ROL_TrustRegionUtilities.hpp

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// @HEADER
// *****************************************************************************
//               Rapid Optimization Library (ROL) Package
//
// Copyright 2014 NTESS and the ROL contributors.
// SPDX-License-Identifier: BSD-3-Clause
// *****************************************************************************
// @HEADER

#ifndef ROL_TRUSTREGIONUTILITIES_U_H
#define ROL_TRUSTREGIONUTILITIES_U_H

#include "ROL_TrustRegionModel_U.hpp"

namespace ROL {
namespace TRUtils {

enum ETRFlag {
  SUCCESS = 0,
  POSPREDNEG,
  NPOSPREDPOS,
  NPOSPREDNEG,
  TRNAN,
  QMINSUFDEC,
  UNDEFINED 
};

inline std::string ETRFlagToString(ETRFlag trf) {
  std::string retString;
  switch(trf) {
    case SUCCESS:  
      retString = "Both actual and predicted reductions are positive (success)";
      break;
    case POSPREDNEG: 
      retString = "Actual reduction is positive and predicted reduction is negative (impossible)";
      break;
    case NPOSPREDPOS: 
      retString = "Actual reduction is nonpositive and predicted reduction is positive";
      break;
    case NPOSPREDNEG:
      retString = "Actual reduction is nonpositive and predicted reduction is negative (impossible)";
      break;
    case TRNAN:
      retString = "Actual and/or predicted reduction is a NaN";
      break;
    case QMINSUFDEC:
      retString = "Subproblem solution did not produce sufficient decrease";
      break;
    default:
      retString = "INVALID ETRFlag";       
  }
  return retString;
}

template<typename Real>
inline Real initialRadius(int &nfval,
                          const Vector<Real> &x,
                          const Vector<Real> &g,
                          Vector<Real> &Bg,
                          const Real fx,
                          const Real gnorm,
                          const Real gtol,
                          Objective<Real> &obj,
                          TrustRegionModel_U<Real> &model,
                          const Real delMax,
                          std::ostream &outStream,
                          const bool print = false) {
  const Real zero(0), half(0.5), one(1), two(2), three(3), six(6);
  const Real eps(ROL_EPSILON<Real>());
  Real del(ROL_INF<Real>());
  Real htol = gtol;
  Ptr<Vector<Real>> xcp = x.clone();
  model.setData(obj,x,g,htol);
  model.hessVec(Bg,g.dual(),x,htol);
  Real gBg = Bg.dot(g);
  Real alpha = (gBg > eps ? gnorm*gnorm/gBg : one);
  // Evaluate the objective function at the Cauchy point
  xcp->set(g.dual());
  xcp->scale(-alpha);
  //Real gs = xcp->dot(g.dual());
  Real gs = xcp->apply(g);
  xcp->plus(x);
  obj.update(*xcp,UpdateType::Temp);
  Real ftol = static_cast<Real>(0.1)*ROL_OVERFLOW<Real>(); 
  Real fnew = obj.value(*xcp,ftol); // MUST DO SOMETHING HERE WITH FTOL
  nfval++;
  // Perform cubic interpolation to determine initial trust region radius
  Real a = fnew - fx - gs - half*alpha*alpha*gBg;
  if ( std::abs(a) < eps ) { 
    // a = 0 implies the objective is quadratic in the negative gradient direction
    del = std::min(alpha*gnorm,delMax);
  }
  else {
    Real b = half*alpha*alpha*gBg;
    Real c = gs;
    if ( b*b-three*a*c > eps ) {
      // There is at least one critical point
      Real t1 = (-b-std::sqrt(b*b-three*a*c))/(three*a);
      Real t2 = (-b+std::sqrt(b*b-three*a*c))/(three*a);
      if ( six*a*t1 + two*b > zero ) {
        // t1 is the minimizer
        del = std::min(t1*alpha*gnorm,delMax);
      }
      else {
        // t2 is the minimizer
        del = std::min(t2*alpha*gnorm,delMax);
      }
    }
    else {
      del = std::min(alpha*gnorm,delMax);
    }
  }
  if (del <= eps*gnorm) {
    del = one;
  }
  obj.update(x,UpdateType::Revert);
  if ( print ) {
    outStream << "  In TrustRegionUtilities::initialRadius"      << std::endl;
    outStream << "    Initial radius:                          " << del << std::endl;
  }
  return del;
}

template<typename Real>
inline void analyzeRatio(Real &rho,
                         ETRFlag &flag,
                   const Real fold,
                   const Real ftrial,
                   const Real pRed,
                   const Real epsi,
                         std::ostream &outStream = std::cout,
                   const bool print = false) {
  const Real zero(0), one(1);
  Real eps       = epsi*std::max(one,fold);
  Real aRed      = fold - ftrial;
  Real aRed_safe = aRed + eps, pRed_safe = pRed + eps;
  if (((std::abs(aRed_safe) < epsi) && (std::abs(pRed_safe) < epsi)) || aRed == pRed) {
    rho  = one;
    flag = SUCCESS;
  }
  else if ( std::isnan(aRed_safe) || std::isnan(pRed_safe) ) {
    rho  = -one;
    flag = TRNAN;
  }
  else {
    rho = aRed_safe/pRed_safe;
    if (pRed_safe < zero && aRed_safe > zero) {
      flag = POSPREDNEG;
    }
    else if (aRed_safe <= zero && pRed_safe > zero) {
      flag = NPOSPREDPOS;
    }
    else if (aRed_safe <= zero && pRed_safe < zero) {
      flag = NPOSPREDNEG;
    }
    else {
      flag = SUCCESS;
    }
  }
  if ( print ) {
    outStream << "  In TrustRegionUtilities::analyzeRatio"       << std::endl;
    outStream << "    Current objective function value:        " << fold      << std::endl;
    outStream << "    New objective function value:            " << ftrial    << std::endl;
    outStream << "    Actual reduction:                        " << aRed      << std::endl;
    outStream << "    Predicted reduction:                     " << pRed      << std::endl;
    outStream << "    Safeguard:                               " << epsi      << std::endl;
    outStream << "    Actual reduction with safeguard:         " << aRed_safe << std::endl;
    outStream << "    Predicted reduction with safeguard:      " << pRed_safe << std::endl;
    outStream << "    Ratio of actual and predicted reduction: " << rho       << std::endl;
    outStream << "    Trust-region flag:                       " << flag      << std::endl;
    outStream << std::endl;
  }
}

template<typename Real>
inline Real interpolateRadius(const Vector<Real> &g,
                              const Vector<Real> &s,
                              const Real snorm,
                              const Real pRed,
                              const Real fold,
                              const Real ftrial,
                              const Real del,
                              const Real gamma0,
                              const Real gamma1,
                              const Real eta2,
                              std::ostream &outStream = std::cout,
                              const bool print = false) {
  const Real one(1);
  //Real gs = g.dot(s.dual());
  Real gs = g.apply(s);
  Real modelVal = fold - pRed;
  Real theta = (one-eta2)*gs/((one-eta2)*(fold+gs)+eta2*modelVal-ftrial);
  if ( print ) {
    outStream << "  In TrustRegionUtilities::interpolateRadius"  << std::endl;
    outStream << "    Interpolation model value:               " << modelVal << std::endl;
    outStream << "    Interpolation step length:               " << theta    << std::endl;
    outStream << std::endl;
  }
  return std::min(gamma1*std::min(snorm,del),std::max(gamma0,theta)*del);
}

} // namespace TrustRegion
} // namespace ROL


#endif