github.com/gonum/lapack@v0.0.0-20181123203213-e4cdc5a0bff9/native/dorg2r.go (about)

     1  // Copyright ©2015 The gonum Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package native
     6  
     7  import (
     8  	"github.com/gonum/blas"
     9  	"github.com/gonum/blas/blas64"
    10  )
    11  
    12  // Dorg2r generates an m×n matrix Q with orthonormal columns defined by the
    13  // product of elementary reflectors as computed by Dgeqrf.
    14  //  Q = H_0 * H_1 * ... * H_{k-1}
    15  // len(tau) >= k, 0 <= k <= n, 0 <= n <= m, len(work) >= n.
    16  // Dorg2r will panic if these conditions are not met.
    17  //
    18  // Dorg2r is an internal routine. It is exported for testing purposes.
    19  func (impl Implementation) Dorg2r(m, n, k int, a []float64, lda int, tau []float64, work []float64) {
    20  	checkMatrix(m, n, a, lda)
    21  	if len(tau) < k {
    22  		panic(badTau)
    23  	}
    24  	if len(work) < n {
    25  		panic(badWork)
    26  	}
    27  	if k > n {
    28  		panic(kGTN)
    29  	}
    30  	if n > m {
    31  		panic(mLTN)
    32  	}
    33  	if len(work) < n {
    34  		panic(badWork)
    35  	}
    36  	if n == 0 {
    37  		return
    38  	}
    39  	bi := blas64.Implementation()
    40  	// Initialize columns k+1:n to columns of the unit matrix.
    41  	for l := 0; l < m; l++ {
    42  		for j := k; j < n; j++ {
    43  			a[l*lda+j] = 0
    44  		}
    45  	}
    46  	for j := k; j < n; j++ {
    47  		a[j*lda+j] = 1
    48  	}
    49  	for i := k - 1; i >= 0; i-- {
    50  		for i := range work {
    51  			work[i] = 0
    52  		}
    53  		if i < n-1 {
    54  			a[i*lda+i] = 1
    55  			impl.Dlarf(blas.Left, m-i, n-i-1, a[i*lda+i:], lda, tau[i], a[i*lda+i+1:], lda, work)
    56  		}
    57  		if i < m-1 {
    58  			bi.Dscal(m-i-1, -tau[i], a[(i+1)*lda+i:], lda)
    59  		}
    60  		a[i*lda+i] = 1 - tau[i]
    61  		for l := 0; l < i; l++ {
    62  			a[l*lda+i] = 0
    63  		}
    64  	}
    65  }