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    1: *> \brief \b DDOT
    2: *
    3: *  =========== DOCUMENTATION ===========
    4: *
    5: * Online html documentation available at
    6: *            http://www.netlib.org/lapack/explore-html/
    7: *
    8: *  Definition:
    9: *  ===========
   10: *
   11: *       DOUBLE PRECISION FUNCTION DDOT(N,DX,INCX,DY,INCY)
   12: *
   13: *       .. Scalar Arguments ..
   14: *       INTEGER INCX,INCY,N
   15: *       ..
   16: *       .. Array Arguments ..
   17: *       DOUBLE PRECISION DX(*),DY(*)
   18: *       ..
   19: *
   20: *
   21: *> \par Purpose:
   22: *  =============
   23: *>
   24: *> \verbatim
   25: *>
   26: *>    DDOT forms the dot product of two vectors.
   27: *>    uses unrolled loops for increments equal to one.
   28: *> \endverbatim
   29: *
   30: *  Arguments:
   31: *  ==========
   32: *
   33: *> \param[in] N
   34: *> \verbatim
   35: *>          N is INTEGER
   36: *>         number of elements in input vector(s)
   37: *> \endverbatim
   38: *>
   39: *> \param[in] DX
   40: *> \verbatim
   41: *>          DX is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCX ) )
   42: *> \endverbatim
   43: *>
   44: *> \param[in] INCX
   45: *> \verbatim
   46: *>          INCX is INTEGER
   47: *>         storage spacing between elements of DX
   48: *> \endverbatim
   49: *>
   50: *> \param[in] DY
   51: *> \verbatim
   52: *>          DY is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCY ) )
   53: *> \endverbatim
   54: *>
   55: *> \param[in] INCY
   56: *> \verbatim
   57: *>          INCY is INTEGER
   58: *>         storage spacing between elements of DY
   59: *> \endverbatim
   60: *
   61: *  Authors:
   62: *  ========
   63: *
   64: *> \author Univ. of Tennessee
   65: *> \author Univ. of California Berkeley
   66: *> \author Univ. of Colorado Denver
   67: *> \author NAG Ltd.
   68: *
   69: *> \date November 2017
   70: *
   71: *> \ingroup double_blas_level1
   72: *
   73: *> \par Further Details:
   74: *  =====================
   75: *>
   76: *> \verbatim
   77: *>
   78: *>     jack dongarra, linpack, 3/11/78.
   79: *>     modified 12/3/93, array(1) declarations changed to array(*)
   80: *> \endverbatim
   81: *>
   82: *  =====================================================================
   83:       DOUBLE PRECISION FUNCTION DDOT(N,DX,INCX,DY,INCY)
   84: *
   85: *  -- Reference BLAS level1 routine (version 3.8.0) --
   86: *  -- Reference BLAS is a software package provided by Univ. of Tennessee,    --
   87: *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
   88: *     November 2017
   89: *
   90: *     .. Scalar Arguments ..
   91:       INTEGER INCX,INCY,N
   92: *     ..
   93: *     .. Array Arguments ..
   94:       DOUBLE PRECISION DX(*),DY(*)
   95: *     ..
   96: *
   97: *  =====================================================================
   98: *
   99: *     .. Local Scalars ..
  100:       DOUBLE PRECISION DTEMP
  101:       INTEGER I,IX,IY,M,MP1
  102: *     ..
  103: *     .. Intrinsic Functions ..
  104:       INTRINSIC MOD
  105: *     ..
  106:       DDOT = 0.0d0
  107:       DTEMP = 0.0d0
  108:       IF (N.LE.0) RETURN
  109:       IF (INCX.EQ.1 .AND. INCY.EQ.1) THEN
  110: *
  111: *        code for both increments equal to 1
  112: *
  113: *
  114: *        clean-up loop
  115: *
  116:          M = MOD(N,5)
  117:          IF (M.NE.0) THEN
  118:             DO I = 1,M
  119:                DTEMP = DTEMP + DX(I)*DY(I)
  120:             END DO
  121:             IF (N.LT.5) THEN
  122:                DDOT=DTEMP
  123:             RETURN
  124:             END IF
  125:          END IF
  126:          MP1 = M + 1
  127:          DO I = MP1,N,5
  128:           DTEMP = DTEMP + DX(I)*DY(I) + DX(I+1)*DY(I+1) +
  129:      $            DX(I+2)*DY(I+2) + DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
  130:          END DO
  131:       ELSE
  132: *
  133: *        code for unequal increments or equal increments
  134: *          not equal to 1
  135: *
  136:          IX = 1
  137:          IY = 1
  138:          IF (INCX.LT.0) IX = (-N+1)*INCX + 1
  139:          IF (INCY.LT.0) IY = (-N+1)*INCY + 1
  140:          DO I = 1,N
  141:             DTEMP = DTEMP + DX(IX)*DY(IY)
  142:             IX = IX + INCX
  143:             IY = IY + INCY
  144:          END DO
  145:       END IF
  146:       DDOT = DTEMP
  147:       RETURN
  148:       END