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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"
)
// Dorgl2 generates an m×n matrix Q with orthonormal rows defined by the
// first m rows product of elementary reflectors as computed by Dgelqf.
// Q = H_0 * H_1 * ... * H_{k-1}
// len(tau) >= k, 0 <= k <= m, 0 <= m <= n, len(work) >= m.
// Dorgl2 will panic if these conditions are not met.
//
// Dorgl2 is an internal routine. It is exported for testing purposes.
func (impl Implementation) Dorgl2(m, n, k int, a []float64, lda int, tau, work []float64) {
switch {
case m < 0:
panic(mLT0)
case n < m:
panic(nLTM)
case k < 0:
panic(kLT0)
case k > m:
panic(kGTM)
case lda < max(1, m):
panic(badLdA)
}
if m == 0 {
return
}
switch {
case len(a) < (m-1)*lda+n:
panic(shortA)
case len(tau) < k:
panic(shortTau)
case len(work) < m:
panic(shortWork)
}
bi := blas64.Implementation()
if k < m {
for i := k; i < m; i++ {
for j := 0; j < n; j++ {
a[i*lda+j] = 0
}
}
for j := k; j < m; j++ {
a[j*lda+j] = 1
}
}
for i := k - 1; i >= 0; i-- {
if i < n-1 {
if i < m-1 {
a[i*lda+i] = 1
impl.Dlarf(blas.Right, m-i-1, n-i, a[i*lda+i:], 1, tau[i], a[(i+1)*lda+i:], lda, work)
}
bi.Dscal(n-i-1, -tau[i], a[i*lda+i+1:], 1)
}
a[i*lda+i] = 1 - tau[i]
for l := 0; l < i; l++ {
a[i*lda+l] = 0
}
}
}