text: /* Copyright (c) 2007 Scott Lembcke
   * 
   * 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"
  #include "constraints/util.h"
  
  static void
  preStep(cpSimpleMotor *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 + b->i_inv);
  }
  
  static void
  applyCachedImpulse(cpSimpleMotor *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;
  	b->w += j*b->i_inv;
  }
  
  static void
  applyImpulse(cpSimpleMotor *joint, cpFloat dt)
  {
  	cpBody *a = joint->constraint.a;
  	cpBody *b = joint->constraint.b;
  	
  	// compute relative rotational velocity
  	cpFloat wr = b->w - a->w + joint->rate;
  	
  	cpFloat jMax = joint->constraint.maxForce*dt;
  	
  	// compute normal impulse	
  	cpFloat j = -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;
  	b->w += j*b->i_inv;
  }
  
  static cpFloat
  getImpulse(cpSimpleMotor *joint)
  {
  	return cpfabs(joint->jAcc);
  }
  
  static const cpConstraintClass klass = {
  	(cpConstraintPreStepImpl)preStep,
  	(cpConstraintApplyCachedImpulseImpl)applyCachedImpulse,
  	(cpConstraintApplyImpulseImpl)applyImpulse,
  	(cpConstraintGetImpulseImpl)getImpulse,
  };
  CP_DefineClassGetter(cpSimpleMotor)
  
  cpSimpleMotor *
  cpSimpleMotorAlloc(void)
  {
  	return (cpSimpleMotor *)cpcalloc(1, sizeof(cpSimpleMotor));
  }
  
  cpSimpleMotor *
  cpSimpleMotorInit(cpSimpleMotor *joint, cpBody *a, cpBody *b, cpFloat rate)
  {
  	cpConstraintInit((cpConstraint *)joint, &klass, a, b);
  	
  	joint->rate = rate;
  	
  	joint->jAcc = 0.0f;
  	
  	return joint;
  }
  
  cpConstraint *
  cpSimpleMotorNew(cpBody *a, cpBody *b, cpFloat rate)
  {
  	return (cpConstraint *)cpSimpleMotorInit(cpSimpleMotorAlloc(), a, b, rate);
  }
  