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// Copyright ©2018 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"
)
// Dlauu2 computes the product
//
// U * Uᵀ if uplo is blas.Upper
// Lᵀ * L if uplo is blas.Lower
//
// where U or L is stored in the upper or lower triangular part of A.
// Only the upper or lower triangle of the result is stored, overwriting
// the corresponding factor in A.
func (impl Implementation) Dlauu2(uplo blas.Uplo, n int, a []float64, lda int) {
switch {
case uplo != blas.Upper && uplo != blas.Lower:
panic(badUplo)
case n < 0:
panic(nLT0)
case lda < max(1, n):
panic(badLdA)
}
// Quick return if possible.
if n == 0 {
return
}
if len(a) < (n-1)*lda+n {
panic(shortA)
}
bi := blas64.Implementation()
if uplo == blas.Upper {
// Compute the product U*Uᵀ.
for i := 0; i < n; i++ {
aii := a[i*lda+i]
if i < n-1 {
a[i*lda+i] = bi.Ddot(n-i, a[i*lda+i:], 1, a[i*lda+i:], 1)
bi.Dgemv(blas.NoTrans, i, n-i-1, 1, a[i+1:], lda, a[i*lda+i+1:], 1,
aii, a[i:], lda)
} else {
bi.Dscal(i+1, aii, a[i:], lda)
}
}
} else {
// Compute the product Lᵀ*L.
for i := 0; i < n; i++ {
aii := a[i*lda+i]
if i < n-1 {
a[i*lda+i] = bi.Ddot(n-i, a[i*lda+i:], lda, a[i*lda+i:], lda)
bi.Dgemv(blas.Trans, n-i-1, i, 1, a[(i+1)*lda:], lda, a[(i+1)*lda+i:], lda,
aii, a[i*lda:], 1)
} else {
bi.Dscal(i+1, aii, a[i*lda:], 1)
}
}
}
}
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