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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"
)
// Dtrti2 computes the inverse of a triangular matrix, storing the result in place
// into a. This is the BLAS level 2 version of the algorithm.
//
// Dtrti2 is an internal routine. It is exported for testing purposes.
func (impl Implementation) Dtrti2(uplo blas.Uplo, diag blas.Diag, n int, a []float64, lda int) {
switch {
case uplo != blas.Upper && uplo != blas.Lower:
panic(badUplo)
case diag != blas.NonUnit && diag != blas.Unit:
panic(badDiag)
case n < 0:
panic(nLT0)
case lda < max(1, n):
panic(badLdA)
}
if n == 0 {
return
}
if len(a) < (n-1)*lda+n {
panic(shortA)
}
bi := blas64.Implementation()
nonUnit := diag == blas.NonUnit
// TODO(btracey): Replace this with a row-major ordering.
if uplo == blas.Upper {
for j := 0; j < n; j++ {
var ajj float64
if nonUnit {
ajj = 1 / a[j*lda+j]
a[j*lda+j] = ajj
ajj *= -1
} else {
ajj = -1
}
bi.Dtrmv(blas.Upper, blas.NoTrans, diag, j, a, lda, a[j:], lda)
bi.Dscal(j, ajj, a[j:], lda)
}
return
}
for j := n - 1; j >= 0; j-- {
var ajj float64
if nonUnit {
ajj = 1 / a[j*lda+j]
a[j*lda+j] = ajj
ajj *= -1
} else {
ajj = -1
}
if j < n-1 {
bi.Dtrmv(blas.Lower, blas.NoTrans, diag, n-j-1, a[(j+1)*lda+j+1:], lda, a[(j+1)*lda+j:], lda)
bi.Dscal(n-j-1, ajj, a[(j+1)*lda+j:], lda)
}
}
}
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