text: /* Copyright (c) 2013 Scott Lembcke and Howling Moon Software
   * 
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to deal
   * in the Software without restriction, including without limitation the rights
   * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   * copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   * 
   * The above copyright notice and this permission notice shall be included in
   * all copies or substantial portions of the Software.
   * 
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
   * SOFTWARE.
   */
  
  #include "chipmunk_private.h"
  
  static void
  preStep(cpGearJoint *joint, cpFloat dt)
  {
  	cpBody *a = joint->constraint.a;
  	cpBody *b = joint->constraint.b;
  	
  	// calculate moment of inertia coefficient.
  	joint->iSum = 1.0f/(a->i_inv*joint->ratio_inv + joint->ratio*b->i_inv);
  	
  	// calculate bias velocity
  	cpFloat maxBias = joint->constraint.maxBias;
  	joint->bias = cpfclamp(-bias_coef(joint->constraint.errorBias, dt)*(b->a*joint->ratio - a->a - joint->phase)/dt, -maxBias, maxBias);
  }
  
  static void
  applyCachedImpulse(cpGearJoint *joint, cpFloat dt_coef)
  {
  	cpBody *a = joint->constraint.a;
  	cpBody *b = joint->constraint.b;
  	
  	cpFloat j = joint->jAcc*dt_coef;
  	a->w -= j*a->i_inv*joint->ratio_inv;
  	b->w += j*b->i_inv;
  }
  
  static void
  applyImpulse(cpGearJoint *joint, cpFloat dt)
  {
  	cpBody *a = joint->constraint.a;
  	cpBody *b = joint->constraint.b;
  	
  	// compute relative rotational velocity
  	cpFloat wr = b->w*joint->ratio - a->w;
  	
  	cpFloat jMax = joint->constraint.maxForce*dt;
  	
  	// compute normal impulse	
  	cpFloat j = (joint->bias - wr)*joint->iSum;
  	cpFloat jOld = joint->jAcc;
  	joint->jAcc = cpfclamp(jOld + j, -jMax, jMax);
  	j = joint->jAcc - jOld;
  	
  	// apply impulse
  	a->w -= j*a->i_inv*joint->ratio_inv;
  	b->w += j*b->i_inv;
  }
  
  static cpFloat
  getImpulse(cpGearJoint *joint)
  {
  	return cpfabs(joint->jAcc);
  }
  
  static const cpConstraintClass klass = {
  	(cpConstraintPreStepImpl)preStep,
  	(cpConstraintApplyCachedImpulseImpl)applyCachedImpulse,
  	(cpConstraintApplyImpulseImpl)applyImpulse,
  	(cpConstraintGetImpulseImpl)getImpulse,
  };
  
  cpGearJoint *
  cpGearJointAlloc(void)
  {
  	return (cpGearJoint *)cpcalloc(1, sizeof(cpGearJoint));
  }
  
  cpGearJoint *
  cpGearJointInit(cpGearJoint *joint, cpBody *a, cpBody *b, cpFloat phase, cpFloat ratio)
  {
  	cpConstraintInit((cpConstraint *)joint, &klass, a, b);
  	
  	joint->phase = phase;
  	joint->ratio = ratio;
  	joint->ratio_inv = 1.0f/ratio;
  	
  	joint->jAcc = 0.0f;
  	
  	return joint;
  }
  
  cpConstraint *
  cpGearJointNew(cpBody *a, cpBody *b, cpFloat phase, cpFloat ratio)
  {
  	return (cpConstraint *)cpGearJointInit(cpGearJointAlloc(), a, b, phase, ratio);
  }
  
  cpBool
  cpConstraintIsGearJoint(const cpConstraint *constraint)
  {
  	return (constraint->klass == &klass);
  }
  
  cpFloat
  cpGearJointGetPhase(const cpConstraint *constraint)
  {
  	cpAssertHard(cpConstraintIsGearJoint(constraint), "Constraint is not a ratchet joint.");
  	return ((cpGearJoint *)constraint)->phase;
  }
  
  void
  cpGearJointSetPhase(cpConstraint *constraint, cpFloat phase)
  {
  	cpAssertHard(cpConstraintIsGearJoint(constraint), "Constraint is not a ratchet joint.");
  	cpConstraintActivateBodies(constraint);
  	((cpGearJoint *)constraint)->phase = phase;
  }
  
  cpFloat
  cpGearJointGetRatio(const cpConstraint *constraint)
  {
  	cpAssertHard(cpConstraintIsGearJoint(constraint), "Constraint is not a ratchet joint.");
  	return ((cpGearJoint *)constraint)->ratio;
  }
  
  void
  cpGearJointSetRatio(cpConstraint *constraint, cpFloat ratio)
  {
  	cpAssertHard(cpConstraintIsGearJoint(constraint), "Constraint is not a ratchet joint.");
  	cpConstraintActivateBodies(constraint);
  	((cpGearJoint *)constraint)->ratio = ratio;
  	((cpGearJoint *)constraint)->ratio_inv = 1.0f/ratio;
  }
  