Annotation of rpl/lapack/lapack/dlartg.f90, revision 1.1
1.1 ! bertrand 1: !> \brief \b DLARTG generates a plane rotation with real cosine and real sine.
! 2: !
! 3: ! =========== DOCUMENTATION ===========
! 4: !
! 5: ! Online html documentation available at
! 6: ! http://www.netlib.org/lapack/explore-html/
! 7: !
! 8: ! Definition:
! 9: ! ===========
! 10: !
! 11: ! SUBROUTINE DLARTG( F, G, C, S, R )
! 12: !
! 13: ! .. Scalar Arguments ..
! 14: ! REAL(wp) C, F, G, R, S
! 15: ! ..
! 16: !
! 17: !> \par Purpose:
! 18: ! =============
! 19: !>
! 20: !> \verbatim
! 21: !>
! 22: !> DLARTG generates a plane rotation so that
! 23: !>
! 24: !> [ C S ] . [ F ] = [ R ]
! 25: !> [ -S C ] [ G ] [ 0 ]
! 26: !>
! 27: !> where C**2 + S**2 = 1.
! 28: !>
! 29: !> The mathematical formulas used for C and S are
! 30: !> R = sign(F) * sqrt(F**2 + G**2)
! 31: !> C = F / R
! 32: !> S = G / R
! 33: !> Hence C >= 0. The algorithm used to compute these quantities
! 34: !> incorporates scaling to avoid overflow or underflow in computing the
! 35: !> square root of the sum of squares.
! 36: !>
! 37: !> This version is discontinuous in R at F = 0 but it returns the same
! 38: !> C and S as ZLARTG for complex inputs (F,0) and (G,0).
! 39: !>
! 40: !> This is a more accurate version of the BLAS1 routine DROTG,
! 41: !> with the following other differences:
! 42: !> F and G are unchanged on return.
! 43: !> If G=0, then C=1 and S=0.
! 44: !> If F=0 and (G .ne. 0), then C=0 and S=sign(1,G) without doing any
! 45: !> floating point operations (saves work in DBDSQR when
! 46: !> there are zeros on the diagonal).
! 47: !>
! 48: !> Below, wp=>dp stands for double precision from LA_CONSTANTS module.
! 49: !> \endverbatim
! 50: !
! 51: ! Arguments:
! 52: ! ==========
! 53: !
! 54: !> \param[in] F
! 55: !> \verbatim
! 56: !> F is REAL(wp)
! 57: !> The first component of vector to be rotated.
! 58: !> \endverbatim
! 59: !>
! 60: !> \param[in] G
! 61: !> \verbatim
! 62: !> G is REAL(wp)
! 63: !> The second component of vector to be rotated.
! 64: !> \endverbatim
! 65: !>
! 66: !> \param[out] C
! 67: !> \verbatim
! 68: !> C is REAL(wp)
! 69: !> The cosine of the rotation.
! 70: !> \endverbatim
! 71: !>
! 72: !> \param[out] S
! 73: !> \verbatim
! 74: !> S is REAL(wp)
! 75: !> The sine of the rotation.
! 76: !> \endverbatim
! 77: !>
! 78: !> \param[out] R
! 79: !> \verbatim
! 80: !> R is REAL(wp)
! 81: !> The nonzero component of the rotated vector.
! 82: !> \endverbatim
! 83: !
! 84: ! Authors:
! 85: ! ========
! 86: !
! 87: !> \author Edward Anderson, Lockheed Martin
! 88: !
! 89: !> \date July 2016
! 90: !
! 91: !> \ingroup OTHERauxiliary
! 92: !
! 93: !> \par Contributors:
! 94: ! ==================
! 95: !>
! 96: !> Weslley Pereira, University of Colorado Denver, USA
! 97: !
! 98: !> \par Further Details:
! 99: ! =====================
! 100: !>
! 101: !> \verbatim
! 102: !>
! 103: !> Anderson E. (2017)
! 104: !> Algorithm 978: Safe Scaling in the Level 1 BLAS
! 105: !> ACM Trans Math Softw 44:1--28
! 106: !> https://doi.org/10.1145/3061665
! 107: !>
! 108: !> \endverbatim
! 109: !
! 110: subroutine DLARTG( f, g, c, s, r )
! 111: use LA_CONSTANTS, &
! 112: only: wp=>dp, zero=>dzero, half=>dhalf, one=>done, &
! 113: safmin=>dsafmin, safmax=>dsafmax
! 114: !
! 115: ! -- LAPACK auxiliary routine --
! 116: ! -- LAPACK is a software package provided by Univ. of Tennessee, --
! 117: ! -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
! 118: ! February 2021
! 119: !
! 120: ! .. Scalar Arguments ..
! 121: real(wp) :: c, f, g, r, s
! 122: ! ..
! 123: ! .. Local Scalars ..
! 124: real(wp) :: d, f1, fs, g1, gs, u, rtmin, rtmax
! 125: ! ..
! 126: ! .. Intrinsic Functions ..
! 127: intrinsic :: abs, sign, sqrt
! 128: ! ..
! 129: ! .. Constants ..
! 130: rtmin = sqrt( safmin )
! 131: rtmax = sqrt( safmax/2 )
! 132: ! ..
! 133: ! .. Executable Statements ..
! 134: !
! 135: f1 = abs( f )
! 136: g1 = abs( g )
! 137: if( g == zero ) then
! 138: c = one
! 139: s = zero
! 140: r = f
! 141: else if( f == zero ) then
! 142: c = zero
! 143: s = sign( one, g )
! 144: r = g1
! 145: else if( f1 > rtmin .and. f1 < rtmax .and. &
! 146: g1 > rtmin .and. g1 < rtmax ) then
! 147: d = sqrt( f*f + g*g )
! 148: c = f1 / d
! 149: r = sign( d, f )
! 150: s = g / r
! 151: else
! 152: u = min( safmax, max( safmin, f1, g1 ) )
! 153: fs = f / u
! 154: gs = g / u
! 155: d = sqrt( fs*fs + gs*gs )
! 156: c = abs( fs ) / d
! 157: r = sign( d, f )
! 158: s = gs / r
! 159: r = r*u
! 160: end if
! 161: return
! 162: end subroutine
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