C HL XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
C HL X
C HL X   LBFGS
C HL X
C HL X   This code was downloaded from J. Nocedal's web site
C HL X   URL: http://www.eecs.northwestern.edu/~nocedal/lbfgs.html
C HL X
C HL X   Author: J. Nocedal
C HL X
C HL X   Condition for Use:
C HL X     This software is freely available for educational or commercial 
C HL X     purposes. We expect that all publications describing work using this 
C HL X     software quote at least one of the references given below.  This 
C HL X     software is released under the GNU Public License (GPL)
C HL X
C HL X   References:
C HL X     J. Nocedal. Updating Quasi-Newton Matrices with Limited Storage 
C HL X       (1980), Mathematics of Computation 35, pp. 773-782.
C HL X     D.C. Liu and J. Nocedal. On the Limited Memory Method for Large 
C HL X       Scale Optimization (1989), Mathematical Programming B, 45, 3, pp. 
C HL X       503-528.
C HL X
C HL XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

C     ----------------------------------------------------------------------
C     This file contains the LBFGS algorithm and supporting routines
C
C     ****************
C     LBFGS SUBROUTINE
C     ****************
C
      SUBROUTINE LBFGS(N,M,X,F,G,DIAGCO,DIAG,IPRINT,EPS,XTOL,W,IFLAG)
C
      INTEGER N,M,IPRINT(2),IFLAG
      DOUBLE PRECISION X(N),G(N),DIAG(N),W(N*(2*M+1)+2*M)
      DOUBLE PRECISION F,EPS,XTOL
      LOGICAL DIAGCO
C
C        LIMITED MEMORY BFGS METHOD FOR LARGE SCALE OPTIMIZATION
C                          JORGE NOCEDAL
C                        *** July 1990 ***
C
C 
C     This subroutine solves the unconstrained minimization problem
C 
C                      min F(x),    x= (x1,x2,...,xN),
C
C      using the limited memory BFGS method. The routine is especially
C      effective on problems involving a large number of variables. In
C      a typical iteration of this method an approximation Hk to the
C      inverse of the Hessian is obtained by applying M BFGS updates to
C      a diagonal matrix Hk0, using information from the previous M steps.
C      The user specifies the number M, which determines the amount of
C      storage required by the routine. The user may also provide the
C      diagonal matrices Hk0 if not satisfied with the default choice.
C      The algorithm is described in "On the limited memory BFGS method
C      for large scale optimization", by D. Liu and J. Nocedal,
C      Mathematical Programming B 45 (1989) 503-528.
C 
C      The user is required to calculate the function value F and its
C      gradient G. In order to allow the user complete control over
C      these computations, reverse  communication is used. The routine
C      must be called repeatedly under the control of the parameter
C      IFLAG. 
C
C      The steplength is determined at each iteration by means of the
C      line search routine MCVSRCH, which is a slight modification of
C      the routine CSRCH written by More' and Thuente.
C 
C      The calling statement is 
C 
C          CALL LBFGS(N,M,X,F,G,DIAGCO,DIAG,IPRINT,EPS,XTOL,W,IFLAG)
C 
C      where
C 
C     N       is an INTEGER variable that must be set by the user to the
C             number of variables. It is not altered by the routine.
C             Restriction: N>0.
C 
C     M       is an INTEGER variable that must be set by the user to
C             the number of corrections used in the BFGS update. It
C             is not altered by the routine. Values of M less than 3 are
C             not recommended; large values of M will result in excessive
C             computing time. 3<= M <=7 is recommended. Restriction: M>0.
C 
C     X       is a DOUBLE PRECISION array of length N. On initial entry
C             it must be set by the user to the values of the initial
C             estimate of the solution vector. On exit with IFLAG=0, it
C             contains the values of the variables at the best point
C             found (usually a solution).
C 
C     F       is a DOUBLE PRECISION variable. Before initial entry and on
C             a re-entry with IFLAG=1, it must be set by the user to
C             contain the value of the function F at the point X.
C 
C     G       is a DOUBLE PRECISION array of length N. Before initial
C             entry and on a re-entry with IFLAG=1, it must be set by
C             the user to contain the components of the gradient G at
C             the point X.
C 
C     DIAGCO  is a LOGICAL variable that must be set to .TRUE. if the
C             user  wishes to provide the diagonal matrix Hk0 at each
C             iteration. Otherwise it should be set to .FALSE., in which
C             case  LBFGS will use a default value described below. If
C             DIAGCO is set to .TRUE. the routine will return at each
C             iteration of the algorithm with IFLAG=2, and the diagonal
C              matrix Hk0  must be provided in the array DIAG.
C 
C 
C     DIAG    is a DOUBLE PRECISION array of length N. If DIAGCO=.TRUE.,
C             then on initial entry or on re-entry with IFLAG=2, DIAG
C             it must be set by the user to contain the values of the 
C             diagonal matrix Hk0.  Restriction: all elements of DIAG
C             must be positive.
C 
C     IPRINT  is an INTEGER array of length two which must be set by the
C             user.
C 
C             IPRINT(1) specifies the frequency of the output:
C                IPRINT(1) < 0 : no output is generated,
C                IPRINT(1) = 0 : output only at first and last iteration,
C                IPRINT(1) > 0 : output every IPRINT(1) iterations.
C 
C             IPRINT(2) specifies the type of output generated:
C                IPRINT(2) = 0 : iteration count, number of function 
C                                evaluations, function value, norm of the
C                                gradient, and steplength,
C                IPRINT(2) = 1 : same as IPRINT(2)=0, plus vector of
C                                variables and  gradient vector at the
C                                initial point,
C                IPRINT(2) = 2 : same as IPRINT(2)=1, plus vector of
C                                variables,
C                IPRINT(2) = 3 : same as IPRINT(2)=2, plus gradient vector.
C 
C 
C     EPS     is a positive DOUBLE PRECISION variable that must be set by
C             the user, and determines the accuracy with which the solution
C             is to be found. The subroutine terminates when
C
C                         ||G|| < EPS max(1,||X||),
C
C             where ||.|| denotes the Euclidean norm.
C 
C     XTOL    is a  positive DOUBLE PRECISION variable that must be set by
C             the user to an estimate of the machine precision (e.g.
C             10**(-16) on a SUN station 3/60). The line search routine will
C             terminate if the relative width of the interval of uncertainty
C             is less than XTOL.
C 
C     W       is a DOUBLE PRECISION array of length N(2M+1)+2M used as
C             workspace for LBFGS. This array must not be altered by the
C             user.
C 
C     IFLAG   is an INTEGER variable that must be set to 0 on initial entry
C             to the subroutine. A return with IFLAG<0 indicates an error,
C             and IFLAG=0 indicates that the routine has terminated without
C             detecting errors. On a return with IFLAG=1, the user must
C             evaluate the function F and gradient G. On a return with
C             IFLAG=2, the user must provide the diagonal matrix Hk0.
C 
C             The following negative values of IFLAG, detecting an error,
C             are possible:
C 
C              IFLAG=-1  The line search routine MCSRCH failed. The
C                        parameter INFO provides more detailed information
C                        (see also the documentation of MCSRCH):
C
C                       INFO = 0  IMPROPER INPUT PARAMETERS.
C
C                       INFO = 2  RELATIVE WIDTH OF THE INTERVAL OF
C                                 UNCERTAINTY IS AT MOST XTOL.
C
C                       INFO = 3  MORE THAN 20 FUNCTION EVALUATIONS WERE
C                                 REQUIRED AT THE PRESENT ITERATION.
C
C                       INFO = 4  THE STEP IS TOO SMALL.
C
C                       INFO = 5  THE STEP IS TOO LARGE.
C
C                       INFO = 6  ROUNDING ERRORS PREVENT FURTHER PROGRESS. 
C                                 THERE MAY NOT BE A STEP WHICH SATISFIES
C                                 THE SUFFICIENT DECREASE AND CURVATURE
C                                 CONDITIONS. TOLERANCES MAY BE TOO SMALL.
C
C 
C              IFLAG=-2  The i-th diagonal element of the diagonal inverse
C                        Hessian approximation, given in DIAG, is not
C                        positive.
C           
C              IFLAG=-3  Improper input parameters for LBFGS (N or M are
C                        not positive).
C 
C
C
C    ON THE DRIVER:
C
C    The program that calls LBFGS must contain the declaration:
C
C                       EXTERNAL QUIPLB2
C
C    QUIPLB2 is a BLOCK DATA that defines the default values of several
C    parameters described in the COMMON section. 
C
C 
C 
C    COMMON:
C 
C     The subroutine contains one common area, which the user may wish to
C    reference:
C 
         COMMON /QUIPLB3/MP,LP,GTOL,STPMIN,STPMAX
C 
C    MP  is an INTEGER variable with default value 6. It is used as the
C        unit number for the printing of the monitoring information
C        controlled by IPRINT.
C 
C    LP  is an INTEGER variable with default value 6. It is used as the
C        unit number for the printing of error messages. This printing
C        may be suppressed by setting LP to a non-positive value.
C 
C    GTOL is a DOUBLE PRECISION variable with default value 0.9, which
C        controls the accuracy of the line search routine MCSRCH. If the
C        function and gradient evaluations are inexpensive with respect
C        to the cost of the iteration (which is sometimes the case when
C        solving very large problems) it may be advantageous to set GTOL
C        to a small value. A typical small value is 0.1.  Restriction:
C        GTOL should be greater than 1.D-04.
C 
C    STPMIN and STPMAX are non-negative DOUBLE PRECISION variables which
C        specify lower and uper bounds for the step in the line search.
C        Their default values are 1.D-20 and 1.D+20, respectively. These
C        values need not be modified unless the exponents are too large
C        for the machine being used, or unless the problem is extremely
C        badly scaled (in which case the exponents should be increased).
C 
C
C  MACHINE DEPENDENCIES
C
C        The only variables that are machine-dependent are XTOL,
C        STPMIN and STPMAX.
C 
C
C  GENERAL INFORMATION
C 
C    Other routines called directly:  DAXPY, DDOT, QUIPLB1, MCSRCH
C 
C    Input/Output  :  No input; diagnostic messages on unit MP and
C                     error messages on unit LP.
C 
C 
C     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
C
      DOUBLE PRECISION GTOL,ONE,ZERO,GNORM,lbfgs_DDOT,STP1,FTOL,STPMIN,
     .                 STPMAX,STP,YS,YY,SQ,YR,BETA,XNORM
      INTEGER MP,LP,ITER,NFUN,POINT,ISPT,IYPT,MAXFEV,INFO,
     .        BOUND,NPT,CP,I,NFEV,INMC,IYCN,ISCN
      LOGICAL FINISH
C
      SAVE
      DATA ONE,ZERO/1.0D+0,0.0D+0/
C
C     INITIALIZE
C     ----------
C
      IF(IFLAG.EQ.0) GO TO 10
      GO TO (172,100) IFLAG
  10  ITER= 0
      IF(N.LE.0.OR.M.LE.0) GO TO 196
      IF(GTOL.LE.1.D-04) THEN
        IF(LP.GT.0) WRITE(LP,245)
        GTOL=9.D-01
      ENDIF
      NFUN= 1
      POINT= 0
      FINISH= .FALSE.
      IF(DIAGCO) THEN
         DO 30 I=1,N
 30      IF (DIAG(I).LE.ZERO) GO TO 195
      ELSE
         DO 40 I=1,N
 40      DIAG(I)= 1.0D0
      ENDIF
C
C     THE WORK VECTOR W IS DIVIDED AS FOLLOWS:
C     ---------------------------------------
C     THE FIRST N LOCATIONS ARE USED TO STORE THE GRADIENT AND
C         OTHER TEMPORARY INFORMATION.
C     LOCATIONS (N+1)...(N+M) STORE THE SCALARS RHO.
C     LOCATIONS (N+M+1)...(N+2M) STORE THE NUMBERS ALPHA USED
C         IN THE FORMULA THAT COMPUTES H*G.
C     LOCATIONS (N+2M+1)...(N+2M+NM) STORE THE LAST M SEARCH
C         STEPS.
C     LOCATIONS (N+2M+NM+1)...(N+2M+2NM) STORE THE LAST M
C         GRADIENT DIFFERENCES.
C
C     THE SEARCH STEPS AND GRADIENT DIFFERENCES ARE STORED IN A
C     CIRCULAR ORDER CONTROLLED BY THE PARAMETER POINT.
C
      ISPT= N+2*M
      IYPT= ISPT+N*M     
      DO 50 I=1,N
 50   W(ISPT+I)= -G(I)*DIAG(I)
      GNORM= DSQRT(lbfgs_DDOT(N,G,1,G,1))
      STP1= ONE/GNORM
C
C     PARAMETERS FOR LINE SEARCH ROUTINE
C     
      FTOL= 1.0D-4
      MAXFEV= 20
C
      IF(IPRINT(1).GE.0) CALL QUIPLB1(IPRINT,ITER,NFUN,
     *                     GNORM,N,M,X,F,G,STP,FINISH)
C
C    --------------------
C     MAIN ITERATION LOOP
C    --------------------
C
 80   ITER= ITER+1
      INFO=0
      BOUND=ITER-1
      IF(ITER.EQ.1) GO TO 165
      IF (ITER .GT. M)BOUND=M
C
         YS= lbfgs_DDOT(N,W(IYPT+NPT+1),1,W(ISPT+NPT+1),1)
      IF(.NOT.DIAGCO) THEN
         YY= lbfgs_DDOT(N,W(IYPT+NPT+1),1,W(IYPT+NPT+1),1)
         DO 90 I=1,N
   90    DIAG(I)= YS/YY
      ELSE
         IFLAG=2
         RETURN
      ENDIF
 100  CONTINUE
      IF(DIAGCO) THEN
        DO 110 I=1,N
 110    IF (DIAG(I).LE.ZERO) GO TO 195
      ENDIF
C
C     COMPUTE -H*G USING THE FORMULA GIVEN IN: Nocedal, J. 1980,
C     "Updating quasi-Newton matrices with limited storage",
C     Mathematics of Computation, Vol.24, No.151, pp. 773-782.
C     ---------------------------------------------------------
C
      CP= POINT
      IF (POINT.EQ.0) CP=M
      W(N+CP)= ONE/YS
      DO 112 I=1,N
 112  W(I)= -G(I)
      CP= POINT
      DO 125 I= 1,BOUND
         CP=CP-1
         IF (CP.EQ. -1)CP=M-1
         SQ= lbfgs_DDOT(N,W(ISPT+CP*N+1),1,W,1)
         INMC=N+M+CP+1
         IYCN=IYPT+CP*N
         W(INMC)= W(N+CP+1)*SQ
         CALL lbfgs_DAXPY(N,-W(INMC),W(IYCN+1),1,W,1)
 125  CONTINUE
C
      DO 130 I=1,N
 130  W(I)=DIAG(I)*W(I)
C
      DO 145 I=1,BOUND
         YR= lbfgs_DDOT(N,W(IYPT+CP*N+1),1,W,1)
         BETA= W(N+CP+1)*YR
         INMC=N+M+CP+1
         BETA= W(INMC)-BETA
         ISCN=ISPT+CP*N
         CALL lbfgs_DAXPY(N,BETA,W(ISCN+1),1,W,1)
         CP=CP+1
         IF (CP.EQ.M)CP=0
 145  CONTINUE
C
C     STORE THE NEW SEARCH DIRECTION
C     ------------------------------
C
       DO 160 I=1,N
 160   W(ISPT+POINT*N+I)= W(I)
C
C     OBTAIN THE ONE-DIMENSIONAL MINIMIZER OF THE FUNCTION 
C     BY USING THE LINE SEARCH ROUTINE MCSRCH
C     ----------------------------------------------------
 165  NFEV=0
      STP=ONE
      IF (ITER.EQ.1) STP=STP1
      DO 170 I=1,N
 170  W(I)=G(I)
 172  CONTINUE
      CALL MCSRCH(N,X,F,G,W(ISPT+POINT*N+1),STP,FTOL,
     *            XTOL,MAXFEV,INFO,NFEV,DIAG)
      IF (INFO .EQ. -1) THEN
        IFLAG=1
        RETURN
      ENDIF
      IF (INFO .NE. 1) GO TO 190
      NFUN= NFUN + NFEV
C
C     COMPUTE THE NEW STEP AND GRADIENT CHANGE 
C     -----------------------------------------
C
      NPT=POINT*N
      DO 175 I=1,N
      W(ISPT+NPT+I)= STP*W(ISPT+NPT+I)
 175  W(IYPT+NPT+I)= G(I)-W(I)
      POINT=POINT+1
      IF (POINT.EQ.M)POINT=0
C
C     TERMINATION TEST
C     ----------------
C
      GNORM= DSQRT(lbfgs_DDOT(N,G,1,G,1))
      XNORM= DSQRT(lbfgs_DDOT(N,X,1,X,1))
      XNORM= DMAX1(1.0D0,XNORM)
      IF (GNORM/XNORM .LE. EPS) FINISH=.TRUE.
C
      IF(IPRINT(1).GE.0) CALL QUIPLB1(IPRINT,ITER,NFUN,
     *               GNORM,N,M,X,F,G,STP,FINISH)
      IF (FINISH) THEN
         IFLAG=0
         RETURN
      ENDIF
      GO TO 80
C
C     ------------------------------------------------------------
C     END OF MAIN ITERATION LOOP. ERROR EXITS.
C     ------------------------------------------------------------
C
 190  IFLAG=-1
      IF(LP.GT.0) WRITE(LP,200) INFO
      RETURN
 195  IFLAG=-2
      IF(LP.GT.0) WRITE(LP,235) I
      RETURN
 196  IFLAG= -3
      IF(LP.GT.0) WRITE(LP,240)
C
C     FORMATS
C     -------
C
 200  FORMAT(/' IFLAG= -1 ',/' LINE SEARCH FAILED. SEE'
     .          ' DOCUMENTATION OF ROUTINE MCSRCH',/' ERROR RETURN'
     .          ' OF LINE SEARCH: INFO= ',I2,/
     .          ' POSSIBLE CAUSES: FUNCTION OR GRADIENT ARE INCORRECT',/,
     .          ' OR INCORRECT TOLERANCES')
 235  FORMAT(/' IFLAG= -2',/' THE',I5,'-TH DIAGONAL ELEMENT OF THE',/,
     .       ' INVERSE HESSIAN APPROXIMATION IS NOT POSITIVE')
 240  FORMAT(/' IFLAG= -3',/' IMPROPER INPUT PARAMETERS (N OR M',
     .       ' ARE NOT POSITIVE)')
 245  FORMAT(/'  GTOL IS LESS THAN OR EQUAL TO 1.D-04',
     .       / ' IT HAS BEEN RESET TO 9.D-01')
      RETURN
      END
C
C     LAST LINE OF SUBROUTINE LBFGS
C
C
      SUBROUTINE QUIPLB1(IPRINT,ITER,NFUN,
     *                     GNORM,N,M,X,F,G,STP,FINISH)
C
C     -------------------------------------------------------------
C     THIS ROUTINE PRINTS MONITORING INFORMATION. THE FREQUENCY AND
C     AMOUNT OF OUTPUT ARE CONTROLLED BY IPRINT.
C     -------------------------------------------------------------
C
      INTEGER IPRINT(2),ITER,NFUN,LP,MP,N,M,I
      DOUBLE PRECISION X(N),G(N),F,GNORM,STP,GTOL,STPMIN,STPMAX
      LOGICAL FINISH
      COMMON /QUIPLB3/MP,LP,GTOL,STPMIN,STPMAX
C
      IF (ITER.EQ.0)THEN
           WRITE(MP,10)
           WRITE(MP,20) N,M
           WRITE(MP,30)F,GNORM
                 IF (IPRINT(2).GE.1)THEN
                     WRITE(MP,40)
                     WRITE(MP,50) (X(I),I=1,N)
                     WRITE(MP,60)
                     WRITE(MP,50) (G(I),I=1,N)
                  ENDIF
           WRITE(MP,10)
           WRITE(MP,70)
      ELSE
          IF ((IPRINT(1).EQ.0).AND.(ITER.NE.1.AND..NOT.FINISH))RETURN
              IF (IPRINT(1).NE.0)THEN
                   IF(MOD(ITER-1,IPRINT(1)).EQ.0.OR.FINISH)THEN
                         IF(IPRINT(2).GT.1.AND.ITER.GT.1) WRITE(MP,70)
                         WRITE(MP,80)ITER,NFUN,F,GNORM,STP
                   ELSE
                         RETURN
                   ENDIF
              ELSE
                   IF( IPRINT(2).GT.1.AND.FINISH) WRITE(MP,70)
                   WRITE(MP,80)ITER,NFUN,F,GNORM,STP
              ENDIF
              IF (IPRINT(2).EQ.2.OR.IPRINT(2).EQ.3)THEN
                    IF (FINISH)THEN
                        WRITE(MP,90)
                    ELSE
                        WRITE(MP,40)
                    ENDIF
                      WRITE(MP,50)(X(I),I=1,N)
                  IF (IPRINT(2).EQ.3)THEN
                      WRITE(MP,60)
                      WRITE(MP,50)(G(I),I=1,N)
                  ENDIF
              ENDIF
            IF (FINISH) WRITE(MP,100)
      ENDIF
C
 10   FORMAT('*************************************************')
 20   FORMAT('  N=',I5,'   NUMBER OF CORRECTIONS=',I2,
     .       /,  '       INITIAL VALUES')
 30   FORMAT(' F= ',1PD10.3,'   GNORM= ',1PD10.3)
 40   FORMAT(' VECTOR X= ')
 50   FORMAT(6(2X,1PD10.3))
 60   FORMAT(' GRADIENT VECTOR G= ')
 70   FORMAT(/'   I   NFN',4X,'FUNC',8X,'GNORM',7X,'STEPLENGTH'/)
 80   FORMAT(2(I4,1X),3X,3(1PD10.3,2X))
 90   FORMAT(' FINAL POINT X= ')
 100  FORMAT(/' THE MINIMIZATION TERMINATED WITHOUT DETECTING ERRORS.',
     .       /' IFLAG = 0')
C
      RETURN
      END
C     ******
C
C
C   ----------------------------------------------------------
C     DATA 
C   ----------------------------------------------------------
C
      BLOCK DATA QUIPLB2
      INTEGER LP,MP
      DOUBLE PRECISION GTOL,STPMIN,STPMAX
      COMMON /QUIPLB3/MP,LP,GTOL,STPMIN,STPMAX
      DATA MP,LP,GTOL,STPMIN,STPMAX/6,6,9.0D-01,1.0D-20,1.0D+20/
      END
C
C
C   ----------------------------------------------------------
C
      subroutine lbfgs_daxpy(n,da,dx,incx,dy,incy)
c
c     constant times a vector plus a vector.
c     uses unrolled loops for increments equal to one.
c     jack dongarra, linpack, 3/11/78.
c
      double precision dx(1),dy(1),da
      integer i,incx,incy,ix,iy,m,mp1,n
c
      if(n.le.0)return
      if (da .eq. 0.0d0) return
      if(incx.eq.1.and.incy.eq.1)go to 20
c
c        code for unequal increments or equal increments
c          not equal to 1
c
      ix = 1
      iy = 1
      if(incx.lt.0)ix = (-n+1)*incx + 1
      if(incy.lt.0)iy = (-n+1)*incy + 1
      do 10 i = 1,n
        dy(iy) = dy(iy) + da*dx(ix)
        ix = ix + incx
        iy = iy + incy
   10 continue
      return
c
c        code for both increments equal to 1
c
c
c        clean-up loop
c
   20 m = mod(n,4)
      if( m .eq. 0 ) go to 40
      do 30 i = 1,m
        dy(i) = dy(i) + da*dx(i)
   30 continue
      if( n .lt. 4 ) return
   40 mp1 = m + 1
      do 50 i = mp1,n,4
        dy(i) = dy(i) + da*dx(i)
        dy(i + 1) = dy(i + 1) + da*dx(i + 1)
        dy(i + 2) = dy(i + 2) + da*dx(i + 2)
        dy(i + 3) = dy(i + 3) + da*dx(i + 3)
   50 continue
      return
      end
C
C
C   ----------------------------------------------------------
C
      double precision function lbfgs_ddot(n,dx,incx,dy,incy)
c
c     forms the dot product of two vectors.
c     uses unrolled loops for increments equal to one.
c     jack dongarra, linpack, 3/11/78.
c
      double precision dx(1),dy(1),dtemp
      integer i,incx,incy,ix,iy,m,mp1,n
c
      lbfgs_ddot = 0.0d0
      dtemp = 0.0d0
      if(n.le.0)return
      if(incx.eq.1.and.incy.eq.1)go to 20
c
c        code for unequal increments or equal increments
c          not equal to 1
c
      ix = 1
      iy = 1
      if(incx.lt.0)ix = (-n+1)*incx + 1
      if(incy.lt.0)iy = (-n+1)*incy + 1
      do 10 i = 1,n
        dtemp = dtemp + dx(ix)*dy(iy)
        ix = ix + incx
        iy = iy + incy
   10 continue
      lbfgs_ddot = dtemp
      return
c
c        code for both increments equal to 1
c
c
c        clean-up loop
c
   20 m = mod(n,5)
      if( m .eq. 0 ) go to 40
      do 30 i = 1,m
        dtemp = dtemp + dx(i)*dy(i)
   30 continue
      if( n .lt. 5 ) go to 60
   40 mp1 = m + 1
      do 50 i = mp1,n,5
        dtemp = dtemp + dx(i)*dy(i) + dx(i + 1)*dy(i + 1) +
     *   dx(i + 2)*dy(i + 2) + dx(i + 3)*dy(i + 3) + dx(i + 4)*dy(i + 4)
   50 continue
   60 lbfgs_ddot = dtemp
      return
      end
C    ------------------------------------------------------------------
C
C     **************************
C     LINE SEARCH ROUTINE MCSRCH
C     **************************
C
      SUBROUTINE MCSRCH(N,X,F,G,S,STP,FTOL,XTOL,MAXFEV,INFO,NFEV,WA)
      INTEGER N,MAXFEV,INFO,NFEV
      DOUBLE PRECISION F,STP,FTOL,GTOL,XTOL,STPMIN,STPMAX
      DOUBLE PRECISION X(N),G(N),S(N),WA(N)
      INTEGER MP,LP
      COMMON /QUIPLB3/MP,LP,GTOL,STPMIN,STPMAX
      SAVE
C
C                     SUBROUTINE MCSRCH
C                
C     A slight modification of the subroutine CSRCH of More' and Thuente.
C     The changes are to allow reverse communication, and do not affect
C     the performance of the routine. 
C
C     THE PURPOSE OF MCSRCH IS TO FIND A STEP WHICH SATISFIES
C     A SUFFICIENT DECREASE CONDITION AND A CURVATURE CONDITION.
C
C     AT EACH STAGE THE SUBROUTINE UPDATES AN INTERVAL OF
C     UNCERTAINTY WITH ENDPOINTS STX AND STY. THE INTERVAL OF
C     UNCERTAINTY IS INITIALLY CHOSEN SO THAT IT CONTAINS A
C     MINIMIZER OF THE MODIFIED FUNCTION
C
C          F(X+STP*S) - F(X) - FTOL*STP*(GRADF(X)'S).
C
C     IF A STEP IS OBTAINED FOR WHICH THE MODIFIED FUNCTION
C     HAS A NONPOSITIVE FUNCTION VALUE AND NONNEGATIVE DERIVATIVE,
C     THEN THE INTERVAL OF UNCERTAINTY IS CHOSEN SO THAT IT
C     CONTAINS A MINIMIZER OF F(X+STP*S).
C
C     THE ALGORITHM IS DESIGNED TO FIND A STEP WHICH SATISFIES
C     THE SUFFICIENT DECREASE CONDITION
C
C           F(X+STP*S) .LE. F(X) + FTOL*STP*(GRADF(X)'S),
C
C     AND THE CURVATURE CONDITION
C
C           ABS(GRADF(X+STP*S)'S)) .LE. GTOL*ABS(GRADF(X)'S).
C
C     IF FTOL IS LESS THAN GTOL AND IF, FOR EXAMPLE, THE FUNCTION
C     IS BOUNDED BELOW, THEN THERE IS ALWAYS A STEP WHICH SATISFIES
C     BOTH CONDITIONS. IF NO STEP CAN BE FOUND WHICH SATISFIES BOTH
C     CONDITIONS, THEN THE ALGORITHM USUALLY STOPS WHEN ROUNDING
C     ERRORS PREVENT FURTHER PROGRESS. IN THIS CASE STP ONLY
C     SATISFIES THE SUFFICIENT DECREASE CONDITION.
C
C     THE SUBROUTINE STATEMENT IS
C
C        SUBROUTINE MCSRCH(N,X,F,G,S,STP,FTOL,XTOL, MAXFEV,INFO,NFEV,WA)
C     WHERE
C
C       N IS A POSITIVE INTEGER INPUT VARIABLE SET TO THE NUMBER
C         OF VARIABLES.
C
C       X IS AN ARRAY OF LENGTH N. ON INPUT IT MUST CONTAIN THE
C         BASE POINT FOR THE LINE SEARCH. ON OUTPUT IT CONTAINS
C         X + STP*S.
C
C       F IS A VARIABLE. ON INPUT IT MUST CONTAIN THE VALUE OF F
C         AT X. ON OUTPUT IT CONTAINS THE VALUE OF F AT X + STP*S.
C
C       G IS AN ARRAY OF LENGTH N. ON INPUT IT MUST CONTAIN THE
C         GRADIENT OF F AT X. ON OUTPUT IT CONTAINS THE GRADIENT
C         OF F AT X + STP*S.
C
C       S IS AN INPUT ARRAY OF LENGTH N WHICH SPECIFIES THE
C         SEARCH DIRECTION.
C
C       STP IS A NONNEGATIVE VARIABLE. ON INPUT STP CONTAINS AN
C         INITIAL ESTIMATE OF A SATISFACTORY STEP. ON OUTPUT
C         STP CONTAINS THE FINAL ESTIMATE.
C
C       FTOL AND GTOL ARE NONNEGATIVE INPUT VARIABLES. (In this reverse
C         communication implementation GTOL is defined in a COMMON
C         statement.) TERMINATION OCCURS WHEN THE SUFFICIENT DECREASE
C         CONDITION AND THE DIRECTIONAL DERIVATIVE CONDITION ARE
C         SATISFIED.
C
C       XTOL IS A NONNEGATIVE INPUT VARIABLE. TERMINATION OCCURS
C         WHEN THE RELATIVE WIDTH OF THE INTERVAL OF UNCERTAINTY
C         IS AT MOST XTOL.
C
C       STPMIN AND STPMAX ARE NONNEGATIVE INPUT VARIABLES WHICH
C         SPECIFY LOWER AND UPPER BOUNDS FOR THE STEP. (In this reverse
C         communication implementatin they are defined in a COMMON
C         statement).
C
C       MAXFEV IS A POSITIVE INTEGER INPUT VARIABLE. TERMINATION
C         OCCURS WHEN THE NUMBER OF CALLS TO FCN IS AT LEAST
C         MAXFEV BY THE END OF AN ITERATION.
C
C       INFO IS AN INTEGER OUTPUT VARIABLE SET AS FOLLOWS:
C
C         INFO = 0  IMPROPER INPUT PARAMETERS.
C
C         INFO =-1  A RETURN IS MADE TO COMPUTE THE FUNCTION AND GRADIENT.
C
C         INFO = 1  THE SUFFICIENT DECREASE CONDITION AND THE
C                   DIRECTIONAL DERIVATIVE CONDITION HOLD.
C
C         INFO = 2  RELATIVE WIDTH OF THE INTERVAL OF UNCERTAINTY
C                   IS AT MOST XTOL.
C
C         INFO = 3  NUMBER OF CALLS TO FCN HAS REACHED MAXFEV.
C
C         INFO = 4  THE STEP IS AT THE LOWER BOUND STPMIN.
C
C         INFO = 5  THE STEP IS AT THE UPPER BOUND STPMAX.
C
C         INFO = 6  ROUNDING ERRORS PREVENT FURTHER PROGRESS.
C                   THERE MAY NOT BE A STEP WHICH SATISFIES THE
C                   SUFFICIENT DECREASE AND CURVATURE CONDITIONS.
C                   TOLERANCES MAY BE TOO SMALL.
C
C       NFEV IS AN INTEGER OUTPUT VARIABLE SET TO THE NUMBER OF
C         CALLS TO FCN.
C
C       WA IS A WORK ARRAY OF LENGTH N.
C
C     SUBPROGRAMS CALLED
C
C       MCSTEP
C
C       FORTRAN-SUPPLIED...ABS,MAX,MIN
C
C     ARGONNE NATIONAL LABORATORY. MINPACK PROJECT. JUNE 1983
C     JORGE J. MORE', DAVID J. THUENTE
C
C     **********
      INTEGER INFOC,J
      LOGICAL BRACKT,STAGE1
      DOUBLE PRECISION DG,DGM,DGINIT,DGTEST,DGX,DGXM,DGY,DGYM,
     *       FINIT,FTEST1,FM,FX,FXM,FY,FYM,P5,P66,STX,STY,
     *       STMIN,STMAX,WIDTH,WIDTH1,XTRAPF,ZERO
      DATA P5,P66,XTRAPF,ZERO /0.5D0,0.66D0,4.0D0,0.0D0/
      IF(INFO.EQ.-1) GO TO 45
      INFOC = 1
C
C     CHECK THE INPUT PARAMETERS FOR ERRORS.
C
      IF (N .LE. 0 .OR. STP .LE. ZERO .OR. FTOL .LT. ZERO .OR.
     *    GTOL .LT. ZERO .OR. XTOL .LT. ZERO .OR. STPMIN .LT. ZERO
     *    .OR. STPMAX .LT. STPMIN .OR. MAXFEV .LE. 0) RETURN
C
C     COMPUTE THE INITIAL GRADIENT IN THE SEARCH DIRECTION
C     AND CHECK THAT S IS A DESCENT DIRECTION.
C
      DGINIT = ZERO
      DO 10 J = 1, N
         DGINIT = DGINIT + G(J)*S(J)
   10    CONTINUE
      IF (DGINIT .GE. ZERO) then
         write(LP,15)
   15    FORMAT(/'  THE SEARCH DIRECTION IS NOT A DESCENT DIRECTION')
         RETURN
         ENDIF
C
C     INITIALIZE LOCAL VARIABLES.
C
      BRACKT = .FALSE.
      STAGE1 = .TRUE.
      NFEV = 0
      FINIT = F
      DGTEST = FTOL*DGINIT
      WIDTH = STPMAX - STPMIN
      WIDTH1 = WIDTH/P5
      DO 20 J = 1, N
         WA(J) = X(J)
   20    CONTINUE
C
C     THE VARIABLES STX, FX, DGX CONTAIN THE VALUES OF THE STEP,
C     FUNCTION, AND DIRECTIONAL DERIVATIVE AT THE BEST STEP.
C     THE VARIABLES STY, FY, DGY CONTAIN THE VALUE OF THE STEP,
C     FUNCTION, AND DERIVATIVE AT THE OTHER ENDPOINT OF
C     THE INTERVAL OF UNCERTAINTY.
C     THE VARIABLES STP, F, DG CONTAIN THE VALUES OF THE STEP,
C     FUNCTION, AND DERIVATIVE AT THE CURRENT STEP.
C
      STX = ZERO
      FX = FINIT
      DGX = DGINIT
      STY = ZERO
      FY = FINIT
      DGY = DGINIT
C
C     START OF ITERATION.
C
   30 CONTINUE
C
C        SET THE MINIMUM AND MAXIMUM STEPS TO CORRESPOND
C        TO THE PRESENT INTERVAL OF UNCERTAINTY.
C
         IF (BRACKT) THEN
            STMIN = MIN(STX,STY)
            STMAX = MAX(STX,STY)
         ELSE
            STMIN = STX
            STMAX = STP + XTRAPF*(STP - STX)
            END IF
C
C        FORCE THE STEP TO BE WITHIN THE BOUNDS STPMAX AND STPMIN.
C
         STP = MAX(STP,STPMIN)
         STP = MIN(STP,STPMAX)
C
C        IF AN UNUSUAL TERMINATION IS TO OCCUR THEN LET
C        STP BE THE LOWEST POINT OBTAINED SO FAR.
C
         IF ((BRACKT .AND. (STP .LE. STMIN .OR. STP .GE. STMAX))
     *      .OR. NFEV .GE. MAXFEV-1 .OR. INFOC .EQ. 0
     *      .OR. (BRACKT .AND. STMAX-STMIN .LE. XTOL*STMAX)) STP = STX
C
C        EVALUATE THE FUNCTION AND GRADIENT AT STP
C        AND COMPUTE THE DIRECTIONAL DERIVATIVE.
C        We return to main program to obtain F and G.
C
         DO 40 J = 1, N
            X(J) = WA(J) + STP*S(J)
   40       CONTINUE
         INFO=-1
         RETURN
C
   45    INFO=0
         NFEV = NFEV + 1
         DG = ZERO
         DO 50 J = 1, N
            DG = DG + G(J)*S(J)
   50       CONTINUE
         FTEST1 = FINIT + STP*DGTEST
C
C        TEST FOR CONVERGENCE.
C
         IF ((BRACKT .AND. (STP .LE. STMIN .OR. STP .GE. STMAX))
     *      .OR. INFOC .EQ. 0) INFO = 6
         IF (STP .EQ. STPMAX .AND.
     *       F .LE. FTEST1 .AND. DG .LE. DGTEST) INFO = 5
         IF (STP .EQ. STPMIN .AND.
     *       (F .GT. FTEST1 .OR. DG .GE. DGTEST)) INFO = 4
         IF (NFEV .GE. MAXFEV) INFO = 3
         IF (BRACKT .AND. STMAX-STMIN .LE. XTOL*STMAX) INFO = 2
         IF (F .LE. FTEST1 .AND. ABS(DG) .LE. GTOL*(-DGINIT)) INFO = 1
C
C        CHECK FOR TERMINATION.
C
         IF (INFO .NE. 0) RETURN
C
C        IN THE FIRST STAGE WE SEEK A STEP FOR WHICH THE MODIFIED
C        FUNCTION HAS A NONPOSITIVE VALUE AND NONNEGATIVE DERIVATIVE.
C
         IF (STAGE1 .AND. F .LE. FTEST1 .AND.
     *       DG .GE. MIN(FTOL,GTOL)*DGINIT) STAGE1 = .FALSE.
C
C        A MODIFIED FUNCTION IS USED TO PREDICT THE STEP ONLY IF
C        WE HAVE NOT OBTAINED A STEP FOR WHICH THE MODIFIED
C        FUNCTION HAS A NONPOSITIVE FUNCTION VALUE AND NONNEGATIVE
C        DERIVATIVE, AND IF A LOWER FUNCTION VALUE HAS BEEN
C        OBTAINED BUT THE DECREASE IS NOT SUFFICIENT.
C
         IF (STAGE1 .AND. F .LE. FX .AND. F .GT. FTEST1) THEN
C
C           DEFINE THE MODIFIED FUNCTION AND DERIVATIVE VALUES.
C
            FM = F - STP*DGTEST
            FXM = FX - STX*DGTEST
            FYM = FY - STY*DGTEST
            DGM = DG - DGTEST
            DGXM = DGX - DGTEST
            DGYM = DGY - DGTEST
C
C           CALL CSTEP TO UPDATE THE INTERVAL OF UNCERTAINTY
C           AND TO COMPUTE THE NEW STEP.
C
            CALL MCSTEP(STX,FXM,DGXM,STY,FYM,DGYM,STP,FM,DGM,
     *                 BRACKT,STMIN,STMAX,INFOC)
C
C           RESET THE FUNCTION AND GRADIENT VALUES FOR F.
C
            FX = FXM + STX*DGTEST
            FY = FYM + STY*DGTEST
            DGX = DGXM + DGTEST
            DGY = DGYM + DGTEST
         ELSE
C
C           CALL MCSTEP TO UPDATE THE INTERVAL OF UNCERTAINTY
C           AND TO COMPUTE THE NEW STEP.
C
            CALL MCSTEP(STX,FX,DGX,STY,FY,DGY,STP,F,DG,
     *                 BRACKT,STMIN,STMAX,INFOC)
            END IF
C
C        FORCE A SUFFICIENT DECREASE IN THE SIZE OF THE
C        INTERVAL OF UNCERTAINTY.
C
         IF (BRACKT) THEN
            IF (ABS(STY-STX) .GE. P66*WIDTH1)
     *         STP = STX + P5*(STY - STX)
            WIDTH1 = WIDTH
            WIDTH = ABS(STY-STX)
            END IF
C
C        END OF ITERATION.
C
         GO TO 30
C
C     LAST LINE OF SUBROUTINE MCSRCH.
C
      END
      SUBROUTINE MCSTEP(STX,FX,DX,STY,FY,DY,STP,FP,DP,BRACKT,
     *                 STPMIN,STPMAX,INFO)
      INTEGER INFO
      DOUBLE PRECISION STX,FX,DX,STY,FY,DY,STP,FP,DP,STPMIN,STPMAX
      LOGICAL BRACKT,BOUND
C
C     SUBROUTINE MCSTEP
C
C     THE PURPOSE OF MCSTEP IS TO COMPUTE A SAFEGUARDED STEP FOR
C     A LINESEARCH AND TO UPDATE AN INTERVAL OF UNCERTAINTY FOR
C     A MINIMIZER OF THE FUNCTION.
C
C     THE PARAMETER STX CONTAINS THE STEP WITH THE LEAST FUNCTION
C     VALUE. THE PARAMETER STP CONTAINS THE CURRENT STEP. IT IS
C     ASSUMED THAT THE DERIVATIVE AT STX IS NEGATIVE IN THE
C     DIRECTION OF THE STEP. IF BRACKT IS SET TRUE THEN A
C     MINIMIZER HAS BEEN BRACKETED IN AN INTERVAL OF UNCERTAINTY
C     WITH ENDPOINTS STX AND STY.
C
C     THE SUBROUTINE STATEMENT IS
C
C       SUBROUTINE MCSTEP(STX,FX,DX,STY,FY,DY,STP,FP,DP,BRACKT,
C                        STPMIN,STPMAX,INFO)
C
C     WHERE
C
C       STX, FX, AND DX ARE VARIABLES WHICH SPECIFY THE STEP,
C         THE FUNCTION, AND THE DERIVATIVE AT THE BEST STEP OBTAINED
C         SO FAR. THE DERIVATIVE MUST BE NEGATIVE IN THE DIRECTION
C         OF THE STEP, THAT IS, DX AND STP-STX MUST HAVE OPPOSITE
C         SIGNS. ON OUTPUT THESE PARAMETERS ARE UPDATED APPROPRIATELY.
C
C       STY, FY, AND DY ARE VARIABLES WHICH SPECIFY THE STEP,
C         THE FUNCTION, AND THE DERIVATIVE AT THE OTHER ENDPOINT OF
C         THE INTERVAL OF UNCERTAINTY. ON OUTPUT THESE PARAMETERS ARE
C         UPDATED APPROPRIATELY.
C
C       STP, FP, AND DP ARE VARIABLES WHICH SPECIFY THE STEP,
C         THE FUNCTION, AND THE DERIVATIVE AT THE CURRENT STEP.
C         IF BRACKT IS SET TRUE THEN ON INPUT STP MUST BE
C         BETWEEN STX AND STY. ON OUTPUT STP IS SET TO THE NEW STEP.
C
C       BRACKT IS A LOGICAL VARIABLE WHICH SPECIFIES IF A MINIMIZER
C         HAS BEEN BRACKETED. IF THE MINIMIZER HAS NOT BEEN BRACKETED
C         THEN ON INPUT BRACKT MUST BE SET FALSE. IF THE MINIMIZER
C         IS BRACKETED THEN ON OUTPUT BRACKT IS SET TRUE.
C
C       STPMIN AND STPMAX ARE INPUT VARIABLES WHICH SPECIFY LOWER
C         AND UPPER BOUNDS FOR THE STEP.
C
C       INFO IS AN INTEGER OUTPUT VARIABLE SET AS FOLLOWS:
C         IF INFO = 1,2,3,4,5, THEN THE STEP HAS BEEN COMPUTED
C         ACCORDING TO ONE OF THE FIVE CASES BELOW. OTHERWISE
C         INFO = 0, AND THIS INDICATES IMPROPER INPUT PARAMETERS.
C
C     SUBPROGRAMS CALLED
C
C       FORTRAN-SUPPLIED ... ABS,MAX,MIN,SQRT
C
C     ARGONNE NATIONAL LABORATORY. MINPACK PROJECT. JUNE 1983
C     JORGE J. MORE', DAVID J. THUENTE
C
      DOUBLE PRECISION GAMMA,P,Q,R,S,SGND,STPC,STPF,STPQ,THETA
      INFO = 0
C
C     CHECK THE INPUT PARAMETERS FOR ERRORS.
C
      IF ((BRACKT .AND. (STP .LE. MIN(STX,STY) .OR.
     *    STP .GE. MAX(STX,STY))) .OR.
     *    DX*(STP-STX) .GE. 0.0 .OR. STPMAX .LT. STPMIN) RETURN
C
C     DETERMINE IF THE DERIVATIVES HAVE OPPOSITE SIGN.
C
      SGND = DP*(DX/ABS(DX))
C
C     FIRST CASE. A HIGHER FUNCTION VALUE.
C     THE MINIMUM IS BRACKETED. IF THE CUBIC STEP IS CLOSER
C     TO STX THAN THE QUADRATIC STEP, THE CUBIC STEP IS TAKEN,
C     ELSE THE AVERAGE OF THE CUBIC AND QUADRATIC STEPS IS TAKEN.
C
      IF (FP .GT. FX) THEN
         INFO = 1
         BOUND = .TRUE.
         THETA = 3*(FX - FP)/(STP - STX) + DX + DP
         S = MAX(ABS(THETA),ABS(DX),ABS(DP))
         GAMMA = S*SQRT((THETA/S)**2 - (DX/S)*(DP/S))
         IF (STP .LT. 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 (ABS(STPC-STX) .LT. ABS(STPQ-STX)) THEN
            STPF = STPC
         ELSE
           STPF = STPC + (STPQ - STPC)/2
           END IF
         BRACKT = .TRUE.
C
C     SECOND CASE. A LOWER FUNCTION VALUE AND DERIVATIVES OF
C     OPPOSITE SIGN. THE MINIMUM IS BRACKETED. IF THE CUBIC
C     STEP IS CLOSER TO STX THAN THE QUADRATIC (SECANT) STEP,
C     THE CUBIC STEP IS TAKEN, ELSE THE QUADRATIC STEP IS TAKEN.
C
      ELSE IF (SGND .LT. 0.0) THEN
         INFO = 2
         BOUND = .FALSE.
         THETA = 3*(FX - FP)/(STP - STX) + DX + DP
         S = MAX(ABS(THETA),ABS(DX),ABS(DP))
         GAMMA = S*SQRT((THETA/S)**2 - (DX/S)*(DP/S))
         IF (STP .GT. 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 (ABS(STPC-STP) .GT. ABS(STPQ-STP)) THEN
            STPF = STPC
         ELSE
            STPF = STPQ
            END IF
         BRACKT = .TRUE.
C
C     THIRD CASE. A LOWER FUNCTION VALUE, DERIVATIVES OF THE
C     SAME SIGN, AND THE MAGNITUDE OF THE DERIVATIVE DECREASES.
C     THE CUBIC STEP IS ONLY USED IF THE CUBIC TENDS TO INFINITY
C     IN THE DIRECTION OF THE STEP OR IF THE MINIMUM OF THE CUBIC
C     IS BEYOND STP. OTHERWISE THE CUBIC STEP IS DEFINED TO BE
C     EITHER STPMIN OR STPMAX. THE QUADRATIC (SECANT) STEP IS ALSO
C     COMPUTED AND IF THE MINIMUM IS BRACKETED THEN THE THE STEP
C     CLOSEST TO STX IS TAKEN, ELSE THE STEP FARTHEST AWAY IS TAKEN.
C
      ELSE IF (ABS(DP) .LT. ABS(DX)) THEN
         INFO = 3
         BOUND = .TRUE.
         THETA = 3*(FX - FP)/(STP - STX) + DX + DP
         S = MAX(ABS(THETA),ABS(DX),ABS(DP))
C
C        THE CASE GAMMA = 0 ONLY ARISES IF THE CUBIC DOES NOT TEND
C        TO INFINITY IN THE DIRECTION OF THE STEP.
C
         GAMMA = S*SQRT(MAX(0.0D0,(THETA/S)**2 - (DX/S)*(DP/S)))
         IF (STP .GT. STX) GAMMA = -GAMMA
         P = (GAMMA - DP) + THETA
         Q = (GAMMA + (DX - DP)) + GAMMA
         R = P/Q
         IF (R .LT. 0.0 .AND. GAMMA .NE. 0.0) THEN
            STPC = STP + R*(STX - STP)
         ELSE IF (STP .GT. STX) THEN
            STPC = STPMAX
         ELSE
            STPC = STPMIN
            END IF
         STPQ = STP + (DP/(DP-DX))*(STX - STP)
         IF (BRACKT) THEN
            IF (ABS(STP-STPC) .LT. ABS(STP-STPQ)) THEN
               STPF = STPC
            ELSE
               STPF = STPQ
               END IF
         ELSE
            IF (ABS(STP-STPC) .GT. ABS(STP-STPQ)) THEN
               STPF = STPC
            ELSE
               STPF = STPQ
               END IF
            END IF
C
C     FOURTH CASE. A LOWER FUNCTION VALUE, DERIVATIVES OF THE
C     SAME SIGN, AND THE MAGNITUDE OF THE DERIVATIVE DOES
C     NOT DECREASE. IF THE MINIMUM IS NOT BRACKETED, THE STEP
C     IS EITHER STPMIN OR STPMAX, ELSE THE CUBIC STEP IS TAKEN.
C
      ELSE
         INFO = 4
         BOUND = .FALSE.
         IF (BRACKT) THEN
            THETA = 3*(FP - FY)/(STY - STP) + DY + DP
            S = MAX(ABS(THETA),ABS(DY),ABS(DP))
            GAMMA = S*SQRT((THETA/S)**2 - (DY/S)*(DP/S))
            IF (STP .GT. 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 .GT. STX) THEN
            STPF = STPMAX
         ELSE
            STPF = STPMIN
            END IF
         END IF
C
C     UPDATE THE INTERVAL OF UNCERTAINTY. THIS UPDATE DOES NOT
C     DEPEND ON THE NEW STEP OR THE CASE ANALYSIS ABOVE.
C
      IF (FP .GT. FX) THEN
         STY = STP
         FY = FP
         DY = DP
      ELSE
         IF (SGND .LT. 0.0) THEN
            STY = STX
            FY = FX
            DY = DX
            END IF
         STX = STP
         FX = FP
         DX = DP
         END IF
C
C     COMPUTE THE NEW STEP AND SAFEGUARD IT.
C
      STPF = MIN(STPMAX,STPF)
      STPF = MAX(STPMIN,STPF)
      STP = STPF
      IF (BRACKT .AND. BOUND) THEN
         IF (STY .GT. STX) THEN
            STP = MIN(STX+0.66*(STY-STX),STP)
         ELSE
            STP = MAX(STX+0.66*(STY-STX),STP)
            END IF
         END IF
      RETURN
C
C     LAST LINE OF SUBROUTINE MCSTEP.
C
      END
