      SUBROUTINE ZGELSY( M, N, NRHS, A, LDA, B, LDB, JPVT, RCOND, RANK,
     $                   WORK, LWORK, RWORK, INFO )
*
*  -- LAPACK driver routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     .. Scalar Arguments ..
      INTEGER            INFO, LDA, LDB, LWORK, M, N, NRHS, RANK
      DOUBLE PRECISION   RCOND
*     ..
*     .. Array Arguments ..
      INTEGER            JPVT( * )
      DOUBLE PRECISION   RWORK( * )
      COMPLEX*16         A( LDA, * ), B( LDB, * ), WORK( * )
*     ..
*
*  Purpose
*  =======
*
*  ZGELSY computes the minimum-norm solution to a complex linear least
*  squares problem:
*      minimize || A * X - B ||
*  using a complete orthogonal factorization of A.  A is an M-by-N
*  matrix which may be rank-deficient.
*
*  Several right hand side vectors b and solution vectors x can be
*  handled in a single call; they are stored as the columns of the
*  M-by-NRHS right hand side matrix B and the N-by-NRHS solution
*  matrix X.
*
*  The routine first computes a QR factorization with column pivoting:
*      A * P = Q * [ R11 R12 ]
*                  [  0  R22 ]
*  with R11 defined as the largest leading submatrix whose estimated
*  condition number is less than 1/RCOND.  The order of R11, RANK,
*  is the effective rank of A.
*
*  Then, R22 is considered to be negligible, and R12 is annihilated
*  by unitary transformations from the right, arriving at the
*  complete orthogonal factorization:
*     A * P = Q * [ T11 0 ] * Z
*                 [  0  0 ]
*  The minimum-norm solution is then
*     X = P * Z' [ inv(T11)*Q1'*B ]
*                [        0       ]
*  where Q1 consists of the first RANK columns of Q.
*
*  This routine is basically identical to the original xGELSX except
*  three differences:
*    o The permutation of matrix B (the right hand side) is faster and
*      more simple.
*    o The call to the subroutine xGEQPF has been substituted by the
*      the call to the subroutine xGEQP3. This subroutine is a Blas-3
*      version of the QR factorization with column pivoting.
*    o Matrix B (the right hand side) is updated with Blas-3.
*
*  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.
*
*  NRHS    (input) INTEGER
*          The number of right hand sides, i.e., the number of
*          columns of matrices B and X. NRHS >= 0.
*
*  A       (input/output) COMPLEX*16 array, dimension (LDA,N)
*          On entry, the M-by-N matrix A.
*          On exit, A has been overwritten by details of its
*          complete orthogonal factorization.
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A.  LDA >= max(1,M).
*
*  B       (input/output) COMPLEX*16 array, dimension (LDB,NRHS)
*          On entry, the M-by-NRHS right hand side matrix B.
*          On exit, the N-by-NRHS solution matrix X.
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B. LDB >= max(1,M,N).
*
*  JPVT    (input/output) INTEGER array, dimension (N)
*          On entry, if JPVT(i) .ne. 0, the i-th column of A is permuted
*          to the front of AP, otherwise column i is a free column.
*          On exit, if JPVT(i) = k, then the i-th column of A*P
*          was the k-th column of A.
*
*  RCOND   (input) DOUBLE PRECISION
*          RCOND is used to determine the effective rank of A, which
*          is defined as the order of the largest leading triangular
*          submatrix R11 in the QR factorization with pivoting of A,
*          whose estimated condition number < 1/RCOND.
*
*  RANK    (output) INTEGER
*          The effective rank of A, i.e., the order of the submatrix
*          R11.  This is the same as the order of the submatrix T11
*          in the complete orthogonal factorization of A.
*
*  WORK    (workspace/output) COMPLEX*16 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.
*          The unblocked strategy requires that:
*            LWORK >= MN + MAX( 2*MN, N+1, MN+NRHS )
*          where MN = min(M,N).
*          The block algorithm requires that:
*            LWORK >= MN + MAX( 2*MN, NB*(N+1), MN+MN*NB, MN+NB*NRHS )
*          where NB is an upper bound on the blocksize returned
*          by ILAENV for the routines ZGEQP3, ZTZRZF, CTZRQF, ZUNMQR,
*          and ZUNMRZ.
*
*          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.
*
*  RWORK   (workspace) DOUBLE PRECISION array, dimension (2*N)
*
*  INFO    (output) INTEGER
*          = 0: successful exit
*          < 0: if INFO = -i, the i-th argument had an illegal value
*
*  Further Details
*  ===============
*
*  Based on contributions by
*    A. Petitet, Computer Science Dept., Univ. of Tenn., Knoxville, USA
*    E. Quintana-Orti, Depto. de Informatica, Universidad Jaime I, Spain
*    G. Quintana-Orti, Depto. de Informatica, Universidad Jaime I, Spain
*
*  =====================================================================
*
*     .. Parameters ..
      INTEGER            IMAX, IMIN
      PARAMETER          ( IMAX = 1, IMIN = 2 )
      DOUBLE PRECISION   ZERO, ONE
      PARAMETER          ( ZERO = 0.0D+0, ONE = 1.0D+0 )
      COMPLEX*16         CZERO, CONE
      PARAMETER          ( CZERO = ( 0.0D+0, 0.0D+0 ),
     $                   CONE = ( 1.0D+0, 0.0D+0 ) )
*     ..
*     .. Local Scalars ..
      LOGICAL            LQUERY
      INTEGER            I, IASCL, IBSCL, ISMAX, ISMIN, J, LWKOPT, MN,
     $                   NB, NB1, NB2, NB3, NB4
      DOUBLE PRECISION   ANRM, BIGNUM, BNRM, SMAX, SMAXPR, SMIN, SMINPR,
     $                   SMLNUM, WSIZE
      COMPLEX*16         C1, C2, S1, S2
*     ..
*     .. External Subroutines ..
      EXTERNAL           DLABAD, XERBLA, ZCOPY, ZGEQP3, ZLAIC1, ZLASCL,
     $                   ZLASET, ZTRSM, ZTZRZF, ZUNMQR, ZUNMRZ
*     ..
*     .. External Functions ..
      INTEGER            ILAENV
      DOUBLE PRECISION   DLAMCH, ZLANGE
      EXTERNAL           ILAENV, DLAMCH, ZLANGE
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          ABS, DBLE, DCMPLX, MAX, MIN
*     ..
*     .. Executable Statements ..
*
      MN = MIN( M, N )
      ISMIN = MN + 1
      ISMAX = 2*MN + 1
*
*     Test the input arguments.
*
      INFO = 0
      NB1 = ILAENV( 1, 'ZGEQRF', ' ', M, N, -1, -1 )
      NB2 = ILAENV( 1, 'ZGERQF', ' ', M, N, -1, -1 )
      NB3 = ILAENV( 1, 'ZUNMQR', ' ', M, N, NRHS, -1 )
      NB4 = ILAENV( 1, 'ZUNMRQ', ' ', M, N, NRHS, -1 )
      NB = MAX( NB1, NB2, NB3, NB4 )
      LWKOPT = MAX( 1, MN+2*N+NB*( N+1 ), 2*MN+NB*NRHS )
      WORK( 1 ) = DCMPLX( LWKOPT )
      LQUERY = ( LWORK.EQ.-1 )
      IF( M.LT.0 ) THEN
         INFO = -1
      ELSE IF( N.LT.0 ) THEN
         INFO = -2
      ELSE IF( NRHS.LT.0 ) THEN
         INFO = -3
      ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
         INFO = -5
      ELSE IF( LDB.LT.MAX( 1, M, N ) ) THEN
         INFO = -7
      ELSE IF( LWORK.LT.( MN+MAX( 2*MN, N+1, MN+NRHS ) ) .AND. .NOT.
     $         LQUERY ) THEN
         INFO = -12
      END IF
*
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'ZGELSY', -INFO )
         RETURN
      ELSE IF( LQUERY ) THEN
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( MIN( M, N, NRHS ).EQ.0 ) THEN
         RANK = 0
         RETURN
      END IF
*
*     Get machine parameters
*
      SMLNUM = DLAMCH( 'S' ) / DLAMCH( 'P' )
      BIGNUM = ONE / SMLNUM
      CALL DLABAD( SMLNUM, BIGNUM )
*
*     Scale A, B if max entries outside range [SMLNUM,BIGNUM]
*
      ANRM = ZLANGE( 'M', M, N, A, LDA, RWORK )
      IASCL = 0
      IF( ANRM.GT.ZERO .AND. ANRM.LT.SMLNUM ) THEN
*
*        Scale matrix norm up to SMLNUM
*
         CALL ZLASCL( 'G', 0, 0, ANRM, SMLNUM, M, N, A, LDA, INFO )
         IASCL = 1
      ELSE IF( ANRM.GT.BIGNUM ) THEN
*
*        Scale matrix norm down to BIGNUM
*
         CALL ZLASCL( 'G', 0, 0, ANRM, BIGNUM, M, N, A, LDA, INFO )
         IASCL = 2
      ELSE IF( ANRM.EQ.ZERO ) THEN
*
*        Matrix all zero. Return zero solution.
*
         CALL ZLASET( 'F', MAX( M, N ), NRHS, CZERO, CZERO, B, LDB )
         RANK = 0
         GO TO 70
      END IF
*
      BNRM = ZLANGE( 'M', M, NRHS, B, LDB, RWORK )
      IBSCL = 0
      IF( BNRM.GT.ZERO .AND. BNRM.LT.SMLNUM ) THEN
*
*        Scale matrix norm up to SMLNUM
*
         CALL ZLASCL( 'G', 0, 0, BNRM, SMLNUM, M, NRHS, B, LDB, INFO )
         IBSCL = 1
      ELSE IF( BNRM.GT.BIGNUM ) THEN
*
*        Scale matrix norm down to BIGNUM
*
         CALL ZLASCL( 'G', 0, 0, BNRM, BIGNUM, M, NRHS, B, LDB, INFO )
         IBSCL = 2
      END IF
*
*     Compute QR factorization with column pivoting of A:
*        A * P = Q * R
*
      CALL ZGEQP3( M, N, A, LDA, JPVT, WORK( 1 ), WORK( MN+1 ),
     $             LWORK-MN, RWORK, INFO )
      WSIZE = MN + DBLE( WORK( MN+1 ) )
*
*     complex workspace: MN+NB*(N+1). real workspace 2*N.
*     Details of Householder rotations stored in WORK(1:MN).
*
*     Determine RANK using incremental condition estimation
*
      WORK( ISMIN ) = CONE
      WORK( ISMAX ) = CONE
      SMAX = ABS( A( 1, 1 ) )
      SMIN = SMAX
      IF( ABS( A( 1, 1 ) ).EQ.ZERO ) THEN
         RANK = 0
         CALL ZLASET( 'F', MAX( M, N ), NRHS, CZERO, CZERO, B, LDB )
         GO TO 70
      ELSE
         RANK = 1
      END IF
*
   10 CONTINUE
      IF( RANK.LT.MN ) THEN
         I = RANK + 1
         CALL ZLAIC1( IMIN, RANK, WORK( ISMIN ), SMIN, A( 1, I ),
     $                A( I, I ), SMINPR, S1, C1 )
         CALL ZLAIC1( IMAX, RANK, WORK( ISMAX ), SMAX, A( 1, I ),
     $                A( I, I ), SMAXPR, S2, C2 )
*
         IF( SMAXPR*RCOND.LE.SMINPR ) THEN
            DO 20 I = 1, RANK
               WORK( ISMIN+I-1 ) = S1*WORK( ISMIN+I-1 )
               WORK( ISMAX+I-1 ) = S2*WORK( ISMAX+I-1 )
   20       CONTINUE
            WORK( ISMIN+RANK ) = C1
            WORK( ISMAX+RANK ) = C2
            SMIN = SMINPR
            SMAX = SMAXPR
            RANK = RANK + 1
            GO TO 10
         END IF
      END IF
*
*     complex workspace: 3*MN.
*
*     Logically partition R = [ R11 R12 ]
*                             [  0  R22 ]
*     where R11 = R(1:RANK,1:RANK)
*
*     [R11,R12] = [ T11, 0 ] * Y
*
      IF( RANK.LT.N )
     $   CALL ZTZRZF( RANK, N, A, LDA, WORK( MN+1 ), WORK( 2*MN+1 ),
     $                LWORK-2*MN, INFO )
*
*     complex workspace: 2*MN.
*     Details of Householder rotations stored in WORK(MN+1:2*MN)
*
*     B(1:M,1:NRHS) := Q' * B(1:M,1:NRHS)
*
      CALL ZUNMQR( 'Left', 'Conjugate transpose', M, NRHS, MN, A, LDA,
     $             WORK( 1 ), B, LDB, WORK( 2*MN+1 ), LWORK-2*MN, INFO )
      WSIZE = MAX( WSIZE, 2*MN+DBLE( WORK( 2*MN+1 ) ) )
*
*     complex workspace: 2*MN+NB*NRHS.
*
*     B(1:RANK,1:NRHS) := inv(T11) * B(1:RANK,1:NRHS)
*
      CALL ZTRSM( 'Left', 'Upper', 'No transpose', 'Non-unit', RANK,
     $            NRHS, CONE, A, LDA, B, LDB )
*
      DO 40 J = 1, NRHS
         DO 30 I = RANK + 1, N
            B( I, J ) = CZERO
   30    CONTINUE
   40 CONTINUE
*
*     B(1:N,1:NRHS) := Y' * B(1:N,1:NRHS)
*
      IF( RANK.LT.N ) THEN
         CALL ZUNMRZ( 'Left', 'Conjugate transpose', N, NRHS, RANK,
     $                N-RANK, A, LDA, WORK( MN+1 ), B, LDB,
     $                WORK( 2*MN+1 ), LWORK-2*MN, INFO )
      END IF
*
*     complex workspace: 2*MN+NRHS.
*
*     B(1:N,1:NRHS) := P * B(1:N,1:NRHS)
*
      DO 60 J = 1, NRHS
         DO 50 I = 1, N
            WORK( JPVT( I ) ) = B( I, J )
   50    CONTINUE
         CALL ZCOPY( N, WORK( 1 ), 1, B( 1, J ), 1 )
   60 CONTINUE
*
*     complex workspace: N.
*
*     Undo scaling
*
      IF( IASCL.EQ.1 ) THEN
         CALL ZLASCL( 'G', 0, 0, ANRM, SMLNUM, N, NRHS, B, LDB, INFO )
         CALL ZLASCL( 'U', 0, 0, SMLNUM, ANRM, RANK, RANK, A, LDA,
     $                INFO )
      ELSE IF( IASCL.EQ.2 ) THEN
         CALL ZLASCL( 'G', 0, 0, ANRM, BIGNUM, N, NRHS, B, LDB, INFO )
         CALL ZLASCL( 'U', 0, 0, BIGNUM, ANRM, RANK, RANK, A, LDA,
     $                INFO )
      END IF
      IF( IBSCL.EQ.1 ) THEN
         CALL ZLASCL( 'G', 0, 0, SMLNUM, BNRM, N, NRHS, B, LDB, INFO )
      ELSE IF( IBSCL.EQ.2 ) THEN
         CALL ZLASCL( 'G', 0, 0, BIGNUM, BNRM, N, NRHS, B, LDB, INFO )
      END IF
*
   70 CONTINUE
      WORK( 1 ) = DCMPLX( LWKOPT )
*
      RETURN
*
*     End of ZGELSY
*
      END
