srotg.f

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*> \brief \b SROTG
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at 
*            http://www.netlib.org/lapack/explore-html/ 
*
*  Definition:
*  ===========
*
*       SUBROUTINE SROTG(SA,SB,C,S)
* 
*       .. Scalar Arguments ..
*       REAL C,S,SA,SB
*       ..
*  
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*>    SROTG 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 single_blas_level1
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>     jack dongarra, linpack, 3/11/78.
*> \endverbatim
*>
*  =====================================================================
      SUBROUTINE srotg(SA,SB,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 ..
      REAL c,s,sa,sb
*     ..
*
*  =====================================================================
*
*     .. Local Scalars ..
      REAL r,roe,scale,z
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC abs,sign,sqrt
*     ..
      roe = sb
      IF (abs(sa).GT.abs(sb)) roe = sa
      scale = abs(sa) + abs(sb)
      IF (scale.EQ.0.0) THEN
         c = 1.0
         s = 0.0
         r = 0.0
         z = 0.0
      ELSE
         r = scale*sqrt((sa/scale)**2+ (sb/scale)**2)
         r = sign(1.0,roe)*r
         c = sa/r
         s = sb/r
         z = 1.0
         IF (abs(sa).GT.abs(sb)) z = s
         IF (abs(sb).GE.abs(sa) .AND. c.NE.0.0) z = 1.0/c
      END IF
      sa = r
      sb = z
      return
      END