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c:/home/kevn/src/animaniac/ani/Dynamics/operators/SpringForceOperator.h

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#ifndef SPRING_FORCE_OPERATOR
#define SPRING_FORCE_OPERATOR

#include <boost/smart_ptr.hpp>
#include <gmtl/Vec.h>
#include "ani/Dynamics/Operator.h"
#include "ani/Dynamics/DynamicSystem.h"
#include <ani/Dynamics/operators/BinaryForceOperator.h>

namespace ani
{
   // spring force
   template<class __EntityType>
   class SpringForceOperator : public BinaryForceOperator<__EntityType>
   {
   public:
      typedef boost::shared_ptr<__EntityType> EntityTypePtr;

   public:
      SpringForceOperator() : mA(NULL), mB(NULL) {}
      virtual ~SpringForceOperator()
      {
      }

      //TODO: refactor this, move it to its superclass.
      void setEntities( EntityTypePtr a, EntityTypePtr b ) 
      {
         // take responsibility for a and b
         mA = a;
         mB = b;
      }
      void setSpringConstant( float ks ) { mSpringConstant = ks; }
      void setDampeningConstant( float kd ) { mDampeningConstant = kd; }
      void setRestLength( float length ) { mRestLength = length; }
      virtual void exec( DynamicSystem<__EntityType>& ps, float timeDelta );
   private:
      float mSpringConstant;
      float mDampeningConstant;
      EntityTypePtr mA, mB;
      float mRestLength;
   };

   //: apply this force function to the particle
   template< class __EntityType >
   void SpringForceOperator<__EntityType>::exec( ani::DynamicSystem<__EntityType>& ps, float timeDelta )
   {
      if (mA == NULL || mB == NULL)
         return;

      // does not make sense for this operator to exist if it has nothing 
      // to act upon...
      // note: this could leave the user holding an operator that maybe they thought would stay in the system... oh well, they will have to check the op's isInSystem...
      if (mA->isInSystem() == false || mB->isInSystem() == false)
      {
         ps.remove( this );
         return;
      }

      // ignoring the particle system...
      gmtl::Vec3f spring_offset = mA->position() - mB->position();
      gmtl::Vec3f speed_of_approach = mA->velocity() - mB->velocity();

      // calculate the spring force magnitude.
      float spring_force_mag = mSpringConstant * (spring_offset.length() - mRestLength);

      // calculate the damping force magnitude.
      float damping_force_mag = mDampeningConstant * ( speed_of_approach.dot( spring_offset ) / spring_offset.length() );

      // what direction the force is in
      gmtl::Vec3f direction = (spring_offset / spring_offset.length());

      // calculate the force from the spring and damping forces.
      gmtl::Vec3f force = -( direction * (spring_force_mag + damping_force_mag ));

      mA->applyForce( force );
      mB->applyForce( -force );
   }
} // end of namespace

#endif

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