slaqps.f

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*> \brief \b SLAQPS computes a step of QR factorization with column pivoting of a real m-by-n matrix A by using BLAS level 3.
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
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*
*  Definition:
*  ===========
*
*       SUBROUTINE SLAQPS( M, N, OFFSET, NB, KB, A, LDA, JPVT, TAU, VN1,
*                          VN2, AUXV, F, LDF )
*
*       .. Scalar Arguments ..
*       INTEGER            KB, LDA, LDF, M, N, NB, OFFSET
*       ..
*       .. Array Arguments ..
*       INTEGER            JPVT( * )
*       REAL               A( LDA, * ), AUXV( * ), F( LDF, * ), TAU( * ),
*      $                   VN1( * ), VN2( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> SLAQPS computes a step of QR factorization with column pivoting
*> of a real M-by-N matrix A by using Blas-3.  It tries to factorize
*> NB columns from A starting from the row OFFSET+1, and updates all
*> of the matrix with Blas-3 xGEMM.
*>
*> In some cases, due to catastrophic cancellations, it cannot
*> factorize NB columns.  Hence, the actual number of factorized
*> columns is returned in KB.
*>
*> Block A(1:OFFSET,1:N) is accordingly pivoted, but not factorized.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] M
*> \verbatim
*>          M is INTEGER
*>          The number of rows of the matrix A. M >= 0.
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>          The number of columns of the matrix A. N >= 0
*> \endverbatim
*>
*> \param[in] OFFSET
*> \verbatim
*>          OFFSET is INTEGER
*>          The number of rows of A that have been factorized in
*>          previous steps.
*> \endverbatim
*>
*> \param[in] NB
*> \verbatim
*>          NB is INTEGER
*>          The number of columns to factorize.
*> \endverbatim
*>
*> \param[out] KB
*> \verbatim
*>          KB is INTEGER
*>          The number of columns actually factorized.
*> \endverbatim
*>
*> \param[in,out] A
*> \verbatim
*>          A is REAL array, dimension (LDA,N)
*>          On entry, the M-by-N matrix A.
*>          On exit, block A(OFFSET+1:M,1:KB) is the triangular
*>          factor obtained and block A(1:OFFSET,1:N) has been
*>          accordingly pivoted, but no factorized.
*>          The rest of the matrix, block A(OFFSET+1:M,KB+1:N) has
*>          been updated.
*> \endverbatim
*>
*> \param[in] LDA
*> \verbatim
*>          LDA is INTEGER
*>          The leading dimension of the array A. LDA >= max(1,M).
*> \endverbatim
*>
*> \param[in,out] JPVT
*> \verbatim
*>          JPVT is INTEGER array, dimension (N)
*>          JPVT(I) = K <==> Column K of the full matrix A has been
*>          permuted into position I in AP.
*> \endverbatim
*>
*> \param[out] TAU
*> \verbatim
*>          TAU is REAL array, dimension (KB)
*>          The scalar factors of the elementary reflectors.
*> \endverbatim
*>
*> \param[in,out] VN1
*> \verbatim
*>          VN1 is REAL array, dimension (N)
*>          The vector with the partial column norms.
*> \endverbatim
*>
*> \param[in,out] VN2
*> \verbatim
*>          VN2 is REAL array, dimension (N)
*>          The vector with the exact column norms.
*> \endverbatim
*>
*> \param[in,out] AUXV
*> \verbatim
*>          AUXV is REAL array, dimension (NB)
*>          Auxiliary vector.
*> \endverbatim
*>
*> \param[in,out] F
*> \verbatim
*>          F is REAL array, dimension (LDF,NB)
*>          Matrix F**T = L*Y**T*A.
*> \endverbatim
*>
*> \param[in] LDF
*> \verbatim
*>          LDF is INTEGER
*>          The leading dimension of the array F. LDF >= max(1,N).
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup laqps
*
*> \par Contributors:
*  ==================
*>
*>    G. Quintana-Orti, Depto. de Informatica, Universidad Jaime I, Spain
*>    X. Sun, Computer Science Dept., Duke University, USA
*>
*> \n
*>  Partial column norm updating strategy modified on April 2011
*>    Z. Drmac and Z. Bujanovic, Dept. of Mathematics,
*>    University of Zagreb, Croatia.
*
*> \par References:
*  ================
*>
*> LAPACK Working Note 176
*
*> <a href="http://www.netlib.org/lapack/lawnspdf/lawn176.pdf">[PDF]</a>
*
*  =====================================================================
      SUBROUTINE slaqps( M, N, OFFSET, NB, KB, A, LDA, JPVT, TAU,
     $                   VN1,
     $                   VN2, AUXV, F, LDF )
*
*  -- LAPACK auxiliary routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      INTEGER            KB, LDA, LDF, M, N, NB, OFFSET
*     ..
*     .. Array Arguments ..
      INTEGER            JPVT( * )
      REAL               A( LDA, * ), AUXV( * ), F( LDF, * ), TAU( * ),
     $                   vn1( * ), vn2( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO, ONE
      PARAMETER          ( ZERO = 0.0e+0, one = 1.0e+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            ITEMP, J, K, LASTRK, LSTICC, PVT, RK
      REAL               AKK, TEMP, TEMP2, TOL3Z
*     ..
*     .. External Subroutines ..
      EXTERNAL           sgemm, sgemv, slarfg, sswap
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          abs, max, min, nint, real, sqrt
*     ..
*     .. External Functions ..
      INTEGER            ISAMAX
      REAL               SLAMCH, SNRM2
      EXTERNAL           isamax, slamch, snrm2
*     ..
*     .. Executable Statements ..
*
      lastrk = min( m, n+offset )
      lsticc = 0
      k = 0
      tol3z = sqrt(slamch('Epsilon'))
*
*     Beginning of while loop.
*
   10 CONTINUE
      IF( ( k.LT.nb ) .AND. ( lsticc.EQ.0 ) ) THEN
         k = k + 1
         rk = offset + k
*
*        Determine ith pivot column and swap if necessary
*
         pvt = ( k-1 ) + isamax( n-k+1, vn1( k ), 1 )
         IF( pvt.NE.k ) THEN
            CALL sswap( m, a( 1, pvt ), 1, a( 1, k ), 1 )
            CALL sswap( k-1, f( pvt, 1 ), ldf, f( k, 1 ), ldf )
            itemp = jpvt( pvt )
            jpvt( pvt ) = jpvt( k )
            jpvt( k ) = itemp
            vn1( pvt ) = vn1( k )
            vn2( pvt ) = vn2( k )
         END IF
*
*        Apply previous Householder reflectors to column K:
*        A(RK:M,K) := A(RK:M,K) - A(RK:M,1:K-1)*F(K,1:K-1)**T.
*
         IF( k.GT.1 ) THEN
            CALL sgemv( 'No transpose', m-rk+1, k-1, -one, a( rk,
     $                  1 ),
     $                  lda, f( k, 1 ), ldf, one, a( rk, k ), 1 )
         END IF
*
*        Generate elementary reflector H(k).
*
         IF( rk.LT.m ) THEN
            CALL slarfg( m-rk+1, a( rk, k ), a( rk+1, k ), 1,
     $                   tau( k ) )
         ELSE
            CALL slarfg( 1, a( rk, k ), a( rk, k ), 1, tau( k ) )
         END IF
*
         akk = a( rk, k )
         a( rk, k ) = one
*
*        Compute Kth column of F:
*
*        Compute  F(K+1:N,K) := tau(K)*A(RK:M,K+1:N)**T*A(RK:M,K).
*
         IF( k.LT.n ) THEN
            CALL sgemv( 'Transpose', m-rk+1, n-k, tau( k ),
     $                  a( rk, k+1 ), lda, a( rk, k ), 1, zero,
     $                  f( k+1, k ), 1 )
         END IF
*
*        Padding F(1:K,K) with zeros.
*
         DO 20 j = 1, k
            f( j, k ) = zero
   20    CONTINUE
*
*        Incremental updating of F:
*        F(1:N,K) := F(1:N,K) - tau(K)*F(1:N,1:K-1)*A(RK:M,1:K-1)**T
*                    *A(RK:M,K).
*
         IF( k.GT.1 ) THEN
            CALL sgemv( 'Transpose', m-rk+1, k-1, -tau( k ), a( rk,
     $                  1 ),
     $                  lda, a( rk, k ), 1, zero, auxv( 1 ), 1 )
*
            CALL sgemv( 'No transpose', n, k-1, one, f( 1, 1 ), ldf,
     $                  auxv( 1 ), 1, one, f( 1, k ), 1 )
         END IF
*
*        Update the current row of A:
*        A(RK,K+1:N) := A(RK,K+1:N) - A(RK,1:K)*F(K+1:N,1:K)**T.
*
         IF( k.LT.n ) THEN
            CALL sgemv( 'No transpose', n-k, k, -one, f( k+1, 1 ),
     $                  ldf,
     $                  a( rk, 1 ), lda, one, a( rk, k+1 ), lda )
         END IF
*
*        Update partial column norms.
*
         IF( rk.LT.lastrk ) THEN
            DO 30 j = k + 1, n
               IF( vn1( j ).NE.zero ) THEN
*
*                 NOTE: The following 4 lines follow from the analysis in
*                 Lapack Working Note 176.
*
                  temp = abs( a( rk, j ) ) / vn1( j )
                  temp = max( zero, ( one+temp )*( one-temp ) )
                  temp2 = temp*( vn1( j ) / vn2( j ) )**2
                  IF( temp2 .LE. tol3z ) THEN
                     vn2( j ) = real( lsticc )
                     lsticc = j
                  ELSE
                     vn1( j ) = vn1( j )*sqrt( temp )
                  END IF
               END IF
   30       CONTINUE
         END IF
*
         a( rk, k ) = akk
*
*        End of while loop.
*
         GO TO 10
      END IF
      kb = k
      rk = offset + kb
*
*     Apply the block reflector to the rest of the matrix:
*     A(OFFSET+KB+1:M,KB+1:N) := A(OFFSET+KB+1:M,KB+1:N) -
*                         A(OFFSET+KB+1:M,1:KB)*F(KB+1:N,1:KB)**T.
*
      IF( kb.LT.min( n, m-offset ) ) THEN
         CALL sgemm( 'No transpose', 'Transpose', m-rk, n-kb, kb,
     $               -one,
     $               a( rk+1, 1 ), lda, f( kb+1, 1 ), ldf, one,
     $               a( rk+1, kb+1 ), lda )
      END IF
*
*     Recomputation of difficult columns.
*
   40 CONTINUE
      IF( lsticc.GT.0 ) THEN
         itemp = nint( vn2( lsticc ) )
         vn1( lsticc ) = snrm2( m-rk, a( rk+1, lsticc ), 1 )
*
*        NOTE: The computation of VN1( LSTICC ) relies on the fact that
*        SNRM2 does not fail on vectors with norm below the value of
*        SQRT(DLAMCH('S'))
*
         vn2( lsticc ) = vn1( lsticc )
         lsticc = itemp
         GO TO 40
      END IF
*
      RETURN
*
*     End of SLAQPS
*
      END