OpenBLAS/lapack-netlib/SRC/clartg.f

*> \brief \b CLARTG generates a plane rotation with real cosine and complex sine.
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
*> \htmlonly
*> Download CLARTG + dependencies
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*> [TGZ]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/clartg.f">
*> [ZIP]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/clartg.f">
*> [TXT]</a>
*> \endhtmlonly
*
*  Definition:
*  ===========
*
*       SUBROUTINE CLARTG( F, G, CS, SN, R )
*
*       .. Scalar Arguments ..
*       REAL               CS
*       COMPLEX            F, G, R, SN
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> CLARTG generates a plane rotation so that
*>
*>    [  CS  SN  ]     [ F ]     [ R ]
*>    [  __      ]  .  [   ]  =  [   ]   where CS**2 + |SN|**2 = 1.
*>    [ -SN  CS  ]     [ G ]     [ 0 ]
*>
*> This is a faster version of the BLAS1 routine CROTG, except for
*> the following differences:
*>    F and G are unchanged on return.
*>    If G=0, then CS=1 and SN=0.
*>    If F=0, then CS=0 and SN is chosen so that R is real.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] F
*> \verbatim
*>          F is COMPLEX
*>          The first component of vector to be rotated.
*> \endverbatim
*>
*> \param[in] G
*> \verbatim
*>          G is COMPLEX
*>          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 COMPLEX
*>          The sine of the rotation.
*> \endverbatim
*>
*> \param[out] R
*> \verbatim
*>          R is COMPLEX
*>          The nonzero component of the rotated vector.
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \date December 2016
*
*> \ingroup complexOTHERauxiliary
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>  3-5-96 - Modified with a new algorithm by W. Kahan and J. Demmel
*>
*>  This version has a few statements commented out for thread safety
*>  (machine parameters are computed on each entry). 10 feb 03, SJH.
*> \endverbatim
*>
*  =====================================================================
      SUBROUTINE CLARTG( F, G, CS, SN, R )
*
*  -- LAPACK auxiliary routine (version 3.7.0) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     December 2016
*
*     .. Scalar Arguments ..
      REAL               CS
      COMPLEX            F, G, R, SN
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               TWO, ONE, ZERO
      PARAMETER          ( TWO = 2.0E+0, ONE = 1.0E+0, ZERO = 0.0E+0 )
      COMPLEX            CZERO
      PARAMETER          ( CZERO = ( 0.0E+0, 0.0E+0 ) )
*     ..
*     .. Local Scalars ..
*     LOGICAL            FIRST
      INTEGER            COUNT, I
      REAL               D, DI, DR, EPS, F2, F2S, G2, G2S, SAFMIN,
     $                   SAFMN2, SAFMX2, SCALE
      COMPLEX            FF, FS, GS
*     ..
*     .. External Functions ..
      REAL               SLAMCH, SLAPY2
      LOGICAL            SISNAN
      EXTERNAL           SLAMCH, SLAPY2, SISNAN
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          ABS, AIMAG, CMPLX, CONJG, INT, LOG, MAX, REAL,
     $                   SQRT
*     ..
*     .. Statement Functions ..
      REAL               ABS1, ABSSQ
*     ..
*     .. Statement Function definitions ..
      ABS1( FF ) = MAX( ABS( REAL( FF ) ), ABS( AIMAG( FF ) ) )
      ABSSQ( FF ) = REAL( FF )**2 + AIMAG( FF )**2
*     ..
*     .. Executable Statements ..
*
      SAFMIN = SLAMCH( 'S' )
      EPS = SLAMCH( 'E' )
      SAFMN2 = SLAMCH( 'B' )**INT( LOG( SAFMIN / EPS ) /
     $         LOG( SLAMCH( 'B' ) ) / TWO )
      SAFMX2 = ONE / SAFMN2
      SCALE = MAX( ABS1( F ), ABS1( G ) )
      FS = F
      GS = G
      COUNT = 0
      IF( SCALE.GE.SAFMX2 ) THEN
   10    CONTINUE
         COUNT = COUNT + 1
         FS = FS*SAFMN2
         GS = GS*SAFMN2
         SCALE = SCALE*SAFMN2
         IF( SCALE.GE.SAFMX2 .AND. COUNT .LT. 20)
     $      GO TO 10
      ELSE IF( SCALE.LE.SAFMN2 ) THEN
         IF( G.EQ.CZERO.OR.SISNAN( ABS( G ) ) ) THEN
            CS = ONE
            SN = CZERO
            R = F
            RETURN
         END IF
   20    CONTINUE
         COUNT = COUNT - 1
         FS = FS*SAFMX2
         GS = GS*SAFMX2
         SCALE = SCALE*SAFMX2
         IF( SCALE.LE.SAFMN2 )
     $      GO TO 20
      END IF
      F2 = ABSSQ( FS )
      G2 = ABSSQ( GS )
      IF( F2.LE.MAX( G2, ONE )*SAFMIN ) THEN
*
*        This is a rare case: F is very small.
*
         IF( F.EQ.CZERO ) THEN
            CS = ZERO
            R = SLAPY2( REAL( G ), AIMAG( G ) )
*           Do complex/real division explicitly with two real divisions
            D = SLAPY2( REAL( GS ), AIMAG( GS ) )
            SN = CMPLX( REAL( GS ) / D, -AIMAG( GS ) / D )
            RETURN
         END IF
         F2S = SLAPY2( REAL( FS ), AIMAG( FS ) )
*        G2 and G2S are accurate
*        G2 is at least SAFMIN, and G2S is at least SAFMN2
         G2S = SQRT( G2 )
*        Error in CS from underflow in F2S is at most
*        UNFL / SAFMN2 .lt. sqrt(UNFL*EPS) .lt. EPS
*        If MAX(G2,ONE)=G2, then F2 .lt. G2*SAFMIN,
*        and so CS .lt. sqrt(SAFMIN)
*        If MAX(G2,ONE)=ONE, then F2 .lt. SAFMIN
*        and so CS .lt. sqrt(SAFMIN)/SAFMN2 = sqrt(EPS)
*        Therefore, CS = F2S/G2S / sqrt( 1 + (F2S/G2S)**2 ) = F2S/G2S
         CS = F2S / G2S
*        Make sure abs(FF) = 1
*        Do complex/real division explicitly with 2 real divisions
         IF( ABS1( F ).GT.ONE ) THEN
            D = SLAPY2( REAL( F ), AIMAG( F ) )
            FF = CMPLX( REAL( F ) / D, AIMAG( F ) / D )
         ELSE
            DR = SAFMX2*REAL( F )
            DI = SAFMX2*AIMAG( F )
            D = SLAPY2( DR, DI )
            FF = CMPLX( DR / D, DI / D )
         END IF
         SN = FF*CMPLX( REAL( GS ) / G2S, -AIMAG( GS ) / G2S )
         R = CS*F + SN*G
      ELSE
*
*        This is the most common case.
*        Neither F2 nor F2/G2 are less than SAFMIN
*        F2S cannot overflow, and it is accurate
*
         F2S = SQRT( ONE+G2 / F2 )
*        Do the F2S(real)*FS(complex) multiply with two real multiplies
         R = CMPLX( F2S*REAL( FS ), F2S*AIMAG( FS ) )
         CS = ONE / F2S
         D = F2 + G2
*        Do complex/real division explicitly with two real divisions
         SN = CMPLX( REAL( R ) / D, AIMAG( R ) / D )
         SN = SN*CONJG( GS )
         IF( COUNT.NE.0 ) THEN
            IF( COUNT.GT.0 ) THEN
               DO 30 I = 1, COUNT
                  R = R*SAFMX2
   30          CONTINUE
            ELSE
               DO 40 I = 1, -COUNT
                  R = R*SAFMN2
   40          CONTINUE
            END IF
         END IF
      END IF
      RETURN
*
*     End of CLARTG
*
      END