lammps/lib/linalg/dznrm2.f

*> \brief \b DZNRM2
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
*  Definition:
*  ===========
*
*       DOUBLE PRECISION FUNCTION DZNRM2(N,X,INCX)
*
*       .. Scalar Arguments ..
*       INTEGER INCX,N
*       ..
*       .. Array Arguments ..
*       COMPLEX*16 X(*)
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> DZNRM2 returns the euclidean norm of a vector via the function
*> name, so that
*>
*>    DZNRM2 := sqrt( x**H*x )
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>         number of elements in input vector(s)
*> \endverbatim
*>
*> \param[in] X
*> \verbatim
*>          X is COMPLEX*16 array, dimension (N)
*>         complex vector with N elements
*> \endverbatim
*>
*> \param[in] INCX
*> \verbatim
*>          INCX is INTEGER
*>         storage spacing between elements of X
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \date December 2016
*
*> \ingroup double_blas_level1
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>  -- This version written on 25-October-1982.
*>     Modified on 14-October-1993 to inline the call to ZLASSQ.
*>     Sven Hammarling, Nag Ltd.
*> \endverbatim
*>
*  =====================================================================
      DOUBLE PRECISION FUNCTION DZNRM2(N,X,INCX)
*
*  -- Reference BLAS level1 routine (version 3.7.0) --
*  -- Reference BLAS is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     December 2016
*
*     .. Scalar Arguments ..
      INTEGER INCX,N
*     ..
*     .. Array Arguments ..
      COMPLEX*16 X(*)
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION ONE,ZERO
      PARAMETER (ONE=1.0D+0,ZERO=0.0D+0)
*     ..
*     .. Local Scalars ..
      DOUBLE PRECISION NORM,SCALE,SSQ,TEMP
      INTEGER IX
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC ABS,DBLE,DIMAG,SQRT
*     ..
      IF (N.LT.1 .OR. INCX.LT.1) THEN
          NORM = ZERO
      ELSE
          SCALE = ZERO
          SSQ = ONE
*        The following loop is equivalent to this call to the LAPACK
*        auxiliary routine:
*        CALL ZLASSQ( N, X, INCX, SCALE, SSQ )
*
          DO 10 IX = 1,1 + (N-1)*INCX,INCX
              IF (DBLE(X(IX)).NE.ZERO) THEN
                  TEMP = ABS(DBLE(X(IX)))
                  IF (SCALE.LT.TEMP) THEN
                      SSQ = ONE + SSQ* (SCALE/TEMP)**2
                      SCALE = TEMP
                  ELSE
                      SSQ = SSQ + (TEMP/SCALE)**2
                  END IF
              END IF
              IF (DIMAG(X(IX)).NE.ZERO) THEN
                  TEMP = ABS(DIMAG(X(IX)))
                  IF (SCALE.LT.TEMP) THEN
                      SSQ = ONE + SSQ* (SCALE/TEMP)**2
                      SCALE = TEMP
                  ELSE
                      SSQ = SSQ + (TEMP/SCALE)**2
                  END IF
              END IF
   10     CONTINUE
          NORM = SCALE*SQRT(SSQ)
      END IF
*
      DZNRM2 = NORM
      RETURN
*
*     End of DZNRM2.
*
      END