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// Copyright ©2020 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"
// Dlagtm performs one of the matrix-matrix operations
//
// C = alpha * A * B + beta * C if trans == blas.NoTrans
// C = alpha * Aᵀ * B + beta * C if trans == blas.Trans or blas.ConjTrans
//
// where A is an m×m tridiagonal matrix represented by its diagonals dl, d, du,
// B and C are m×n dense matrices, and alpha and beta are scalars.
func (impl Implementation) Dlagtm(trans blas.Transpose, m, n int, alpha float64, dl, d, du []float64, b []float64, ldb int, beta float64, c []float64, ldc int) {
switch {
case trans != blas.NoTrans && trans != blas.Trans && trans != blas.ConjTrans:
panic(badTrans)
case m < 0:
panic(mLT0)
case n < 0:
panic(nLT0)
case ldb < max(1, n):
panic(badLdB)
case ldc < max(1, n):
panic(badLdC)
}
if m == 0 || n == 0 {
return
}
switch {
case len(dl) < m-1:
panic(shortDL)
case len(d) < m:
panic(shortD)
case len(du) < m-1:
panic(shortDU)
case len(b) < (m-1)*ldb+n:
panic(shortB)
case len(c) < (m-1)*ldc+n:
panic(shortC)
}
if beta != 1 {
if beta == 0 {
for i := 0; i < m; i++ {
ci := c[i*ldc : i*ldc+n]
for j := range ci {
ci[j] = 0
}
}
} else {
for i := 0; i < m; i++ {
ci := c[i*ldc : i*ldc+n]
for j := range ci {
ci[j] *= beta
}
}
}
}
if alpha == 0 {
return
}
if m == 1 {
if alpha == 1 {
for j := 0; j < n; j++ {
c[j] += d[0] * b[j]
}
} else {
for j := 0; j < n; j++ {
c[j] += alpha * d[0] * b[j]
}
}
return
}
if trans != blas.NoTrans {
dl, du = du, dl
}
if alpha == 1 {
for j := 0; j < n; j++ {
c[j] += d[0]*b[j] + du[0]*b[ldb+j]
}
for i := 1; i < m-1; i++ {
for j := 0; j < n; j++ {
c[i*ldc+j] += dl[i-1]*b[(i-1)*ldb+j] + d[i]*b[i*ldb+j] + du[i]*b[(i+1)*ldb+j]
}
}
for j := 0; j < n; j++ {
c[(m-1)*ldc+j] += dl[m-2]*b[(m-2)*ldb+j] + d[m-1]*b[(m-1)*ldb+j]
}
} else {
for j := 0; j < n; j++ {
c[j] += alpha * (d[0]*b[j] + du[0]*b[ldb+j])
}
for i := 1; i < m-1; i++ {
for j := 0; j < n; j++ {
c[i*ldc+j] += alpha * (dl[i-1]*b[(i-1)*ldb+j] + d[i]*b[i*ldb+j] + du[i]*b[(i+1)*ldb+j])
}
}
for j := 0; j < n; j++ {
c[(m-1)*ldc+j] += alpha * (dl[m-2]*b[(m-2)*ldb+j] + d[m-1]*b[(m-1)*ldb+j])
}
}
}
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