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// Copyright ©2015 The Gonum Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package gonum
import (
"gonum.org/v1/gonum/blas"
"gonum.org/v1/gonum/blas/blas64"
)
// Dlarf applies an elementary reflector H to an m×n matrix C:
// C = H * C if side == blas.Left
// C = C * H if side == blas.Right
// H is represented in the form
// H = I - tau * v * vᵀ
// where tau is a scalar and v is a vector.
//
// work must have length at least m if side == blas.Left and
// at least n if side == blas.Right.
//
// Dlarf is an internal routine. It is exported for testing purposes.
func (impl Implementation) Dlarf(side blas.Side, m, n int, v []float64, incv int, tau float64, c []float64, ldc int, work []float64) {
switch {
case side != blas.Left && side != blas.Right:
panic(badSide)
case m < 0:
panic(mLT0)
case n < 0:
panic(nLT0)
case incv == 0:
panic(zeroIncV)
case ldc < max(1, n):
panic(badLdC)
}
if m == 0 || n == 0 {
return
}
applyleft := side == blas.Left
lenV := n
if applyleft {
lenV = m
}
switch {
case len(v) < 1+(lenV-1)*abs(incv):
panic(shortV)
case len(c) < (m-1)*ldc+n:
panic(shortC)
case (applyleft && len(work) < n) || (!applyleft && len(work) < m):
panic(shortWork)
}
lastv := -1 // last non-zero element of v
lastc := -1 // last non-zero row/column of C
if tau != 0 {
if applyleft {
lastv = m - 1
} else {
lastv = n - 1
}
var i int
if incv > 0 {
i = lastv * incv
}
// Look for the last non-zero row in v.
for lastv >= 0 && v[i] == 0 {
lastv--
i -= incv
}
if applyleft {
// Scan for the last non-zero column in C[0:lastv, :]
lastc = impl.Iladlc(lastv+1, n, c, ldc)
} else {
// Scan for the last non-zero row in C[:, 0:lastv]
lastc = impl.Iladlr(m, lastv+1, c, ldc)
}
}
if lastv == -1 || lastc == -1 {
return
}
bi := blas64.Implementation()
if applyleft {
// Form H * C
// w[0:lastc+1] = c[1:lastv+1, 1:lastc+1]ᵀ * v[1:lastv+1,1]
bi.Dgemv(blas.Trans, lastv+1, lastc+1, 1, c, ldc, v, incv, 0, work, 1)
// c[0: lastv, 0: lastc] = c[...] - w[0:lastv, 1] * v[1:lastc, 1]ᵀ
bi.Dger(lastv+1, lastc+1, -tau, v, incv, work, 1, c, ldc)
} else {
// Form C * H
// w[0:lastc+1,1] := c[0:lastc+1,0:lastv+1] * v[0:lastv+1,1]
bi.Dgemv(blas.NoTrans, lastc+1, lastv+1, 1, c, ldc, v, incv, 0, work, 1)
// c[0:lastc+1,0:lastv+1] = c[...] - w[0:lastc+1,0] * v[0:lastv+1,0]ᵀ
bi.Dger(lastc+1, lastv+1, -tau, work, 1, v, incv, c, ldc)
}
}