drotg.f

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*> \brief \b DROTG
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at 
*            http://www.netlib.org/lapack/explore-html/ 
*
*  Definition:
*  ===========
*
*       SUBROUTINE DROTG(DA,DB,C,S)
* 
*       .. Scalar Arguments ..
*       DOUBLE PRECISION C,DA,DB,S
*       ..
*  
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*>    DROTG construct givens plane rotation.
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee 
*> \author Univ. of California Berkeley 
*> \author Univ. of Colorado Denver 
*> \author NAG Ltd. 
*
*> \date November 2011
*
*> \ingroup double_blas_level1
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>     jack dongarra, linpack, 3/11/78.
*> \endverbatim
*>
*  =====================================================================
      SUBROUTINE drotg(DA,DB,C,S)
*
*  -- Reference BLAS level1 routine (version 3.4.0) --
*  -- Reference BLAS is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2011
*
*     .. Scalar Arguments ..
      DOUBLE PRECISION C,DA,DB,S
*     ..
*
*  =====================================================================
*
*     .. Local Scalars ..
      DOUBLE PRECISION R,ROE,SCALE,Z
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC dabs,dsign,dsqrt
*     ..
      roe = db
      IF (dabs(da).GT.dabs(db)) roe = da
      scale = dabs(da) + dabs(db)
      IF (scale.EQ.0.0d0) THEN
         c = 1.0d0
         s = 0.0d0
         r = 0.0d0
         z = 0.0d0
      ELSE
         r = scale*dsqrt((da/scale)**2+ (db/scale)**2)
         r = dsign(1.0d0,roe)*r
         c = da/r
         s = db/r
         z = 1.0d0
         IF (dabs(da).GT.dabs(db)) z = s
         IF (dabs(db).GE.dabs(da) .AND. c.NE.0.0d0) z = 1.0d0/c
      END IF
      da = r
      db = z
      RETURN
      END