/src/Attraction.cpp

// attract positive repel negative



#include <cmath>

#include "cinder/Vector.h"

#include "Attraction.h"



namespace traer { namespace physics {

	

    Attraction::Attraction( Particle* _a, Particle* _b, float _k, float _distanceMin )

	{

		a = _a;

		b = _b;

		k = _k;

		on = true;

		distanceMin = _distanceMin;

		distanceMinSquared = distanceMin*distanceMin;

	}



	float Attraction::getMinimumDistance()

	{

		return distanceMin;

	}



	void Attraction::setMinimumDistance( float d )

	{

		distanceMin = d;

		distanceMinSquared = d*d;

	}



	void Attraction::turnOff()

	{

		on = false;

	}



	void Attraction::turnOn()

	{

		on = true;

	}



	void Attraction::setStrength( float _k )

	{

		k = k;

	}



	Particle* Attraction::getOneEnd()

	{

		return a;

	}



	Particle* Attraction::getTheOtherEnd()

	{

		return b;

	}



	void Attraction::apply()

	{

		if ( on && ( !a->fixed || !b->fixed ) )

		{

            ci::Vec3f a2b = a->position - b->position;



			float a2bDistanceSquared = a2b.lengthSquared();



			if ( a2bDistanceSquared < distanceMinSquared )

				a2bDistanceSquared = distanceMinSquared;



			float force = k * a->mass * b->mass / a2bDistanceSquared;



			// make unit vector

			

            //a2b /= sqrt(a2bDistanceSquared);

			

			// multiply by force 

			

			//a2b *= force;

            

            a2b *= Q_rsqrt(a2bDistanceSquared) * force;



			// apply

			

			if ( !a->fixed )

				a->force -= a2b;

			if ( !b->fixed )

				b->force += a2b;

		}

	}



	float Attraction::getStrength()

	{

		return k;

	}



	bool Attraction::isOn() const

	{

		return on;

	}



	bool Attraction::isOff() const

	{

		return !on;

	}



    float Attraction::Q_rsqrt( float number )

    {

        long i;

        float x2, y;

        const float threehalfs = 1.5F;

        

        x2 = number * 0.5F;

        y  = number;

        i  = * ( long * ) &y;                       // evil floating point bit level hacking [sic]

        i  = 0x5f3759df - ( i >> 1 );               // what the fuck? [sic]

        y  = * ( float * ) &i;

        y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration

        //    y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

        

        return y;

    }



} } // namespace traer::physics