gonum.org/v1/gonum@v0.14.0/lapack/gonum/dlagtm.go (about)

     1  // Copyright ©2020 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 gonum
     6  
     7  import "gonum.org/v1/gonum/blas"
     8  
     9  // Dlagtm performs one of the matrix-matrix operations
    10  //
    11  //	C = alpha * A * B + beta * C   if trans == blas.NoTrans
    12  //	C = alpha * Aᵀ * B + beta * C  if trans == blas.Trans or blas.ConjTrans
    13  //
    14  // where A is an m×m tridiagonal matrix represented by its diagonals dl, d, du,
    15  // B and C are m×n dense matrices, and alpha and beta are scalars.
    16  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) {
    17  	switch {
    18  	case trans != blas.NoTrans && trans != blas.Trans && trans != blas.ConjTrans:
    19  		panic(badTrans)
    20  	case m < 0:
    21  		panic(mLT0)
    22  	case n < 0:
    23  		panic(nLT0)
    24  	case ldb < max(1, n):
    25  		panic(badLdB)
    26  	case ldc < max(1, n):
    27  		panic(badLdC)
    28  	}
    29  
    30  	if m == 0 || n == 0 {
    31  		return
    32  	}
    33  
    34  	switch {
    35  	case len(dl) < m-1:
    36  		panic(shortDL)
    37  	case len(d) < m:
    38  		panic(shortD)
    39  	case len(du) < m-1:
    40  		panic(shortDU)
    41  	case len(b) < (m-1)*ldb+n:
    42  		panic(shortB)
    43  	case len(c) < (m-1)*ldc+n:
    44  		panic(shortC)
    45  	}
    46  
    47  	if beta != 1 {
    48  		if beta == 0 {
    49  			for i := 0; i < m; i++ {
    50  				ci := c[i*ldc : i*ldc+n]
    51  				for j := range ci {
    52  					ci[j] = 0
    53  				}
    54  			}
    55  		} else {
    56  			for i := 0; i < m; i++ {
    57  				ci := c[i*ldc : i*ldc+n]
    58  				for j := range ci {
    59  					ci[j] *= beta
    60  				}
    61  			}
    62  		}
    63  	}
    64  
    65  	if alpha == 0 {
    66  		return
    67  	}
    68  
    69  	if m == 1 {
    70  		if alpha == 1 {
    71  			for j := 0; j < n; j++ {
    72  				c[j] += d[0] * b[j]
    73  			}
    74  		} else {
    75  			for j := 0; j < n; j++ {
    76  				c[j] += alpha * d[0] * b[j]
    77  			}
    78  		}
    79  		return
    80  	}
    81  
    82  	if trans != blas.NoTrans {
    83  		dl, du = du, dl
    84  	}
    85  
    86  	if alpha == 1 {
    87  		for j := 0; j < n; j++ {
    88  			c[j] += d[0]*b[j] + du[0]*b[ldb+j]
    89  		}
    90  		for i := 1; i < m-1; i++ {
    91  			for j := 0; j < n; j++ {
    92  				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]
    93  			}
    94  		}
    95  		for j := 0; j < n; j++ {
    96  			c[(m-1)*ldc+j] += dl[m-2]*b[(m-2)*ldb+j] + d[m-1]*b[(m-1)*ldb+j]
    97  		}
    98  	} else {
    99  		for j := 0; j < n; j++ {
   100  			c[j] += alpha * (d[0]*b[j] + du[0]*b[ldb+j])
   101  		}
   102  		for i := 1; i < m-1; i++ {
   103  			for j := 0; j < n; j++ {
   104  				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])
   105  			}
   106  		}
   107  		for j := 0; j < n; j++ {
   108  			c[(m-1)*ldc+j] += alpha * (dl[m-2]*b[(m-2)*ldb+j] + d[m-1]*b[(m-1)*ldb+j])
   109  		}
   110  	}
   111  }