| // 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 ( |
| "math" |
| |
| "gonum.org/v1/gonum/blas/blas64" |
| ) |
| |
| // Dgetf2 computes the LU decomposition of the m×n matrix A. |
| // The LU decomposition is a factorization of a into |
| // A = P * L * U |
| // where P is a permutation matrix, L is a unit lower triangular matrix, and |
| // U is a (usually) non-unit upper triangular matrix. On exit, L and U are stored |
| // in place into a. |
| // |
| // ipiv is a permutation vector. It indicates that row i of the matrix was |
| // changed with ipiv[i]. ipiv must have length at least min(m,n), and will panic |
| // otherwise. ipiv is zero-indexed. |
| // |
| // Dgetf2 returns whether the matrix A is singular. The LU decomposition will |
| // be computed regardless of the singularity of A, but division by zero |
| // will occur if the false is returned and the result is used to solve a |
| // system of equations. |
| // |
| // Dgetf2 is an internal routine. It is exported for testing purposes. |
| func (Implementation) Dgetf2(m, n int, a []float64, lda int, ipiv []int) (ok bool) { |
| mn := min(m, n) |
| checkMatrix(m, n, a, lda) |
| if len(ipiv) < mn { |
| panic(badIpiv) |
| } |
| if m == 0 || n == 0 { |
| return true |
| } |
| bi := blas64.Implementation() |
| sfmin := dlamchS |
| ok = true |
| for j := 0; j < mn; j++ { |
| // Find a pivot and test for singularity. |
| jp := j + bi.Idamax(m-j, a[j*lda+j:], lda) |
| ipiv[j] = jp |
| if a[jp*lda+j] == 0 { |
| ok = false |
| } else { |
| // Swap the rows if necessary. |
| if jp != j { |
| bi.Dswap(n, a[j*lda:], 1, a[jp*lda:], 1) |
| } |
| if j < m-1 { |
| aj := a[j*lda+j] |
| if math.Abs(aj) >= sfmin { |
| bi.Dscal(m-j-1, 1/aj, a[(j+1)*lda+j:], lda) |
| } else { |
| for i := 0; i < m-j-1; i++ { |
| a[(j+1)*lda+j] = a[(j+1)*lda+j] / a[lda*j+j] |
| } |
| } |
| } |
| } |
| if j < mn-1 { |
| bi.Dger(m-j-1, n-j-1, -1, a[(j+1)*lda+j:], lda, a[j*lda+j+1:], 1, a[(j+1)*lda+j+1:], lda) |
| } |
| } |
| return ok |
| } |