slartgp.f

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*> \brief \b SLARTGP generates a plane rotation so that the diagonal is nonnegative.
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
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*> [TXT]</a>
*
*  Definition:
*  ===========
*
*       SUBROUTINE SLARTGP( F, G, CS, SN, R )
*
*       .. Scalar Arguments ..
*       REAL               CS, F, G, R, SN
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> SLARTGP generates a plane rotation so that
*>
*>    [  CS  SN  ]  .  [ F ]  =  [ R ]   where CS**2 + SN**2 = 1.
*>    [ -SN  CS  ]     [ G ]     [ 0 ]
*>
*> This is a slower, more accurate version of the Level 1 BLAS routine SROTG,
*> with the following other differences:
*>    F and G are unchanged on return.
*>    If G=0, then CS=(+/-)1 and SN=0.
*>    If F=0 and (G .ne. 0), then CS=0 and SN=(+/-)1.
*>
*> The sign is chosen so that R >= 0.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] F
*> \verbatim
*>          F is REAL
*>          The first component of vector to be rotated.
*> \endverbatim
*>
*> \param[in] G
*> \verbatim
*>          G is REAL
*>          The second component of vector to be rotated.
*> \endverbatim
*>
*> \param[out] CS
*> \verbatim
*>          CS is REAL
*>          The cosine of the rotation.
*> \endverbatim
*>
*> \param[out] SN
*> \verbatim
*>          SN is REAL
*>          The sine of the rotation.
*> \endverbatim
*>
*> \param[out] R
*> \verbatim
*>          R is REAL
*>          The nonzero component of the rotated vector.
*>
*>  This version has a few statements commented out for thread safety
*>  (machine parameters are computed on each entry). 10 feb 03, SJH.
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup lartgp
*
*  =====================================================================
      SUBROUTINE slartgp( F, G, CS, SN, R )
*
*  -- LAPACK auxiliary routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      REAL               CS, F, G, R, SN
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO
      parameter( zero = 0.0e0 )
      REAL               ONE
      parameter( one = 1.0e0 )
      REAL               TWO
      parameter( two = 2.0e0 )
*     ..
*     .. Local Scalars ..
*     LOGICAL            FIRST
      INTEGER            COUNT, I
      REAL               EPS, F1, G1, SAFMIN, SAFMN2, SAFMX2, SCALE
*     ..
*     .. External Functions ..
      REAL               SLAMCH
      EXTERNAL           slamch
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs, int, log, max, sign, sqrt
*     ..
*     .. Save statement ..
*     SAVE               FIRST, SAFMX2, SAFMIN, SAFMN2
*     ..
*     .. Data statements ..
*     DATA               FIRST / .TRUE. /
*     ..
*     .. Executable Statements ..
*
*     IF( FIRST ) THEN
         safmin = slamch( 'S' )
         eps = slamch( 'E' )
         safmn2 = slamch( 'B' )**int( log( safmin / eps ) /
     $            log( slamch( 'B' ) ) / two )
         safmx2 = one / safmn2
*        FIRST = .FALSE.
*     END IF
      IF( g.EQ.zero ) THEN
         cs = sign( one, f )
         sn = zero
         r = abs( f )
      ELSE IF( f.EQ.zero ) THEN
         cs = zero
         sn = sign( one, g )
         r = abs( g )
      ELSE
         f1 = f
         g1 = g
         scale = max( abs( f1 ), abs( g1 ) )
         IF( scale.GE.safmx2 ) THEN
            count = 0
   10       CONTINUE
            count = count + 1
            f1 = f1*safmn2
            g1 = g1*safmn2
            scale = max( abs( f1 ), abs( g1 ) )
            IF( scale.GE.safmx2 .AND. count .LT. 20)
     $         GO TO 10
            r = sqrt( f1**2+g1**2 )
            cs = f1 / r
            sn = g1 / r
            DO 20 i = 1, count
               r = r*safmx2
   20       CONTINUE
         ELSE IF( scale.LE.safmn2 ) THEN
            count = 0
   30       CONTINUE
            count = count + 1
            f1 = f1*safmx2
            g1 = g1*safmx2
            scale = max( abs( f1 ), abs( g1 ) )
            IF( scale.LE.safmn2 )
     $         GO TO 30
            r = sqrt( f1**2+g1**2 )
            cs = f1 / r
            sn = g1 / r
            DO 40 i = 1, count
               r = r*safmn2
   40       CONTINUE
         ELSE
            r = sqrt( f1**2+g1**2 )
            cs = f1 / r
            sn = g1 / r
         END IF
         IF( r.LT.zero ) THEN
            cs = -cs
            sn = -sn
            r = -r
         END IF
      END IF
      RETURN
*
*     End of SLARTGP
*
      END