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278 lines
7.6 KiB
278 lines
7.6 KiB
2 years ago
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!> \brief \b ZROTG generates a Givens rotation with real cosine and complex sine.
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!
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! =========== DOCUMENTATION ===========
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!
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! Online html documentation available at
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! http://www.netlib.org/lapack/explore-html/
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!
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! Definition:
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! ===========
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!
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! ZROTG constructs a plane rotation
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! [ c s ] [ a ] = [ r ]
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! [ -conjg(s) c ] [ b ] [ 0 ]
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! where c is real, s is complex, and c**2 + conjg(s)*s = 1.
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!
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!> \par Purpose:
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! =============
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!>
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!> \verbatim
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!>
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!> The computation uses the formulas
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!> |x| = sqrt( Re(x)**2 + Im(x)**2 )
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!> sgn(x) = x / |x| if x /= 0
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!> = 1 if x = 0
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!> c = |a| / sqrt(|a|**2 + |b|**2)
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!> s = sgn(a) * conjg(b) / sqrt(|a|**2 + |b|**2)
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!> r = sgn(a)*sqrt(|a|**2 + |b|**2)
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!> When a and b are real and r /= 0, the formulas simplify to
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!> c = a / r
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!> s = b / r
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!> the same as in DROTG when |a| > |b|. When |b| >= |a|, the
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!> sign of c and s will be different from those computed by DROTG
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!> if the signs of a and b are not the same.
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!>
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!> \endverbatim
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!
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! Arguments:
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! ==========
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!
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!> \param[in,out] A
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!> \verbatim
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!> A is DOUBLE COMPLEX
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!> On entry, the scalar a.
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!> On exit, the scalar r.
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!> \endverbatim
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!>
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!> \param[in] B
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!> \verbatim
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!> B is DOUBLE COMPLEX
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!> The scalar b.
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!> \endverbatim
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!>
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!> \param[out] C
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!> \verbatim
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!> C is DOUBLE PRECISION
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!> The scalar c.
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!> \endverbatim
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!>
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!> \param[out] S
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!> \verbatim
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!> S is DOUBLE COMPLEX
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!> The scalar s.
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!> \endverbatim
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!
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! Authors:
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! ========
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!
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!> \author Weslley Pereira, University of Colorado Denver, USA
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!
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!> \date December 2021
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!
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!> \ingroup single_blas_level1
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!
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!> \par Further Details:
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! =====================
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!>
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!> \verbatim
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!>
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!> Based on the algorithm from
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!>
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!> Anderson E. (2017)
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!> Algorithm 978: Safe Scaling in the Level 1 BLAS
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!> ACM Trans Math Softw 44:1--28
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!> https://doi.org/10.1145/3061665
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!>
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!> \endverbatim
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!
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! =====================================================================
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subroutine ZROTG( a, b, c, s )
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integer, parameter :: wp = kind(1.d0)
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!
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! -- Reference BLAS level1 routine --
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! -- Reference BLAS is a software package provided by Univ. of Tennessee, --
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! -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
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!
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! .. Constants ..
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real(wp), parameter :: zero = 0.0_wp
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real(wp), parameter :: one = 1.0_wp
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complex(wp), parameter :: czero = 0.0_wp
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! ..
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! .. Scaling constants ..
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real(wp), parameter :: safmin = real(radix(0._wp),wp)**max( &
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minexponent(0._wp)-1, &
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1-maxexponent(0._wp) &
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)
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real(wp), parameter :: safmax = real(radix(0._wp),wp)**max( &
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1-minexponent(0._wp), &
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maxexponent(0._wp)-1 &
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)
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real(wp), parameter :: rtmin = sqrt( safmin )
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! ..
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! .. Scalar Arguments ..
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real(wp) :: c
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complex(wp) :: a, b, s
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! ..
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! .. Local Scalars ..
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real(wp) :: d, f1, f2, g1, g2, h2, u, v, w, rtmax
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complex(wp) :: f, fs, g, gs, r, t
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! ..
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! .. Intrinsic Functions ..
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intrinsic :: abs, aimag, conjg, max, min, real, sqrt
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! ..
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! .. Statement Functions ..
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real(wp) :: ABSSQ
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! ..
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! .. Statement Function definitions ..
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ABSSQ( t ) = real( t )**2 + aimag( t )**2
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! ..
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! .. Executable Statements ..
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!
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f = a
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g = b
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if( g == czero ) then
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c = one
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s = czero
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r = f
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else if( f == czero ) then
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c = zero
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if( real(g) == zero ) then
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r = abs(aimag(g))
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s = conjg( g ) / r
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elseif( aimag(g) == zero ) then
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r = abs(real(g))
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s = conjg( g ) / r
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else
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g1 = max( abs(real(g)), abs(aimag(g)) )
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rtmax = sqrt( safmax/2 )
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if( g1 > rtmin .and. g1 < rtmax ) then
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!
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! Use unscaled algorithm
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!
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! The following two lines can be replaced by `d = abs( g )`.
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! This algorithm do not use the intrinsic complex abs.
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g2 = ABSSQ( g )
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d = sqrt( g2 )
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s = conjg( g ) / d
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r = d
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else
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!
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! Use scaled algorithm
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!
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u = min( safmax, max( safmin, g1 ) )
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gs = g / u
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! The following two lines can be replaced by `d = abs( gs )`.
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! This algorithm do not use the intrinsic complex abs.
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g2 = ABSSQ( gs )
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d = sqrt( g2 )
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s = conjg( gs ) / d
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r = d*u
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end if
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end if
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else
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f1 = max( abs(real(f)), abs(aimag(f)) )
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g1 = max( abs(real(g)), abs(aimag(g)) )
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rtmax = sqrt( safmax/4 )
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if( f1 > rtmin .and. f1 < rtmax .and. &
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g1 > rtmin .and. g1 < rtmax ) then
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!
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! Use unscaled algorithm
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!
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f2 = ABSSQ( f )
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g2 = ABSSQ( g )
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h2 = f2 + g2
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! safmin <= f2 <= h2 <= safmax
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if( f2 >= h2 * safmin ) then
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! safmin <= f2/h2 <= 1, and h2/f2 is finite
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c = sqrt( f2 / h2 )
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r = f / c
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rtmax = rtmax * 2
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if( f2 > rtmin .and. h2 < rtmax ) then
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! safmin <= sqrt( f2*h2 ) <= safmax
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s = conjg( g ) * ( f / sqrt( f2*h2 ) )
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else
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s = conjg( g ) * ( r / h2 )
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end if
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else
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! f2/h2 <= safmin may be subnormal, and h2/f2 may overflow.
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! Moreover,
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! safmin <= f2*f2 * safmax < f2 * h2 < h2*h2 * safmin <= safmax,
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! sqrt(safmin) <= sqrt(f2 * h2) <= sqrt(safmax).
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! Also,
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! g2 >> f2, which means that h2 = g2.
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d = sqrt( f2 * h2 )
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c = f2 / d
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if( c >= safmin ) then
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r = f / c
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else
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! f2 / sqrt(f2 * h2) < safmin, then
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! sqrt(safmin) <= f2 * sqrt(safmax) <= h2 / sqrt(f2 * h2) <= h2 * (safmin / f2) <= h2 <= safmax
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r = f * ( h2 / d )
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end if
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s = conjg( g ) * ( f / d )
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end if
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else
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!
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! Use scaled algorithm
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!
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u = min( safmax, max( safmin, f1, g1 ) )
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gs = g / u
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g2 = ABSSQ( gs )
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if( f1 / u < rtmin ) then
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!
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! f is not well-scaled when scaled by g1.
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! Use a different scaling for f.
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!
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v = min( safmax, max( safmin, f1 ) )
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w = v / u
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fs = f / v
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f2 = ABSSQ( fs )
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h2 = f2*w**2 + g2
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else
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!
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! Otherwise use the same scaling for f and g.
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!
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w = one
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fs = f / u
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f2 = ABSSQ( fs )
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h2 = f2 + g2
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end if
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! safmin <= f2 <= h2 <= safmax
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if( f2 >= h2 * safmin ) then
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! safmin <= f2/h2 <= 1, and h2/f2 is finite
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c = sqrt( f2 / h2 )
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r = fs / c
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rtmax = rtmax * 2
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if( f2 > rtmin .and. h2 < rtmax ) then
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! safmin <= sqrt( f2*h2 ) <= safmax
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s = conjg( gs ) * ( fs / sqrt( f2*h2 ) )
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else
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s = conjg( gs ) * ( r / h2 )
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end if
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else
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! f2/h2 <= safmin may be subnormal, and h2/f2 may overflow.
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! Moreover,
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! safmin <= f2*f2 * safmax < f2 * h2 < h2*h2 * safmin <= safmax,
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! sqrt(safmin) <= sqrt(f2 * h2) <= sqrt(safmax).
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! Also,
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! g2 >> f2, which means that h2 = g2.
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d = sqrt( f2 * h2 )
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c = f2 / d
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if( c >= safmin ) then
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r = fs / c
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else
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! f2 / sqrt(f2 * h2) < safmin, then
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! sqrt(safmin) <= f2 * sqrt(safmax) <= h2 / sqrt(f2 * h2) <= h2 * (safmin / f2) <= h2 <= safmax
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r = fs * ( h2 / d )
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end if
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s = conjg( gs ) * ( fs / d )
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end if
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! Rescale c and r
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c = c * w
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r = r * u
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end if
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end if
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a = r
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return
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end subroutine
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