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https://github.com/gonum/gonum.git
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84 lines
2.3 KiB
Go
84 lines
2.3 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 gonum
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import (
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"gonum.org/v1/gonum/blas"
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"gonum.org/v1/gonum/blas/blas64"
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)
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// Dlarf applies an elementary reflector to a general rectangular matrix c.
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// This computes
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// c = h * c if side == Left
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// c = c * h if side == right
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// where
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// h = 1 - tau * v * v^T
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// and c is an m * n matrix.
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//
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// work is temporary storage of length at least m if side == Left and at least
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// n if side == Right. This function will panic if this length requirement is not met.
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//
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// Dlarf is an internal routine. It is exported for testing purposes.
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func (impl Implementation) Dlarf(side blas.Side, m, n int, v []float64, incv int, tau float64, c []float64, ldc int, work []float64) {
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applyleft := side == blas.Left
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if (applyleft && len(work) < n) || (!applyleft && len(work) < m) {
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panic(badWork)
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}
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checkMatrix(m, n, c, ldc)
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// v has length m if applyleft and n otherwise.
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lenV := n
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if applyleft {
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lenV = m
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}
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checkVector(lenV, v, incv)
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lastv := 0 // last non-zero element of v
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lastc := 0 // last non-zero row/column of c
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if tau != 0 {
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var i int
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if applyleft {
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lastv = m - 1
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} else {
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lastv = n - 1
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}
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if incv > 0 {
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i = lastv * incv
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}
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// Look for the last non-zero row in v.
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for lastv >= 0 && v[i] == 0 {
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lastv--
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i -= incv
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}
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if applyleft {
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// Scan for the last non-zero column in C[0:lastv, :]
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lastc = impl.Iladlc(lastv+1, n, c, ldc)
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} else {
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// Scan for the last non-zero row in C[:, 0:lastv]
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lastc = impl.Iladlr(m, lastv+1, c, ldc)
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}
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}
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if lastv == -1 || lastc == -1 {
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return
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}
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// Sometimes 1-indexing is nicer ...
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bi := blas64.Implementation()
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if applyleft {
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// Form H * C
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// w[0:lastc+1] = c[1:lastv+1, 1:lastc+1]^T * v[1:lastv+1,1]
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bi.Dgemv(blas.Trans, lastv+1, lastc+1, 1, c, ldc, v, incv, 0, work, 1)
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// c[0: lastv, 0: lastc] = c[...] - w[0:lastv, 1] * v[1:lastc, 1]^T
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bi.Dger(lastv+1, lastc+1, -tau, v, incv, work, 1, c, ldc)
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return
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}
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// Form C*H
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// w[0:lastc+1,1] := c[0:lastc+1,0:lastv+1] * v[0:lastv+1,1]
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bi.Dgemv(blas.NoTrans, lastc+1, lastv+1, 1, c, ldc, v, incv, 0, work, 1)
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// c[0:lastc+1,0:lastv+1] = c[...] - w[0:lastc+1,0] * v[0:lastv+1,0]^T
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bi.Dger(lastc+1, lastv+1, -tau, work, 1, v, incv, c, ldc)
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}
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