forked from lijiext/lammps
133 lines
3.0 KiB
Fortran
133 lines
3.0 KiB
Fortran
*> \brief \b DNRM2
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*
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* =========== DOCUMENTATION ===========
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*
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* Online html documentation available at
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* http://www.netlib.org/lapack/explore-html/
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*
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* Definition:
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* ===========
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*
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* DOUBLE PRECISION FUNCTION DNRM2(N,X,INCX)
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*
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* .. Scalar Arguments ..
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* INTEGER INCX,N
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* ..
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* .. Array Arguments ..
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* DOUBLE PRECISION X(*)
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* ..
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*
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*
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*> \par Purpose:
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* =============
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*>
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*> \verbatim
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*>
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*> DNRM2 returns the euclidean norm of a vector via the function
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*> name, so that
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*>
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*> DNRM2 := sqrt( x'*x )
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*> \endverbatim
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*
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* Arguments:
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* ==========
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*
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*> \param[in] N
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*> \verbatim
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*> N is INTEGER
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*> number of elements in input vector(s)
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*> \endverbatim
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*>
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*> \param[in] X
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*> \verbatim
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*> X is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCX ) )
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*> \endverbatim
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*>
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*> \param[in] INCX
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*> \verbatim
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*> INCX is INTEGER
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*> storage spacing between elements of DX
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*> \endverbatim
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*
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* Authors:
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* ========
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*
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*> \author Univ. of Tennessee
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*> \author Univ. of California Berkeley
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*> \author Univ. of Colorado Denver
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*> \author NAG Ltd.
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*
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*> \date December 2016
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*
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*> \ingroup double_blas_level1
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*
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*> \par Further Details:
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* =====================
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*>
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*> \verbatim
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*>
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*> -- This version written on 25-October-1982.
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*> Modified on 14-October-1993 to inline the call to DLASSQ.
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*> Sven Hammarling, Nag Ltd.
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*> \endverbatim
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*>
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* =====================================================================
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DOUBLE PRECISION FUNCTION DNRM2(N,X,INCX)
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*
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* -- Reference BLAS level1 routine (version 3.7.0) --
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* -- Reference BLAS is a software package provided by Univ. of Tennessee, --
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* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
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* December 2016
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*
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* .. Scalar Arguments ..
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INTEGER INCX,N
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* ..
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* .. Array Arguments ..
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DOUBLE PRECISION X(*)
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* ..
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*
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* =====================================================================
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*
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* .. Parameters ..
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DOUBLE PRECISION ONE,ZERO
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PARAMETER (ONE=1.0D+0,ZERO=0.0D+0)
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* ..
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* .. Local Scalars ..
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DOUBLE PRECISION ABSXI,NORM,SCALE,SSQ
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INTEGER IX
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC ABS,SQRT
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* ..
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IF (N.LT.1 .OR. INCX.LT.1) THEN
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NORM = ZERO
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ELSE IF (N.EQ.1) THEN
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NORM = ABS(X(1))
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ELSE
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SCALE = ZERO
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SSQ = ONE
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* The following loop is equivalent to this call to the LAPACK
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* auxiliary routine:
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* CALL DLASSQ( N, X, INCX, SCALE, SSQ )
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*
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DO 10 IX = 1,1 + (N-1)*INCX,INCX
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IF (X(IX).NE.ZERO) THEN
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ABSXI = ABS(X(IX))
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IF (SCALE.LT.ABSXI) THEN
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SSQ = ONE + SSQ* (SCALE/ABSXI)**2
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SCALE = ABSXI
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ELSE
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SSQ = SSQ + (ABSXI/SCALE)**2
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END IF
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END IF
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10 CONTINUE
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NORM = SCALE*SQRT(SSQ)
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END IF
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*
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DNRM2 = NORM
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RETURN
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*
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* End of DNRM2.
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*
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END
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