
				derivatives[i].dvdt.x = particles[i].f.x / particles[i].m;
				derivatives[i].dvdt.y = particles[i].f.y / particles[i].m;
				derivatives[i].dvdt.z = particles[i].f.z / particles[i].m;
			}
			
			return  derivatives;
		};

	jQuery.particlePhysics.setGravity = function(val)
		{
			jQuery.particlePhysics.physical.gravitational = val;
		};
		
	jQuery.particlePhysics.particles = [];
	jQuery.particlePhysics.springs = [];
	jQuery.particlePhysics.physical = {gravitational:10, viscousdrag:0.1};

})(jQuery);