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271 lines
7.6 KiB
271 lines
7.6 KiB
2 years ago
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*> \brief \b DLASWLQ
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*
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* Definition:
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* ===========
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*
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* SUBROUTINE DLASWLQ( M, N, MB, NB, A, LDA, T, LDT, WORK,
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* LWORK, INFO)
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*
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* .. Scalar Arguments ..
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* INTEGER INFO, LDA, M, N, MB, NB, LDT, LWORK
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* ..
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* .. Array Arguments ..
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* DOUBLE PRECISION A( LDA, * ), T( LDT, * ), WORK( * )
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* ..
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*
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*
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*> \par Purpose:
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* =============
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*>
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*> \verbatim
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*>
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*> DLASWLQ computes a blocked Tall-Skinny LQ factorization of
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*> a real M-by-N matrix A for M <= N:
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*>
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*> A = ( L 0 ) * Q,
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*>
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*> where:
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*>
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*> Q is a n-by-N orthogonal matrix, stored on exit in an implicit
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*> form in the elements above the diagonal of the array A and in
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*> the elements of the array T;
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*> L is a lower-triangular M-by-M matrix stored on exit in
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*> the elements on and below the diagonal of the array A.
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*> 0 is a M-by-(N-M) zero matrix, if M < N, and is not stored.
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*>
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*> \endverbatim
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*
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* Arguments:
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* ==========
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*
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*> \param[in] M
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*> \verbatim
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*> M is INTEGER
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*> The number of rows of the matrix A. M >= 0.
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*> \endverbatim
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*>
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*> \param[in] N
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*> \verbatim
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*> N is INTEGER
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*> The number of columns of the matrix A. N >= M >= 0.
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*> \endverbatim
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*>
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*> \param[in] MB
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*> \verbatim
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*> MB is INTEGER
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*> The row block size to be used in the blocked QR.
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*> M >= MB >= 1
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*> \endverbatim
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*> \param[in] NB
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*> \verbatim
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*> NB is INTEGER
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*> The column block size to be used in the blocked QR.
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*> NB > 0.
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*> \endverbatim
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*>
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*> \param[in,out] A
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*> \verbatim
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*> A is DOUBLE PRECISION array, dimension (LDA,N)
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*> On entry, the M-by-N matrix A.
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*> On exit, the elements on and below the diagonal
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*> of the array contain the N-by-N lower triangular matrix L;
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*> the elements above the diagonal represent Q by the rows
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*> of blocked V (see Further Details).
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*>
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*> \endverbatim
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*>
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*> \param[in] LDA
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*> \verbatim
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*> LDA is INTEGER
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*> The leading dimension of the array A. LDA >= max(1,M).
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*> \endverbatim
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*>
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*> \param[out] T
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*> \verbatim
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*> T is DOUBLE PRECISION array,
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*> dimension (LDT, N * Number_of_row_blocks)
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*> where Number_of_row_blocks = CEIL((N-M)/(NB-M))
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*> The blocked upper triangular block reflectors stored in compact form
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*> as a sequence of upper triangular blocks.
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*> See Further Details below.
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*> \endverbatim
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*>
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*> \param[in] LDT
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*> \verbatim
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*> LDT is INTEGER
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*> The leading dimension of the array T. LDT >= MB.
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*> \endverbatim
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*>
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*>
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*> \param[out] WORK
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*> \verbatim
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*> (workspace) DOUBLE PRECISION array, dimension (MAX(1,LWORK))
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*>
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*> \endverbatim
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*> \param[in] LWORK
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*> \verbatim
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*> LWORK is INTEGER
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*> The dimension of the array WORK. LWORK >= MB*M.
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*> If LWORK = -1, then a workspace query is assumed; the routine
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*> only calculates the optimal size of the WORK array, returns
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*> this value as the first entry of the WORK array, and no error
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*> message related to LWORK is issued by XERBLA.
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*>
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*> \endverbatim
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*> \param[out] INFO
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*> \verbatim
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*> INFO is INTEGER
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*> = 0: successful exit
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*> < 0: if INFO = -i, the i-th argument had an illegal value
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*> \endverbatim
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*
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* Authors:
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* ========
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*
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*> \author Univ. of Tennessee
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*> \author Univ. of California Berkeley
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*> \author Univ. of Colorado Denver
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*> \author NAG Ltd.
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*
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*> \par Further Details:
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* =====================
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*>
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*> \verbatim
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*> Short-Wide LQ (SWLQ) performs LQ by a sequence of orthogonal transformations,
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*> representing Q as a product of other orthogonal matrices
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*> Q = Q(1) * Q(2) * . . . * Q(k)
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*> where each Q(i) zeros out upper diagonal entries of a block of NB rows of A:
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*> Q(1) zeros out the upper diagonal entries of rows 1:NB of A
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*> Q(2) zeros out the bottom MB-N rows of rows [1:M,NB+1:2*NB-M] of A
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*> Q(3) zeros out the bottom MB-N rows of rows [1:M,2*NB-M+1:3*NB-2*M] of A
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*> . . .
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*>
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*> Q(1) is computed by GELQT, which represents Q(1) by Householder vectors
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*> stored under the diagonal of rows 1:MB of A, and by upper triangular
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*> block reflectors, stored in array T(1:LDT,1:N).
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*> For more information see Further Details in GELQT.
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*>
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*> Q(i) for i>1 is computed by TPLQT, which represents Q(i) by Householder vectors
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*> stored in columns [(i-1)*(NB-M)+M+1:i*(NB-M)+M] of A, and by upper triangular
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*> block reflectors, stored in array T(1:LDT,(i-1)*M+1:i*M).
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*> The last Q(k) may use fewer rows.
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*> For more information see Further Details in TPQRT.
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*>
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*> For more details of the overall algorithm, see the description of
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*> Sequential TSQR in Section 2.2 of [1].
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*>
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*> [1] “Communication-Optimal Parallel and Sequential QR and LU Factorizations,”
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*> J. Demmel, L. Grigori, M. Hoemmen, J. Langou,
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*> SIAM J. Sci. Comput, vol. 34, no. 1, 2012
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*> \endverbatim
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*>
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* =====================================================================
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SUBROUTINE DLASWLQ( M, N, MB, NB, A, LDA, T, LDT, WORK, LWORK,
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$ INFO)
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*
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* -- LAPACK computational routine --
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* -- LAPACK is a software package provided by Univ. of Tennessee, --
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* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd. --
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*
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* .. Scalar Arguments ..
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INTEGER INFO, LDA, M, N, MB, NB, LWORK, LDT
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* ..
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* .. Array Arguments ..
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DOUBLE PRECISION A( LDA, * ), WORK( * ), T( LDT, *)
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* ..
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*
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* =====================================================================
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*
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* ..
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* .. Local Scalars ..
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LOGICAL LQUERY
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INTEGER I, II, KK, CTR
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* ..
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* .. EXTERNAL FUNCTIONS ..
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LOGICAL LSAME
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EXTERNAL LSAME
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* .. EXTERNAL SUBROUTINES ..
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EXTERNAL DGELQT, DTPLQT, XERBLA
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* .. INTRINSIC FUNCTIONS ..
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INTRINSIC MAX, MIN, MOD
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* ..
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* .. EXECUTABLE STATEMENTS ..
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*
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* TEST THE INPUT ARGUMENTS
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*
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INFO = 0
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*
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LQUERY = ( LWORK.EQ.-1 )
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*
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IF( M.LT.0 ) THEN
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INFO = -1
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ELSE IF( N.LT.0 .OR. N.LT.M ) THEN
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INFO = -2
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ELSE IF( MB.LT.1 .OR. ( MB.GT.M .AND. M.GT.0 )) THEN
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INFO = -3
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ELSE IF( NB.LT.0 ) THEN
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INFO = -4
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ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
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INFO = -6
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ELSE IF( LDT.LT.MB ) THEN
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INFO = -8
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ELSE IF( ( LWORK.LT.M*MB) .AND. (.NOT.LQUERY) ) THEN
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INFO = -10
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END IF
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IF( INFO.EQ.0) THEN
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WORK(1) = MB*M
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END IF
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*
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IF( INFO.NE.0 ) THEN
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CALL XERBLA( 'DLASWLQ', -INFO )
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RETURN
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ELSE IF (LQUERY) THEN
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RETURN
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END IF
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*
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* Quick return if possible
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*
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IF( MIN(M,N).EQ.0 ) THEN
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RETURN
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END IF
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*
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* The LQ Decomposition
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*
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IF((M.GE.N).OR.(NB.LE.M).OR.(NB.GE.N)) THEN
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CALL DGELQT( M, N, MB, A, LDA, T, LDT, WORK, INFO)
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RETURN
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END IF
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*
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KK = MOD((N-M),(NB-M))
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II=N-KK+1
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*
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* Compute the LQ factorization of the first block A(1:M,1:NB)
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*
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CALL DGELQT( M, NB, MB, A(1,1), LDA, T, LDT, WORK, INFO)
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CTR = 1
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*
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DO I = NB+1, II-NB+M , (NB-M)
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*
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* Compute the QR factorization of the current block A(1:M,I:I+NB-M)
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*
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CALL DTPLQT( M, NB-M, 0, MB, A(1,1), LDA, A( 1, I ),
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$ LDA, T(1, CTR * M + 1),
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$ LDT, WORK, INFO )
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CTR = CTR + 1
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END DO
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*
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* Compute the QR factorization of the last block A(1:M,II:N)
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*
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IF (II.LE.N) THEN
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CALL DTPLQT( M, KK, 0, MB, A(1,1), LDA, A( 1, II ),
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$ LDA, T(1, CTR * M + 1), LDT,
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$ WORK, INFO )
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END IF
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*
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WORK( 1 ) = M * MB
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RETURN
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*
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* End of DLASWLQ
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*
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END
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