source: ThirdParty/mpqc_open/src/lib/math/optimize/mcsearch.cc@ 860145

// These routines were translated from lbfgs.f by f2c (version 20030320)
// and modified by Curtis Janssen.

#ifdef __GNUC__
#pragma implementation
#endif

#include <math.h>
#include <util/class/scexception.h>
#include <math/optimize/mcsearch.h>

static inline double min(double a, double b)
{
  return (a<b)?a:b;
}

static inline double max(double a, double b)
{
  return (a<b)?b:a;
}


using namespace sc;

namespace sc {

static ClassDesc MCSearch_cd(
  typeid(MCSearch),"MCSearch",1,"public LineOpt",
  0, create<MCSearch>, 0);

MCSearch::MCSearch(const Ref<KeyVal>& keyval) 
  : LineOpt(keyval)
{ 
}

MCSearch::~MCSearch()
{
}

void
MCSearch::mcinit()
{
  info_ = 0;

  // work area
  wa_.reset(new double[function()->dimension()->n()]);
}

void
MCSearch::init(RefSCVector& direction)
{
  LineOpt::init(direction);
  mcinit();
}

void
MCSearch::init(RefSCVector& direction, Ref<Function> function)
{
  LineOpt::init(direction, function);
  mcinit();
}

int
MCSearch::update()
{
  int n = function()->dimension()->n();

  // function coordinate
  auto_vec<double> x(new double[n]);
  function()->get_x()->convert(x.get());

  // gradient
  auto_vec<double> g(new double[n]);
  function()->gradient()->convert(g.get());

  // function value
  double f = function()->value();

  // step direction
  auto_vec<double> s(new double[n]);
  search_direction_->convert(s.get());

  // step size;
  double stp;
  stp = 1.0;

  // value tolerance
  double ftol;
  ftol = 1.0e-4;

  // the machine precision
  double xtol;
  xtol = DBL_EPSILON;

  // maximum number of function evaluations
  int maxfev = 20;

  // number of function evaluations
  int nfev = 0;

  // controls accuracy of line search routine
  gtol_ = 0.9;

  // minimum step size
  stpmin_ = DBL_EPSILON;

  // maximum step size
  stpmax_ = 1.0e20;

  mcsrch(&n, x.get(), &f,g.get(), s.get(), &stp, &ftol,
         &xtol, &maxfev, &info_, &nfev, wa_.get());

//         INFO = 0  IMPROPER INPUT PARAMETERS. 
  if (info_ == 0) {
      throw ProgrammingError("error in MCSearch: info == 0",
                             __FILE__,
                             __LINE__,
                             class_desc());
    }

//         INFO =-1  A RETURN IS MADE TO COMPUTE THE FUNCTION AND GRADIENT. 
  if (info_ == -1) {
      RefSCVector new_x = function()->get_x()->copy();
      new_x->assign(x.get());
      function()->set_x(new_x);
      return 0;
    }

//         INFO = 1  THE SUFFICIENT DECREASE CONDITION AND THE 
//                   DIRECTIONAL DERIVATIVE CONDITION HOLD. 
  if (info_ == 1) {
      return 1;
    }

//         INFO = 2  RELATIVE WIDTH OF THE INTERVAL OF UNCERTAINTY 
//                   IS AT MOST XTOL. 
  if (info_ == 2) {
      throw AlgorithmException("error in MCSearch: info == 2",
                             __FILE__,
                             __LINE__,
                             class_desc());
      return 1;
    }

//         INFO = 3  NUMBER OF CALLS TO FCN HAS REACHED MAXFEV. 
  if (info_ == 3) {
      throw ProgrammingError("error in MCSearch: info == 3",
                             __FILE__,
                             __LINE__,
                             class_desc());
      return 1;
    }

//         INFO = 4  THE STEP IS AT THE LOWER BOUND STPMIN. 
  if (info_ == 4) {
      throw AlgorithmException("error in MCSearch: info == 4",
                             __FILE__,
                             __LINE__,
                             class_desc());
      return 1;
    }

//         INFO = 5  THE STEP IS AT THE UPPER BOUND STPMAX. 
  if (info_ == 5) {
      throw AlgorithmException("error in MCSearch: info == 5",
                             __FILE__,
                             __LINE__,
                             class_desc());
      return 1;
    }

//         INFO = 6  ROUNDING ERRORS PREVENT FURTHER PROGRESS. 
//                   THERE MAY NOT BE A STEP WHICH SATISFIES THE 
//                   SUFFICIENT DECREASE AND CURVATURE CONDITIONS. 
//                   TOLERANCES MAY BE TOO SMALL. 
  if (info_ == 6) {
      throw AlgorithmException("error in MCSearch: info == 6",
                             __FILE__,
                             __LINE__,
                             class_desc());
      return 1;
    }

  throw ProgrammingError("error in MCSearch: unknown info",
                         __FILE__,
                         __LINE__,
                         class_desc());

  return 0;
}

//     **************************
//     LINE SEARCH ROUTINE MCSRCH
//     **************************

void
MCSearch::mcsrch(int *n, double *x, double *f, 
        double *g, double *s, double *stp, double *ftol, 
        double *xtol, int *maxfev, int *info, int *nfev, 
        double *wa)
{
    // Initialized data 

    const double p5 = .5;
    const double p66 = .66;
    const double xtrapf = 4.;
    const double zero = 0.;

    // System generated locals 
    int i__1;
    double d__1;

//                     SUBROUTINE MCSRCH 

//     A slight modification of the subroutine CSRCH of More' and Thuente. 
//     The changes are to allow reverse communication, and do not affect 
//     the performance of the routine. 

//     THE PURPOSE OF MCSRCH IS TO FIND A STEP WHICH SATISFIES 
//     A SUFFICIENT DECREASE CONDITION AND A CURVATURE CONDITION. 

//     AT EACH STAGE THE SUBROUTINE UPDATES AN INTERVAL OF 
//     UNCERTAINTY WITH ENDPOINTS STX AND STY. THE INTERVAL OF 
//     UNCERTAINTY IS INITIALLY CHOSEN SO THAT IT CONTAINS A 
//     MINIMIZER OF THE MODIFIED FUNCTION 

//          F(X+STP*S) - F(X) - FTOL*STP*(GRADF(X)'S). 

//     IF A STEP IS OBTAINED FOR WHICH THE MODIFIED FUNCTION 
//     HAS A NONPOSITIVE FUNCTION VALUE AND NONNEGATIVE DERIVATIVE, 
//     THEN THE INTERVAL OF UNCERTAINTY IS CHOSEN SO THAT IT 
//     CONTAINS A MINIMIZER OF F(X+STP*S). 

//     THE ALGORITHM IS DESIGNED TO FIND A STEP WHICH SATISFIES 
//     THE SUFFICIENT DECREASE CONDITION 

//           F(X+STP*S) .LE. F(X) + FTOL*STP*(GRADF(X)'S), 

//     AND THE CURVATURE CONDITION 

//           ABS(GRADF(X+STP*S)'S)) .LE. GTOL*ABS(GRADF(X)'S). 

//     IF FTOL IS LESS THAN GTOL AND IF, FOR EXAMPLE, THE FUNCTION 
//     IS BOUNDED BELOW, THEN THERE IS ALWAYS A STEP WHICH SATISFIES 
//     BOTH CONDITIONS. IF NO STEP CAN BE FOUND WHICH SATISFIES BOTH 
//     CONDITIONS, THEN THE ALGORITHM USUALLY STOPS WHEN ROUNDING 
//     ERRORS PREVENT FURTHER PROGRESS. IN THIS CASE STP ONLY 
//     SATISFIES THE SUFFICIENT DECREASE CONDITION. 

//     THE SUBROUTINE STATEMENT IS 

//        SUBROUTINE MCSRCH(N,X,F,G,S,STP,FTOL,XTOL, MAXFEV,INFO,NFEV,WA) 
//     WHERE 

//       N IS A POSITIVE INTEGER INPUT VARIABLE SET TO THE NUMBER 
//         OF VARIABLES. 

//       X IS AN ARRAY OF LENGTH N. ON INPUT IT MUST CONTAIN THE 
//         BASE POINT FOR THE LINE SEARCH. ON OUTPUT IT CONTAINS 
//         X + STP*S. 

//       F IS A VARIABLE. ON INPUT IT MUST CONTAIN THE VALUE OF F 
//         AT X. ON OUTPUT IT CONTAINS THE VALUE OF F AT X + STP*S. 

//       G IS AN ARRAY OF LENGTH N. ON INPUT IT MUST CONTAIN THE 
//         GRADIENT OF F AT X. ON OUTPUT IT CONTAINS THE GRADIENT 
//         OF F AT X + STP*S. 

//       S IS AN INPUT ARRAY OF LENGTH N WHICH SPECIFIES THE 
//         SEARCH DIRECTION. 

//       STP IS A NONNEGATIVE VARIABLE. ON INPUT STP CONTAINS AN 
//         INITIAL ESTIMATE OF A SATISFACTORY STEP. ON OUTPUT 
//         STP CONTAINS THE FINAL ESTIMATE. 

//       FTOL AND GTOL ARE NONNEGATIVE INPUT VARIABLES. (In this reverse 
//         communication implementation GTOL is defined in a COMMON 
//         statement.) TERMINATION OCCURS WHEN THE SUFFICIENT DECREASE 
//         CONDITION AND THE DIRECTIONAL DERIVATIVE CONDITION ARE 
//         SATISFIED. 

//       XTOL IS A NONNEGATIVE INPUT VARIABLE. TERMINATION OCCURS 
//         WHEN THE RELATIVE WIDTH OF THE INTERVAL OF UNCERTAINTY 
//         IS AT MOST XTOL. 

//       STPMIN AND STPMAX ARE NONNEGATIVE INPUT VARIABLES WHICH 
//         SPECIFY LOWER AND UPPER BOUNDS FOR THE STEP. (In this reverse 
//         communication implementatin they are defined in a COMMON 
//         statement). 

//       MAXFEV IS A POSITIVE INTEGER INPUT VARIABLE. TERMINATION 
//         OCCURS WHEN THE NUMBER OF CALLS TO FCN IS AT LEAST 
//         MAXFEV BY THE END OF AN ITERATION. 

//       INFO IS AN INTEGER OUTPUT VARIABLE SET AS FOLLOWS: 

//         INFO = 0  IMPROPER INPUT PARAMETERS. 

//         INFO =-1  A RETURN IS MADE TO COMPUTE THE FUNCTION AND GRADIENT. 

//         INFO = 1  THE SUFFICIENT DECREASE CONDITION AND THE 
//                   DIRECTIONAL DERIVATIVE CONDITION HOLD. 

//         INFO = 2  RELATIVE WIDTH OF THE INTERVAL OF UNCERTAINTY 
//                   IS AT MOST XTOL. 

//         INFO = 3  NUMBER OF CALLS TO FCN HAS REACHED MAXFEV. 

//         INFO = 4  THE STEP IS AT THE LOWER BOUND STPMIN. 

//         INFO = 5  THE STEP IS AT THE UPPER BOUND STPMAX. 

//         INFO = 6  ROUNDING ERRORS PREVENT FURTHER PROGRESS. 
//                   THERE MAY NOT BE A STEP WHICH SATISFIES THE 
//                   SUFFICIENT DECREASE AND CURVATURE CONDITIONS. 
//                   TOLERANCES MAY BE TOO SMALL. 

//       NFEV IS AN INTEGER OUTPUT VARIABLE SET TO THE NUMBER OF 
//         CALLS TO FCN. 

//       WA IS A WORK ARRAY OF LENGTH N. 

//     SUBPROGRAMS CALLED 

//       MCSTEP 

//       FORTRAN-SUPPLIED...ABS,MAX,MIN 

//     ARGONNE NATIONAL LABORATORY. MINPACK PROJECT. JUNE 1983 
//     JORGE J. MORE', DAVID J. THUENTE 

//     ********** 
    // Parameter adjustments 
    --wa;
    --s;
    --g;
    --x;

    // Function Body 
    if (*info == -1) {
        goto L45;
    }
    infoc = 1;

//     CHECK THE INPUT PARAMETERS FOR ERRORS. 

    if (*n <= 0 || *stp <= zero || *ftol < zero || gtol_ < zero || *xtol 
            < zero || stpmin_ < zero || stpmax_ < stpmin_ || *
            maxfev <= 0) {
        return;
    }

//     COMPUTE THE INITIAL GRADIENT IN THE SEARCH DIRECTION 
//     AND CHECK THAT S IS A DESCENT DIRECTION. 

    dginit = zero;
    i__1 = *n;
    for (int j = 1; j <= i__1; ++j) {
        dginit += g[j] * s[j];
// L10: 
    }
    if (dginit >= zero) {
        ExEnv::out0() << indent
                      << "MCSearch: "
                      << "The search direction is not a descent direction"
                      << std::endl;
        return;
    }

//     INITIALIZE LOCAL VARIABLES. 

    brackt = false;
    stage1 = true;
    *nfev = 0;
    finit = *f;
    dgtest = *ftol * dginit;
    width = stpmax_ - stpmin_;
    width1 = width / p5;
    i__1 = *n;
    for (int j = 1; j <= i__1; ++j) {
        wa[j] = x[j];
// L20: 
    }

//     THE VARIABLES STX, FX, DGX CONTAIN THE VALUES OF THE STEP, 
//     FUNCTION, AND DIRECTIONAL DERIVATIVE AT THE BEST STEP. 
//     THE VARIABLES STY, FY, DGY CONTAIN THE VALUE OF THE STEP, 
//     FUNCTION, AND DERIVATIVE AT THE OTHER ENDPOINT OF 
//     THE INTERVAL OF UNCERTAINTY. 
//     THE VARIABLES STP, F, DG CONTAIN THE VALUES OF THE STEP, 
//     FUNCTION, AND DERIVATIVE AT THE CURRENT STEP. 

    stx = zero;
    fx = finit;
    dgx = dginit;
    sty = zero;
    fy = finit;
    dgy = dginit;

//     START OF ITERATION. 

L30:

//        SET THE MINIMUM AND MAXIMUM STEPS TO CORRESPOND 
//        TO THE PRESENT INTERVAL OF UNCERTAINTY. 

    if (brackt) {
        stmin = min(stx,sty);
        stmax = max(stx,sty);
    } else {
        stmin = stx;
        stmax = *stp + xtrapf * (*stp - stx);
    }

//        FORCE THE STEP TO BE WITHIN THE BOUNDS STPMAX AND STPMIN. 

    *stp = max(*stp,stpmin_);
    *stp = min(*stp,stpmax_);

//        IF AN UNUSUAL TERMINATION IS TO OCCUR THEN LET 
//        STP BE THE LOWEST POINT OBTAINED SO FAR. 

    if (brackt && (*stp <= stmin || *stp >= stmax) || *nfev >= *maxfev - 1 || 
            infoc == 0 || brackt && stmax - stmin <= *xtol * stmax) {
        *stp = stx;
    }

//        EVALUATE THE FUNCTION AND GRADIENT AT STP 
//        AND COMPUTE THE DIRECTIONAL DERIVATIVE. 
//        We return to main program to obtain F and G. 

    i__1 = *n;
    for (int j = 1; j <= i__1; ++j) {
        x[j] = wa[j] + *stp * s[j];
// L40: 
    }
    *info = -1;
    return;

L45:
    *info = 0;
    ++(*nfev);
    dg = zero;
    i__1 = *n;
    for (int j = 1; j <= i__1; ++j) {
        dg += g[j] * s[j];
// L50: 
    }
    ftest1 = finit + *stp * dgtest;

//        TEST FOR CONVERGENCE. 

    if (brackt && (*stp <= stmin || *stp >= stmax) || infoc == 0) {
        *info = 6;
    }
    if (*stp == stpmax_ && *f <= ftest1 && dg <= dgtest) {
        *info = 5;
    }
    if (*stp == stpmin_ && (*f > ftest1 || dg >= dgtest)) {
        *info = 4;
    }
    if (*nfev >= *maxfev) {
        *info = 3;
    }
    if (brackt && stmax - stmin <= *xtol * stmax) {
        *info = 2;
    }
    if (*f <= ftest1 && fabs(dg) <= gtol_ * (-dginit)) {
        *info = 1;
    }

//        CHECK FOR TERMINATION. 

    if (*info != 0) {
        return;
    }

//        IN THE FIRST STAGE WE SEEK A STEP FOR WHICH THE MODIFIED 
//        FUNCTION HAS A NONPOSITIVE VALUE AND NONNEGATIVE DERIVATIVE. 

    if (stage1 && *f <= ftest1 && dg >= min(*ftol,gtol_) * dginit) {
        stage1 = false;
    }

//        A MODIFIED FUNCTION IS USED TO PREDICT THE STEP ONLY IF 
//        WE HAVE NOT OBTAINED A STEP FOR WHICH THE MODIFIED 
//        FUNCTION HAS A NONPOSITIVE FUNCTION VALUE AND NONNEGATIVE 
//        DERIVATIVE, AND IF A LOWER FUNCTION VALUE HAS BEEN 
//        OBTAINED BUT THE DECREASE IS NOT SUFFICIENT. 

    if (stage1 && *f <= fx && *f > ftest1) {

//           DEFINE THE MODIFIED FUNCTION AND DERIVATIVE VALUES. 

        fm = *f - *stp * dgtest;
        fxm = fx - stx * dgtest;
        fym = fy - sty * dgtest;
        dgm = dg - dgtest;
        dgxm = dgx - dgtest;
        dgym = dgy - dgtest;

//           CALL CSTEP TO UPDATE THE INTERVAL OF UNCERTAINTY 
//           AND TO COMPUTE THE NEW STEP. 

        mcstep(&stx, &fxm, &dgxm, &sty, &fym, &dgym, stp, &fm, &dgm, &brackt,
                 &stmin, &stmax, &infoc);

//           RESET THE FUNCTION AND GRADIENT VALUES FOR F. 

        fx = fxm + stx * dgtest;
        fy = fym + sty * dgtest;
        dgx = dgxm + dgtest;
        dgy = dgym + dgtest;
    } else {

//           CALL MCSTEP TO UPDATE THE INTERVAL OF UNCERTAINTY 
//           AND TO COMPUTE THE NEW STEP. 

        mcstep(&stx, &fx, &dgx, &sty, &fy, &dgy, stp, f, &dg, &brackt, &
                stmin, &stmax, &infoc);
    }

//        FORCE A SUFFICIENT DECREASE IN THE SIZE OF THE 
//        INTERVAL OF UNCERTAINTY. 

    if (brackt) {
        if ((d__1 = sty - stx, fabs(d__1)) >= p66 * width1) {
            *stp = stx + p5 * (sty - stx);
        }
        width1 = width;
        width = (d__1 = sty - stx, fabs(d__1));
    }

//        END OF ITERATION. 

    goto L30;

//     LAST LINE OF SUBROUTINE MCSRCH. 

} // mcsrch_ 

void
MCSearch::mcstep(double *stx, double *fx, double *dx, 
                 double *sty, double *fy, double *dy, double *stp, 
                 double *fp, double *dp, bool *brackt, double *stpmin, 
                 double *stpmax, int *info)
{
    // System generated locals 
    double d__1, d__2, d__3;

//     SUBROUTINE MCSTEP 

//     THE PURPOSE OF MCSTEP IS TO COMPUTE A SAFEGUARDED STEP FOR 
//     A LINESEARCH AND TO UPDATE AN INTERVAL OF UNCERTAINTY FOR 
//     A MINIMIZER OF THE FUNCTION. 

//     THE PARAMETER STX CONTAINS THE STEP WITH THE LEAST FUNCTION 
//     VALUE. THE PARAMETER STP CONTAINS THE CURRENT STEP. IT IS 
//     ASSUMED THAT THE DERIVATIVE AT STX IS NEGATIVE IN THE 
//     DIRECTION OF THE STEP. IF BRACKT IS SET TRUE THEN A 
//     MINIMIZER HAS BEEN BRACKETED IN AN INTERVAL OF UNCERTAINTY 
//     WITH ENDPOINTS STX AND STY. 

//     THE SUBROUTINE STATEMENT IS 

//       SUBROUTINE MCSTEP(STX,FX,DX,STY,FY,DY,STP,FP,DP,BRACKT, 
//                        STPMIN,STPMAX,INFO) 

//     WHERE 

//       STX, FX, AND DX ARE VARIABLES WHICH SPECIFY THE STEP, 
//         THE FUNCTION, AND THE DERIVATIVE AT THE BEST STEP OBTAINED 
//         SO FAR. THE DERIVATIVE MUST BE NEGATIVE IN THE DIRECTION 
//         OF THE STEP, THAT IS, DX AND STP-STX MUST HAVE OPPOSITE 
//         SIGNS. ON OUTPUT THESE PARAMETERS ARE UPDATED APPROPRIATELY. 

//       STY, FY, AND DY ARE VARIABLES WHICH SPECIFY THE STEP, 
//         THE FUNCTION, AND THE DERIVATIVE AT THE OTHER ENDPOINT OF 
//         THE INTERVAL OF UNCERTAINTY. ON OUTPUT THESE PARAMETERS ARE 
//         UPDATED APPROPRIATELY. 

//       STP, FP, AND DP ARE VARIABLES WHICH SPECIFY THE STEP, 
//         THE FUNCTION, AND THE DERIVATIVE AT THE CURRENT STEP. 
//         IF BRACKT IS SET TRUE THEN ON INPUT STP MUST BE 
//         BETWEEN STX AND STY. ON OUTPUT STP IS SET TO THE NEW STEP. 

//       BRACKT IS A LOGICAL VARIABLE WHICH SPECIFIES IF A MINIMIZER 
//         HAS BEEN BRACKETED. IF THE MINIMIZER HAS NOT BEEN BRACKETED 
//         THEN ON INPUT BRACKT MUST BE SET FALSE. IF THE MINIMIZER 
//         IS BRACKETED THEN ON OUTPUT BRACKT IS SET TRUE. 

//       STPMIN AND STPMAX ARE INPUT VARIABLES WHICH SPECIFY LOWER 
//         AND UPPER BOUNDS FOR THE STEP. 

//       INFO IS AN INTEGER OUTPUT VARIABLE SET AS FOLLOWS: 
//         IF INFO = 1,2,3,4,5, THEN THE STEP HAS BEEN COMPUTED 
//         ACCORDING TO ONE OF THE FIVE CASES BELOW. OTHERWISE 
//         INFO = 0, AND THIS INDICATES IMPROPER INPUT PARAMETERS. 

//     SUBPROGRAMS CALLED 

//       FORTRAN-SUPPLIED ... ABS,MAX,MIN,SQRT 

//     ARGONNE NATIONAL LABORATORY. MINPACK PROJECT. JUNE 1983 
//     JORGE J. MORE', DAVID J. THUENTE 

    *info = 0;

//     CHECK THE INPUT PARAMETERS FOR ERRORS. 

    if (*brackt && (*stp <= min(*stx,*sty) || *stp >= max(*stx,*sty)) || *dx *
             (*stp - *stx) >= 0.f || *stpmax < *stpmin) {
        return;
    }

//     DETERMINE IF THE DERIVATIVES HAVE OPPOSITE SIGN. 

    sgnd = *dp * (*dx / fabs(*dx));

//     FIRST CASE. A HIGHER FUNCTION VALUE. 
//     THE MINIMUM IS BRACKETED. IF THE CUBIC STEP IS CLOSER 
//     TO STX THAN THE QUADRATIC STEP, THE CUBIC STEP IS TAKEN, 
//     ELSE THE AVERAGE OF THE CUBIC AND QUADRATIC STEPS IS TAKEN. 

    if (*fp > *fx) {
        *info = 1;
        bound = true;
        theta = (*fx - *fp) * 3 / (*stp - *stx) + *dx + *dp;
// Computing MAX 
        d__1 = fabs(theta), d__2 = fabs(*dx), d__1 = max(d__1,d__2), d__2 = fabs(
                *dp);
        s = max(d__1,d__2);
// Computing 2nd power 
        d__1 = theta / s;
        gamma = s * sqrt(d__1 * d__1 - *dx / s * (*dp / s));
        if (*stp < *stx) {
            gamma = -gamma;
        }
        p = gamma - *dx + theta;
        q = gamma - *dx + gamma + *dp;
        r__ = p / q;
        stpc = *stx + r__ * (*stp - *stx);
        stpq = *stx + *dx / ((*fx - *fp) / (*stp - *stx) + *dx) / 2 * (*stp - 
                *stx);
        if ((d__1 = stpc - *stx, fabs(d__1)) < (d__2 = stpq - *stx, fabs(d__2)))
                 {
            stpf = stpc;
        } else {
            stpf = stpc + (stpq - stpc) / 2;
        }
        *brackt = true;

//     SECOND CASE. A LOWER FUNCTION VALUE AND DERIVATIVES OF 
//     OPPOSITE SIGN. THE MINIMUM IS BRACKETED. IF THE CUBIC 
//     STEP IS CLOSER TO STX THAN THE QUADRATIC (SECANT) STEP, 
//     THE CUBIC STEP IS TAKEN, ELSE THE QUADRATIC STEP IS TAKEN. 

    } else if (sgnd < 0.f) {
        *info = 2;
        bound = false;
        theta = (*fx - *fp) * 3 / (*stp - *stx) + *dx + *dp;
// Computing MAX 
        d__1 = fabs(theta), d__2 = fabs(*dx), d__1 = max(d__1,d__2), d__2 = fabs(
                *dp);
        s = max(d__1,d__2);
// Computing 2nd power 
        d__1 = theta / s;
        gamma = s * sqrt(d__1 * d__1 - *dx / s * (*dp / s));
        if (*stp > *stx) {
            gamma = -gamma;
        }
        p = gamma - *dp + theta;
        q = gamma - *dp + gamma + *dx;
        r__ = p / q;
        stpc = *stp + r__ * (*stx - *stp);
        stpq = *stp + *dp / (*dp - *dx) * (*stx - *stp);
        if ((d__1 = stpc - *stp, fabs(d__1)) > (d__2 = stpq - *stp, fabs(d__2)))
                 {
            stpf = stpc;
        } else {
            stpf = stpq;
        }
        *brackt = true;

//     THIRD CASE. A LOWER FUNCTION VALUE, DERIVATIVES OF THE 
//     SAME SIGN, AND THE MAGNITUDE OF THE DERIVATIVE DECREASES. 
//     THE CUBIC STEP IS ONLY USED IF THE CUBIC TENDS TO INFINITY 
//     IN THE DIRECTION OF THE STEP OR IF THE MINIMUM OF THE CUBIC 
//     IS BEYOND STP. OTHERWISE THE CUBIC STEP IS DEFINED TO BE 
//     EITHER STPMIN OR STPMAX. THE QUADRATIC (SECANT) STEP IS ALSO 
//     COMPUTED AND IF THE MINIMUM IS BRACKETED THEN THE THE STEP 
//     CLOSEST TO STX IS TAKEN, ELSE THE STEP FARTHEST AWAY IS TAKEN. 

    } else if (fabs(*dp) < fabs(*dx)) {
        *info = 3;
        bound = true;
        theta = (*fx - *fp) * 3 / (*stp - *stx) + *dx + *dp;
// Computing MAX 
        d__1 = fabs(theta), d__2 = fabs(*dx), d__1 = max(d__1,d__2), d__2 = fabs(
                *dp);
        s = max(d__1,d__2);

//        THE CASE GAMMA = 0 ONLY ARISES IF THE CUBIC DOES NOT TEND 
//        TO INFINITY IN THE DIRECTION OF THE STEP. 

// Computing MAX 
// Computing 2nd power 
        d__3 = theta / s;
        d__1 = 0., d__2 = d__3 * d__3 - *dx / s * (*dp / s);
        gamma = s * sqrt((max(d__1,d__2)));
        if (*stp > *stx) {
            gamma = -gamma;
        }
        p = gamma - *dp + theta;
        q = gamma + (*dx - *dp) + gamma;
        r__ = p / q;
        if (r__ < 0.f && gamma != 0.f) {
            stpc = *stp + r__ * (*stx - *stp);
        } else if (*stp > *stx) {
            stpc = *stpmax;
        } else {
            stpc = *stpmin;
        }
        stpq = *stp + *dp / (*dp - *dx) * (*stx - *stp);
        if (*brackt) {
            if ((d__1 = *stp - stpc, fabs(d__1)) < (d__2 = *stp - stpq, fabs(
                    d__2))) {
                stpf = stpc;
            } else {
                stpf = stpq;
            }
        } else {
            if ((d__1 = *stp - stpc, fabs(d__1)) > (d__2 = *stp - stpq, fabs(
                    d__2))) {
                stpf = stpc;
            } else {
                stpf = stpq;
            }
        }

//     FOURTH CASE. A LOWER FUNCTION VALUE, DERIVATIVES OF THE 
//     SAME SIGN, AND THE MAGNITUDE OF THE DERIVATIVE DOES 
//     NOT DECREASE. IF THE MINIMUM IS NOT BRACKETED, THE STEP 
//     IS EITHER STPMIN OR STPMAX, ELSE THE CUBIC STEP IS TAKEN. 

    } else {
        *info = 4;
        bound = false;
        if (*brackt) {
            theta = (*fp - *fy) * 3 / (*sty - *stp) + *dy + *dp;
// Computing MAX 
            d__1 = fabs(theta), d__2 = fabs(*dy), d__1 = max(d__1,d__2), d__2 = 
                    fabs(*dp);
            s = max(d__1,d__2);
// Computing 2nd power 
            d__1 = theta / s;
            gamma = s * sqrt(d__1 * d__1 - *dy / s * (*dp / s));
            if (*stp > *sty) {
                gamma = -gamma;
            }
            p = gamma - *dp + theta;
            q = gamma - *dp + gamma + *dy;
            r__ = p / q;
            stpc = *stp + r__ * (*sty - *stp);
            stpf = stpc;
        } else if (*stp > *stx) {
            stpf = *stpmax;
        } else {
            stpf = *stpmin;
        }
    }

//     UPDATE THE INTERVAL OF UNCERTAINTY. THIS UPDATE DOES NOT 
//     DEPEND ON THE NEW STEP OR THE CASE ANALYSIS ABOVE. 

    if (*fp > *fx) {
        *sty = *stp;
        *fy = *fp;
        *dy = *dp;
    } else {
        if (sgnd < 0.) {
            *sty = *stx;
            *fy = *fx;
            *dy = *dx;
        }
        *stx = *stp;
        *fx = *fp;
        *dx = *dp;
    }

//     COMPUTE THE NEW STEP AND SAFEGUARD IT. 

    stpf = min(*stpmax,stpf);
    stpf = max(*stpmin,stpf);
    *stp = stpf;
    if (*brackt && bound) {
        if (*sty > *stx) {
// Computing MIN 
            d__1 = *stx + (*sty - *stx) * .66;
            *stp = min(d__1,*stp);
        } else {
// Computing MAX 
            d__1 = *stx + (*sty - *stx) * .66;
            *stp = max(d__1,*stp);
        }
    }
    return;

//     LAST LINE OF SUBROUTINE MCSTEP. 

} // mcstep_ 

}