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83 lines
1.7 KiB
Go
83 lines
1.7 KiB
Go
// Copyright ©2015 The gonum Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package native
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import "math"
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// Dlartg generates a plane rotation so that
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// [ cs sn] * [f] = [r]
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// [-sn cs] [g] = [0]
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// This is a more accurate version of BLAS drotg, with the other differences that
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// if g = 0, then cs = 1 and sn = 0, and if f = 0 and g != 0, then cs = 0 and sn = 1.
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// If abs(f) > abs(g), cs will be positive.
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//
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// Dlartg is an internal routine. It is exported for testing purposes.
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func (impl Implementation) Dlartg(f, g float64) (cs, sn, r float64) {
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safmin := dlamchS
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eps := dlamchE
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safmn2 := math.Pow(float64(dlamchB), math.Log(safmin/eps))
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safmx2 := 1 / safmn2
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if g == 0 {
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cs = 1
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sn = 0
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r = f
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return cs, sn, r
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}
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if f == 0 {
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cs = 0
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sn = 1
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r = g
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return cs, sn, r
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}
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f1 := f
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g1 := g
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scale := math.Max(math.Abs(f1), math.Abs(g1))
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if scale >= safmx2 {
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var count int
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for {
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count++
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f1 *= safmn2
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g1 *= safmn2
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scale = math.Max(math.Abs(f1), math.Abs(g1))
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if scale < safmx2 {
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break
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}
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}
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r = math.Sqrt(f1*f1 + g1*g1)
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cs = f1 / r
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sn = g1 / r
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for i := 0; i < count; i++ {
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r *= safmx2
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}
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} else if scale <= safmn2 {
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var count int
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for {
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count++
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f1 *= safmx2
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g1 *= safmx2
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scale = math.Max(math.Abs(f1), math.Abs(g1))
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if scale >= safmn2 {
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break
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}
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}
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r = math.Sqrt(f1*f1 + g1*g1)
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cs = f1 / r
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sn = g1 / r
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for i := 0; i < count; i++ {
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r *= safmn2
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}
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} else {
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r = math.Sqrt(f1*f1 + g1*g1)
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cs = f1 / r
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sn = g1 / r
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}
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if math.Abs(f) > math.Abs(g) && cs < 0 {
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cs *= -1
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sn *= -1
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r *= -1
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}
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return cs, sn, r
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}
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