dladiv.f

*> \brief \b DLADIV performs complex division in real arithmetic, avoiding unnecessary overflow.
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
*> \htmlonly
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*> [TXT]</a>
*> \endhtmlonly
*
*  Definition:
*  ===========
*
*       SUBROUTINE DLADIV( A, B, C, D, P, Q )
*
*       .. Scalar Arguments ..
*       DOUBLE PRECISION   A, B, C, D, P, Q
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> DLADIV performs complex division in  real arithmetic
*>
*>                       a + i*b
*>            p + i*q = ---------
*>                       c + i*d
*>
*> The algorithm is due to Michael Baudin and Robert L. Smith
*> and can be found in the paper
*> "A Robust Complex Division in Scilab"
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] A
*> \verbatim
*>          A is DOUBLE PRECISION
*> \endverbatim
*>
*> \param[in] B
*> \verbatim
*>          B is DOUBLE PRECISION
*> \endverbatim
*>
*> \param[in] C
*> \verbatim
*>          C is DOUBLE PRECISION
*> \endverbatim
*>
*> \param[in] D
*> \verbatim
*>          D is DOUBLE PRECISION
*>          The scalars a, b, c, and d in the above expression.
*> \endverbatim
*>
*> \param[out] P
*> \verbatim
*>          P is DOUBLE PRECISION
*> \endverbatim
*>
*> \param[out] Q
*> \verbatim
*>          Q is DOUBLE PRECISION
*>          The scalars p and q in the above expression.
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup ladiv
*
*  =====================================================================
      SUBROUTINE dladiv( A, B, C, D, P, Q )
*
*  -- 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 ..
      DOUBLE PRECISION   A, B, C, D, P, Q
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   BS
      parameter( bs = 2.0d0 )
      DOUBLE PRECISION   HALF
      parameter( half = 0.5d0 )
      DOUBLE PRECISION   TWO
      parameter( two = 2.0d0 )
*
*     .. Local Scalars ..
      DOUBLE PRECISION   AA, BB, CC, DD, AB, CD, S, OV, UN, BE, EPS
*     ..
*     .. External Functions ..
      DOUBLE PRECISION   DLAMCH
      EXTERNAL           dlamch
*     ..
*     .. External Subroutines ..
      EXTERNAL           dladiv1
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs, max
*     ..
*     .. Executable Statements ..
*
      aa = a
      bb = b
      cc = c
      dd = d
      ab = max( abs(a), abs(b) )
      cd = max( abs(c), abs(d) )
      s = 1.0d0

      ov = dlamch( 'Overflow threshold' )
      un = dlamch( 'Safe minimum' )
      eps = dlamch( 'Epsilon' )
      be = bs / (eps*eps)

      IF( ab >= half*ov ) THEN
         aa = half * aa
         bb = half * bb
         s  = two * s
      END IF
      IF( cd >= half*ov ) THEN
         cc = half * cc
         dd = half * dd
         s  = half * s
      END IF
      IF( ab <= un*bs/eps ) THEN
         aa = aa * be
         bb = bb * be
         s  = s / be
      END IF
      IF( cd <= un*bs/eps ) THEN
         cc = cc * be
         dd = dd * be
         s  = s * be
      END IF
      IF( abs( d ).LE.abs( c ) ) THEN
         CALL dladiv1(aa, bb, cc, dd, p, q)
      ELSE
         CALL dladiv1(bb, aa, dd, cc, p, q)
         q = -q
      END IF
      p = p * s
      q = q * s
*
      RETURN
*
*     End of DLADIV
*
      END

*> \ingroup ladiv


      SUBROUTINE dladiv1( A, B, C, D, P, Q )
*
*  -- 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 ..
      DOUBLE PRECISION   A, B, C, D, P, Q
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   ONE
      parameter( one = 1.0d0 )
*
*     .. Local Scalars ..
      DOUBLE PRECISION   R, T
*     ..
*     .. External Functions ..
      DOUBLE PRECISION   DLADIV2
      EXTERNAL           dladiv2
*     ..
*     .. Executable Statements ..
*
      r = d / c
      t = one / (c + d * r)
      p = dladiv2(a, b, c, d, r, t)
      a = -a
      q = dladiv2(b, a, c, d, r, t)
*
      RETURN
*
*     End of DLADIV1
*
      END

*> \ingroup ladiv

      DOUBLE PRECISION FUNCTION dladiv2( A, B, C, D, R, T )
*
*  -- 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 ..
      DOUBLE PRECISION   A, B, C, D, R, T
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   ZERO
      parameter( zero = 0.0d0 )
*
*     .. Local Scalars ..
      DOUBLE PRECISION   BR
*     ..
*     .. Executable Statements ..
*
      IF( r.NE.zero ) THEN
         br = b * r
         IF( br.NE.zero ) THEN
            dladiv2 = (a + br) * t
         ELSE
            dladiv2 = a * t + (b * t) * r
         END IF
      ELSE
         dladiv2 = (a + d * (b / c)) * t
      END IF
*
      RETURN
*
*     End of DLADIV2
*
      END