public interface Integrator 
{

	public void step( float t );

}


public class ModifiedEulerIntegrator implements Integrator

{

	ParticleSystem s;

	

	public ModifiedEulerIntegrator( ParticleSystem s )

	{

		this.s = s;

	}

	

	public void step( float t )

	{

		s.clearForces();

		s.applyForces();

		

		float halftt = 0.5f*t*t;

		

		for ( int i = 0; i < s.numberOfParticles(); i++ )

		{

			Particle p = s.getParticle( i );

			if ( p.isFree() )

			{

				float ax = p.force().x()/p.mass();

				float ay = p.force().y()/p.mass();

				float az = p.force().z()/p.mass();

				

				p.position().add( p.velocity().x()/t, p.velocity().y()/t, p.velocity().z()/t );

				p.position().add( ax*halftt, ay*halftt, az*halftt );

				p.velocity().add( ax/t, ay/t, az/t );

			}

		}

	}



}


public class EulerIntegrator implements Integrator

{

	ParticleSystem s;

	

	public EulerIntegrator( ParticleSystem s )

	{

		this.s = s;

	}

	

	public void step( float t )

	{

		s.clearForces();

		s.applyForces();

		

		for ( int i = 0; i < s.numberOfParticles(); i++ )

		{

			Particle p = (Particle)s.getParticle( i );

			if ( p.isFree() )

			{

				p.velocity().add( p.force().x()/(p.mass()*t), p.force().y()/(p.mass()*t), p.force().z()/(p.mass()*t) );

				p.position().add( p.velocity().x()/t, p.velocity().y()/t, p.velocity().z()/t );

			}

		}

	}
}


public interface Force
{
	public void turnOn();
	public void turnOff();
	public boolean isOn();
	public boolean isOff();
	public void apply();
}