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| 1 | +!> \brief \b DNRM2 |
| 2 | +! |
| 3 | +! =========== DOCUMENTATION =========== |
| 4 | +! |
| 5 | +! Online html documentation available at |
| 6 | +! http://www.netlib.org/lapack/explore-html/ |
| 7 | +! |
| 8 | +! Definition: |
| 9 | +! =========== |
| 10 | +! |
| 11 | +! DOUBLE PRECISION FUNCTION DNRM2(N,X,INCX) |
| 12 | +! |
| 13 | +! .. Scalar Arguments .. |
| 14 | +! INTEGER INCX,N |
| 15 | +! .. |
| 16 | +! .. Array Arguments .. |
| 17 | +! DOUBLE PRECISION X(*) |
| 18 | +! .. |
| 19 | +! |
| 20 | +! |
| 21 | +!> \par Purpose: |
| 22 | +! ============= |
| 23 | +!> |
| 24 | +!> \verbatim |
| 25 | +!> |
| 26 | +!> DNRM2 returns the euclidean norm of a vector via the function |
| 27 | +!> name, so that |
| 28 | +!> |
| 29 | +!> DNRM2 := sqrt( x'*x ) |
| 30 | +!> \endverbatim |
| 31 | +! |
| 32 | +! Arguments: |
| 33 | +! ========== |
| 34 | +! |
| 35 | +!> \param[in] N |
| 36 | +!> \verbatim |
| 37 | +!> N is INTEGER |
| 38 | +!> number of elements in input vector(s) |
| 39 | +!> \endverbatim |
| 40 | +!> |
| 41 | +!> \param[in] X |
| 42 | +!> \verbatim |
| 43 | +!> X is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCX ) ) |
| 44 | +!> \endverbatim |
| 45 | +!> |
| 46 | +!> \param[in] INCX |
| 47 | +!> \verbatim |
| 48 | +!> INCX is INTEGER, storage spacing between elements of X |
| 49 | +!> If INCX > 0, X(1+(i-1)*INCX) = x(i) for 1 <= i <= n |
| 50 | +!> If INCX < 0, X(1-(n-i)*INCX) = x(i) for 1 <= i <= n |
| 51 | +!> If INCX = 0, x isn't a vector so there is no need to call |
| 52 | +!> this subroutine. If you call it anyway, it will count x(1) |
| 53 | +!> in the vector norm N times. |
| 54 | +!> \endverbatim |
| 55 | +! |
| 56 | +! Authors: |
| 57 | +! ======== |
| 58 | +! |
| 59 | +!> \author Edward Anderson, Lockheed Martin |
| 60 | +! |
| 61 | +!> \date August 2016 |
| 62 | +! |
| 63 | +!> \ingroup single_blas_level1 |
| 64 | +! |
| 65 | +!> \par Contributors: |
| 66 | +! ================== |
| 67 | +!> |
| 68 | +!> Weslley Pereira, University of Colorado Denver, USA |
| 69 | +! |
| 70 | +!> \par Further Details: |
| 71 | +! ===================== |
| 72 | +!> |
| 73 | +!> \verbatim |
| 74 | +!> |
| 75 | +!> Anderson E. (2017) |
| 76 | +!> Algorithm 978: Safe Scaling in the Level 1 BLAS |
| 77 | +!> ACM Trans Math Softw 44:1--28 |
| 78 | +!> https://doi.org/10.1145/3061665 |
| 79 | +!> |
| 80 | +!> Blue, James L. (1978) |
| 81 | +!> A Portable Fortran Program to Find the Euclidean Norm of a Vector |
| 82 | +!> ACM Trans Math Softw 4:15--23 |
| 83 | +!> https://doi.org/10.1145/355769.355771 |
| 84 | +!> |
| 85 | +!> \endverbatim |
| 86 | +!> |
| 87 | +! ===================================================================== |
| 88 | +function DNRM2( n, x, incx ) |
| 89 | + integer, parameter :: wp = kind(1.d0) |
| 90 | + real(wp) :: DNRM2 |
| 91 | +! |
| 92 | +! -- Reference BLAS level1 routine (version 3.9.1) -- |
| 93 | +! -- Reference BLAS is a software package provided by Univ. of Tennessee, -- |
| 94 | +! -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- |
| 95 | +! March 2021 |
| 96 | +! |
| 97 | +! .. Constants .. |
| 98 | + real(wp), parameter :: zero = 0.0_wp |
| 99 | + real(wp), parameter :: one = 1.0_wp |
| 100 | + real(wp), parameter :: maxN = huge(0.0_wp) |
| 101 | +! .. |
| 102 | +! .. Blue's ccaling constants .. |
| 103 | + real(wp), parameter :: tsml = real(radix(0._wp), wp)**ceiling( & |
| 104 | + (minexponent(0._wp) - 1) * 0.5_wp) |
| 105 | + real(wp), parameter :: tbig = real(radix(0._wp), wp)**floor( & |
| 106 | + (maxexponent(0._wp) - digits(0._wp) + 1) * 0.5_wp) |
| 107 | + real(wp), parameter :: ssml = real(radix(0._wp), wp)**( - floor( & |
| 108 | + (minexponent(0._wp) - 1) * 0.5_wp)) |
| 109 | + real(wp), parameter :: sbig = real(radix(0._wp), wp)**( - ceiling( & |
| 110 | + (maxexponent(0._wp) - digits(0._wp) + 1) * 0.5_wp)) |
| 111 | +! .. |
| 112 | +! .. Scalar Arguments .. |
| 113 | + integer :: incx, n |
| 114 | +! .. |
| 115 | +! .. Array Arguments .. |
| 116 | + real(wp) :: x(*) |
| 117 | +! .. |
| 118 | +! .. Local Scalars .. |
| 119 | + integer :: i, ix |
| 120 | + logical :: notbig |
| 121 | + real(wp) :: abig, amed, asml, ax, scl, sumsq, ymax, ymin |
| 122 | +! |
| 123 | +! Quick return if possible |
| 124 | +! |
| 125 | + DNRM2 = zero |
| 126 | + if( n <= 0 ) return |
| 127 | +! |
| 128 | + scl = one |
| 129 | + sumsq = zero |
| 130 | +! |
| 131 | +! Compute the sum of squares in 3 accumulators: |
| 132 | +! abig -- sums of squares scaled down to avoid overflow |
| 133 | +! asml -- sums of squares scaled up to avoid underflow |
| 134 | +! amed -- sums of squares that do not require scaling |
| 135 | +! The thresholds and multipliers are |
| 136 | +! tbig -- values bigger than this are scaled down by sbig |
| 137 | +! tsml -- values smaller than this are scaled up by ssml |
| 138 | +! |
| 139 | + notbig = .true. |
| 140 | + asml = zero |
| 141 | + amed = zero |
| 142 | + abig = zero |
| 143 | + ix = 1 |
| 144 | + if( incx < 0 ) ix = 1 - (n-1)*incx |
| 145 | + do i = 1, n |
| 146 | + ax = abs(x(ix)) |
| 147 | + if (ax > tbig) then |
| 148 | + abig = abig + (ax*sbig)**2 |
| 149 | + notbig = .false. |
| 150 | + else if (ax < tsml) then |
| 151 | + if (notbig) asml = asml + (ax*ssml)**2 |
| 152 | + else |
| 153 | + amed = amed + ax**2 |
| 154 | + end if |
| 155 | + ix = ix + incx |
| 156 | + end do |
| 157 | +! |
| 158 | +! Combine abig and amed or amed and asml if more than one |
| 159 | +! accumulator was used. |
| 160 | +! |
| 161 | + if (abig > zero) then |
| 162 | +! |
| 163 | +! Combine abig and amed if abig > 0. |
| 164 | +! |
| 165 | + if ( (amed > zero) .or. (amed > maxN) .or. (amed /= amed) ) then |
| 166 | + abig = abig + (amed*sbig)*sbig |
| 167 | + end if |
| 168 | + scl = one / sbig |
| 169 | + sumsq = abig |
| 170 | + else if (asml > zero) then |
| 171 | +! |
| 172 | +! Combine amed and asml if asml > 0. |
| 173 | +! |
| 174 | + if ( (amed > zero) .or. (amed > maxN) .or. (amed /= amed) ) then |
| 175 | + amed = sqrt(amed) |
| 176 | + asml = sqrt(asml) / ssml |
| 177 | + if (asml > amed) then |
| 178 | + ymin = amed |
| 179 | + ymax = asml |
| 180 | + else |
| 181 | + ymin = asml |
| 182 | + ymax = amed |
| 183 | + end if |
| 184 | + scl = one |
| 185 | + sumsq = ymax**2*( one + (ymin/ymax)**2 ) |
| 186 | + else |
| 187 | + scl = one / ssml |
| 188 | + sumsq = asml |
| 189 | + end if |
| 190 | + else |
| 191 | +! |
| 192 | +! Otherwise all values are mid-range |
| 193 | +! |
| 194 | + scl = one |
| 195 | + sumsq = amed |
| 196 | + end if |
| 197 | + DNRM2 = scl*sqrt( sumsq ) |
| 198 | + return |
| 199 | +end function |
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