SUBROUTINE SGERQF( M, N, A, LDA, TAU, WORK, LWORK, INFO )
*
* -- LAPACK routine (version 3.1) --
* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
* November 2006
*
* .. Scalar Arguments ..
INTEGER INFO, LDA, LWORK, M, N
* ..
* .. Array Arguments ..
REAL A( LDA, * ), TAU( * ), WORK( * )
* ..
*
* Purpose
* =======
*
* SGERQF computes an RQ factorization of a real M-by-N matrix A:
* A = R * Q.
*
* Arguments
* =========
*
* M (input) INTEGER
* The number of rows of the matrix A. M >= 0.
*
* N (input) INTEGER
* The number of columns of the matrix A. N >= 0.
*
* A (input/output) REAL array, dimension (LDA,N)
* On entry, the M-by-N matrix A.
* On exit,
* if m <= n, the upper triangle of the subarray
* A(1:m,n-m+1:n) contains the M-by-M upper triangular matrix R;
* if m >= n, the elements on and above the (m-n)-th subdiagonal
* contain the M-by-N upper trapezoidal matrix R;
* the remaining elements, with the array TAU, represent the
* orthogonal matrix Q as a product of min(m,n) elementary
* reflectors (see Further Details).
*
* LDA (input) INTEGER
* The leading dimension of the array A. LDA >= max(1,M).
*
* TAU (output) REAL array, dimension (min(M,N))
* The scalar factors of the elementary reflectors (see Further
* Details).
*
* WORK (workspace/output) REAL array, dimension (MAX(1,LWORK))
* On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
*
* LWORK (input) INTEGER
* The dimension of the array WORK. LWORK >= max(1,M).
* For optimum performance LWORK >= M*NB, where NB is
* the optimal blocksize.
*
* If LWORK = -1, then a workspace query is assumed; the routine
* only calculates the optimal size of the WORK array, returns
* this value as the first entry of the WORK array, and no error
* message related to LWORK is issued by XERBLA.
*
* INFO (output) INTEGER
* = 0: successful exit
* < 0: if INFO = -i, the i-th argument had an illegal value
*
* Further Details
* ===============
*
* The matrix Q is represented as a product of elementary reflectors
*
* Q = H(1) H(2) . . . H(k), where k = min(m,n).
*
* Each H(i) has the form
*
* H(i) = I - tau * v * v'
*
* where tau is a real scalar, and v is a real vector with
* v(n-k+i+1:n) = 0 and v(n-k+i) = 1; v(1:n-k+i-1) is stored on exit in
* A(m-k+i,1:n-k+i-1), and tau in TAU(i).
*
* =====================================================================
*
* .. Local Scalars ..
LOGICAL LQUERY
INTEGER I, IB, IINFO, IWS, K, KI, KK, LDWORK, LWKOPT,
$ MU, NB, NBMIN, NU, NX
* ..
* .. External Subroutines ..
EXTERNAL SGERQ2, SLARFB, SLARFT, XERBLA
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX, MIN
* ..
* .. External Functions ..
INTEGER ILAENV
EXTERNAL ILAENV
* ..
* .. Executable Statements ..
*
* Test the input arguments
*
INFO = 0
LQUERY = ( LWORK.EQ.-1 )
IF( M.LT.0 ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
INFO = -4
ELSE IF( LWORK.LT.MAX( 1, M ) .AND. .NOT.LQUERY ) THEN
INFO = -7
END IF
*
IF( INFO.EQ.0 ) THEN
K = MIN( M, N )
IF( K.EQ.0 ) THEN
LWKOPT = 1
ELSE
NB = ILAENV( 1, 'SGERQF', ' ', M, N, -1, -1 )
LWKOPT = M*NB
WORK( 1 ) = LWKOPT
END IF
WORK( 1 ) = LWKOPT
*
IF( LWORK.LT.MAX( 1, M ) .AND. .NOT.LQUERY ) THEN
INFO = -7
END IF
END IF
*
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'SGERQF', -INFO )
RETURN
ELSE IF( LQUERY ) THEN
RETURN
END IF
*
* Quick return if possible
*
IF( K.EQ.0 ) THEN
RETURN
END IF
*
NBMIN = 2
NX = 1
IWS = M
IF( NB.GT.1 .AND. NB.LT.K ) THEN
*
* Determine when to cross over from blocked to unblocked code.
*
NX = MAX( 0, ILAENV( 3, 'SGERQF', ' ', M, N, -1, -1 ) )
IF( NX.LT.K ) THEN
*
* Determine if workspace is large enough for blocked code.
*
LDWORK = M
IWS = LDWORK*NB
IF( LWORK.LT.IWS ) THEN
*
* Not enough workspace to use optimal NB: reduce NB and
* determine the minimum value of NB.
*
NB = LWORK / LDWORK
NBMIN = MAX( 2, ILAENV( 2, 'SGERQF', ' ', M, N, -1,
$ -1 ) )
END IF
END IF
END IF
*
IF( NB.GE.NBMIN .AND. NB.LT.K .AND. NX.LT.K ) THEN
*
* Use blocked code initially.
* The last kk rows are handled by the block method.
*
KI = ( ( K-NX-1 ) / NB )*NB
KK = MIN( K, KI+NB )
*
DO 10 I = K - KK + KI + 1, K - KK + 1, -NB
IB = MIN( K-I+1, NB )
*
* Compute the RQ factorization of the current block
* A(m-k+i:m-k+i+ib-1,1:n-k+i+ib-1)
*
CALL SGERQ2( IB, N-K+I+IB-1, A( M-K+I, 1 ), LDA, TAU( I ),
$ WORK, IINFO )
IF( M-K+I.GT.1 ) THEN
*
* Form the triangular factor of the block reflector
* H = H(i+ib-1) . . . H(i+1) H(i)
*
CALL SLARFT( 'Backward', 'Rowwise', N-K+I+IB-1, IB,
$ A( M-K+I, 1 ), LDA, TAU( I ), WORK, LDWORK )
*
* Apply H to A(1:m-k+i-1,1:n-k+i+ib-1) from the right
*
CALL SLARFB( 'Right', 'No transpose', 'Backward',
$ 'Rowwise', M-K+I-1, N-K+I+IB-1, IB,
$ A( M-K+I, 1 ), LDA, WORK, LDWORK, A, LDA,
$ WORK( IB+1 ), LDWORK )
END IF
10 CONTINUE
MU = M - K + I + NB - 1
NU = N - K + I + NB - 1
ELSE
MU = M
NU = N
END IF
*
* Use unblocked code to factor the last or only block
*
IF( MU.GT.0 .AND. NU.GT.0 )
$ CALL SGERQ2( MU, NU, A, LDA, TAU, WORK, IINFO )
*
WORK( 1 ) = IWS
RETURN
*
* End of SGERQF
*
END