// $Id$
//
//  Copyright (C) 2013 Paolo Tosco
//
//  Copyright (C) 2004-2006 Rational Discovery LLC
//
//   @@ All Rights Reserved @@
//  This file is part of the RDKit.
//  The contents are covered by the terms of the BSD license
//  which is included in the file license.txt, found at the root
//  of the RDKit source tree.
//
#include "DistanceConstraint.h"
#include <cmath>
#include <ForceField/ForceField.h>
#include <RDGeneral/Invariant.h>

namespace ForceFields {
  namespace MMFF {
    DistanceConstraintContrib::DistanceConstraintContrib(ForceField *owner,
      unsigned int idx1, unsigned int idx2, double minLen, double maxLen, double forceConst)
    {
      PRECONDITION(owner,"bad owner");
      RANGE_CHECK(0, idx1, owner->positions().size() - 1);
      RANGE_CHECK(0, idx2, owner->positions().size() - 1);
      PRECONDITION(maxLen >= minLen, "bad bounds");

      dp_forceField = owner;
      d_end1Idx = idx1;
      d_end2Idx = idx2;
      d_minLen = minLen;
      d_maxLen = maxLen;
      d_forceConstant = forceConst;
    }

    DistanceConstraintContrib::DistanceConstraintContrib(ForceField *owner,
      unsigned int idx1, unsigned int idx2, bool relative, double minLen,
      double maxLen, double forceConst)
    {
      PRECONDITION(owner,"bad owner");
      const RDGeom::PointPtrVect &pos = owner->positions();
      RANGE_CHECK(0, idx1, pos.size() - 1);
      RANGE_CHECK(0, idx2, pos.size() - 1);
      PRECONDITION(maxLen >= minLen, "bad bounds");

      double dist = 0.0;
      if (relative) {
        RDGeom::Point3D p1 = *((RDGeom::Point3D *)pos[idx1]);
        RDGeom::Point3D p2 = *((RDGeom::Point3D *)pos[idx2]);
        dist = (p1 - p2).length();
      }
      dp_forceField = owner;
      d_end1Idx = idx1;
      d_end2Idx = idx2;
      d_minLen = std::max(dist + minLen, 0.0);
      d_maxLen = std::max(dist + maxLen, 0.0);
      d_forceConstant = forceConst;
    }

    double DistanceConstraintContrib::getEnergy(double *pos) const
    {
      PRECONDITION(dp_forceField, "no owner");
      PRECONDITION(pos, "bad vector");

      double dist = dp_forceField->distance(d_end1Idx, d_end2Idx, pos);
      double distTerm = 0.0;
      if (dist < d_minLen) {
        distTerm = d_minLen - dist;
      }
      else if (dist > d_maxLen) {
        distTerm = dist - d_maxLen;
      }
      double res = 0.5 * d_forceConstant * distTerm * distTerm;

      return res;
    }
    
    void DistanceConstraintContrib::getGrad(double *pos, double *grad) const
    {
      PRECONDITION(dp_forceField,"no owner");
      PRECONDITION(pos,"bad vector");
      PRECONDITION(grad,"bad vector");

      double dist = dp_forceField->distance(d_end1Idx, d_end2Idx,pos);

      double preFactor = 0.0;
      if (dist < d_minLen) {
        preFactor = dist - d_minLen;
      }
      else if (dist > d_maxLen) {
        preFactor = dist - d_maxLen;
      }
      else {
        return;
      }
      preFactor *= d_forceConstant;
    
      double *end1Coords = &(pos[3 * d_end1Idx]);
      double *end2Coords = &(pos[3 * d_end2Idx]);
      for (unsigned int i = 0; i < 3; ++i) {
        double dGrad;
        dGrad = preFactor * (end1Coords[i] - end2Coords[i]) / std::max(dist, 1.0e-8);
        grad[3 * d_end1Idx + i] += dGrad;
        grad[3 * d_end2Idx + i] -= dGrad;
      }    
    }
  }
}  
