LAPACK 3.3.1
Linear Algebra PACKage

spstrf.f

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00001       SUBROUTINE SPSTRF( UPLO, N, A, LDA, PIV, RANK, TOL, WORK, INFO )
00002 *
00003 *  -- LAPACK routine (version 3.2.2) --
00004 *     Craig Lucas, University of Manchester / NAG Ltd.
00005 *     October, 2008
00006 *
00007 *     .. Scalar Arguments ..
00008       REAL               TOL
00009       INTEGER            INFO, LDA, N, RANK
00010       CHARACTER          UPLO
00011 *     ..
00012 *     .. Array Arguments ..
00013       REAL               A( LDA, * ), WORK( 2*N )
00014       INTEGER            PIV( N )
00015 *     ..
00016 *
00017 *  Purpose
00018 *  =======
00019 *
00020 *  SPSTRF computes the Cholesky factorization with complete
00021 *  pivoting of a real symmetric positive semidefinite matrix A.
00022 *
00023 *  The factorization has the form
00024 *     P**T * A * P = U**T * U ,  if UPLO = 'U',
00025 *     P**T * A * P = L  * L**T,  if UPLO = 'L',
00026 *  where U is an upper triangular matrix and L is lower triangular, and
00027 *  P is stored as vector PIV.
00028 *
00029 *  This algorithm does not attempt to check that A is positive
00030 *  semidefinite. This version of the algorithm calls level 3 BLAS.
00031 *
00032 *  Arguments
00033 *  =========
00034 *
00035 *  UPLO    (input) CHARACTER*1
00036 *          Specifies whether the upper or lower triangular part of the
00037 *          symmetric matrix A is stored.
00038 *          = 'U':  Upper triangular
00039 *          = 'L':  Lower triangular
00040 *
00041 *  N       (input) INTEGER
00042 *          The order of the matrix A.  N >= 0.
00043 *
00044 *  A       (input/output) REAL array, dimension (LDA,N)
00045 *          On entry, the symmetric matrix A.  If UPLO = 'U', the leading
00046 *          n by n upper triangular part of A contains the upper
00047 *          triangular part of the matrix A, and the strictly lower
00048 *          triangular part of A is not referenced.  If UPLO = 'L', the
00049 *          leading n by n lower triangular part of A contains the lower
00050 *          triangular part of the matrix A, and the strictly upper
00051 *          triangular part of A is not referenced.
00052 *
00053 *          On exit, if INFO = 0, the factor U or L from the Cholesky
00054 *          factorization as above.
00055 *
00056 *  LDA     (input) INTEGER
00057 *          The leading dimension of the array A.  LDA >= max(1,N).
00058 *
00059 *  PIV     (output) INTEGER array, dimension (N)
00060 *          PIV is such that the nonzero entries are P( PIV(K), K ) = 1.
00061 *
00062 *  RANK    (output) INTEGER
00063 *          The rank of A given by the number of steps the algorithm
00064 *          completed.
00065 *
00066 *  TOL     (input) REAL
00067 *          User defined tolerance. If TOL < 0, then N*U*MAX( A(K,K) )
00068 *          will be used. The algorithm terminates at the (K-1)st step
00069 *          if the pivot <= TOL.
00070 *
00071 *  WORK    (workspace) REAL array, dimension (2*N)
00072 *          Work space.
00073 *
00074 *  INFO    (output) INTEGER
00075 *          < 0: If INFO = -K, the K-th argument had an illegal value,
00076 *          = 0: algorithm completed successfully, and
00077 *          > 0: the matrix A is either rank deficient with computed rank
00078 *               as returned in RANK, or is indefinite.  See Section 7 of
00079 *               LAPACK Working Note #161 for further information.
00080 *
00081 *  =====================================================================
00082 *
00083 *     .. Parameters ..
00084       REAL               ONE, ZERO
00085       PARAMETER          ( ONE = 1.0E+0, ZERO = 0.0E+0 )
00086 *     ..
00087 *     .. Local Scalars ..
00088       REAL               AJJ, SSTOP, STEMP
00089       INTEGER            I, ITEMP, J, JB, K, NB, PVT
00090       LOGICAL            UPPER
00091 *     ..
00092 *     .. External Functions ..
00093       REAL               SLAMCH
00094       INTEGER            ILAENV
00095       LOGICAL            LSAME, SISNAN
00096       EXTERNAL           SLAMCH, ILAENV, LSAME, SISNAN
00097 *     ..
00098 *     .. External Subroutines ..
00099       EXTERNAL           SGEMV, SPSTF2, SSCAL, SSWAP, SSYRK, XERBLA
00100 *     ..
00101 *     .. Intrinsic Functions ..
00102       INTRINSIC          MAX, MIN, SQRT, MAXLOC
00103 *     ..
00104 *     .. Executable Statements ..
00105 *
00106 *     Test the input parameters.
00107 *
00108       INFO = 0
00109       UPPER = LSAME( UPLO, 'U' )
00110       IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
00111          INFO = -1
00112       ELSE IF( N.LT.0 ) THEN
00113          INFO = -2
00114       ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
00115          INFO = -4
00116       END IF
00117       IF( INFO.NE.0 ) THEN
00118          CALL XERBLA( 'SPSTRF', -INFO )
00119          RETURN
00120       END IF
00121 *
00122 *     Quick return if possible
00123 *
00124       IF( N.EQ.0 )
00125      $   RETURN
00126 *
00127 *     Get block size
00128 *
00129       NB = ILAENV( 1, 'SPOTRF', UPLO, N, -1, -1, -1 )
00130       IF( NB.LE.1 .OR. NB.GE.N ) THEN
00131 *
00132 *        Use unblocked code
00133 *
00134          CALL SPSTF2( UPLO, N, A( 1, 1 ), LDA, PIV, RANK, TOL, WORK,
00135      $                INFO )
00136          GO TO 200
00137 *
00138       ELSE
00139 *
00140 *     Initialize PIV
00141 *
00142          DO 100 I = 1, N
00143             PIV( I ) = I
00144   100    CONTINUE
00145 *
00146 *     Compute stopping value
00147 *
00148          PVT = 1
00149          AJJ = A( PVT, PVT )
00150          DO I = 2, N
00151             IF( A( I, I ).GT.AJJ ) THEN
00152                PVT = I
00153                AJJ = A( PVT, PVT )
00154             END IF
00155          END DO
00156          IF( AJJ.EQ.ZERO.OR.SISNAN( AJJ ) ) THEN
00157             RANK = 0
00158             INFO = 1
00159             GO TO 200
00160          END IF
00161 *
00162 *     Compute stopping value if not supplied
00163 *
00164          IF( TOL.LT.ZERO ) THEN
00165             SSTOP = N * SLAMCH( 'Epsilon' ) * AJJ
00166          ELSE
00167             SSTOP = TOL
00168          END IF
00169 *
00170 *
00171          IF( UPPER ) THEN
00172 *
00173 *           Compute the Cholesky factorization P**T * A * P = U**T * U
00174 *
00175             DO 140 K = 1, N, NB
00176 *
00177 *              Account for last block not being NB wide
00178 *
00179                JB = MIN( NB, N-K+1 )
00180 *
00181 *              Set relevant part of first half of WORK to zero,
00182 *              holds dot products
00183 *
00184                DO 110 I = K, N
00185                   WORK( I ) = 0
00186   110          CONTINUE
00187 *
00188                DO 130 J = K, K + JB - 1
00189 *
00190 *              Find pivot, test for exit, else swap rows and columns
00191 *              Update dot products, compute possible pivots which are
00192 *              stored in the second half of WORK
00193 *
00194                   DO 120 I = J, N
00195 *
00196                      IF( J.GT.K ) THEN
00197                         WORK( I ) = WORK( I ) + A( J-1, I )**2
00198                      END IF
00199                      WORK( N+I ) = A( I, I ) - WORK( I )
00200 *
00201   120             CONTINUE
00202 *
00203                   IF( J.GT.1 ) THEN
00204                      ITEMP = MAXLOC( WORK( (N+J):(2*N) ), 1 )
00205                      PVT = ITEMP + J - 1
00206                      AJJ = WORK( N+PVT )
00207                      IF( AJJ.LE.SSTOP.OR.SISNAN( AJJ ) ) THEN
00208                         A( J, J ) = AJJ
00209                         GO TO 190
00210                      END IF
00211                   END IF
00212 *
00213                   IF( J.NE.PVT ) THEN
00214 *
00215 *                    Pivot OK, so can now swap pivot rows and columns
00216 *
00217                      A( PVT, PVT ) = A( J, J )
00218                      CALL SSWAP( J-1, A( 1, J ), 1, A( 1, PVT ), 1 )
00219                      IF( PVT.LT.N )
00220      $                  CALL SSWAP( N-PVT, A( J, PVT+1 ), LDA,
00221      $                              A( PVT, PVT+1 ), LDA )
00222                      CALL SSWAP( PVT-J-1, A( J, J+1 ), LDA,
00223      $                           A( J+1, PVT ), 1 )
00224 *
00225 *                    Swap dot products and PIV
00226 *
00227                      STEMP = WORK( J )
00228                      WORK( J ) = WORK( PVT )
00229                      WORK( PVT ) = STEMP
00230                      ITEMP = PIV( PVT )
00231                      PIV( PVT ) = PIV( J )
00232                      PIV( J ) = ITEMP
00233                   END IF
00234 *
00235                   AJJ = SQRT( AJJ )
00236                   A( J, J ) = AJJ
00237 *
00238 *                 Compute elements J+1:N of row J.
00239 *
00240                   IF( J.LT.N ) THEN
00241                      CALL SGEMV( 'Trans', J-K, N-J, -ONE, A( K, J+1 ),
00242      $                           LDA, A( K, J ), 1, ONE, A( J, J+1 ),
00243      $                           LDA )
00244                      CALL SSCAL( N-J, ONE / AJJ, A( J, J+1 ), LDA )
00245                   END IF
00246 *
00247   130          CONTINUE
00248 *
00249 *              Update trailing matrix, J already incremented
00250 *
00251                IF( K+JB.LE.N ) THEN
00252                   CALL SSYRK( 'Upper', 'Trans', N-J+1, JB, -ONE,
00253      $                        A( K, J ), LDA, ONE, A( J, J ), LDA )
00254                END IF
00255 *
00256   140       CONTINUE
00257 *
00258          ELSE
00259 *
00260 *        Compute the Cholesky factorization P**T * A * P = L * L**T
00261 *
00262             DO 180 K = 1, N, NB
00263 *
00264 *              Account for last block not being NB wide
00265 *
00266                JB = MIN( NB, N-K+1 )
00267 *
00268 *              Set relevant part of first half of WORK to zero,
00269 *              holds dot products
00270 *
00271                DO 150 I = K, N
00272                   WORK( I ) = 0
00273   150          CONTINUE
00274 *
00275                DO 170 J = K, K + JB - 1
00276 *
00277 *              Find pivot, test for exit, else swap rows and columns
00278 *              Update dot products, compute possible pivots which are
00279 *              stored in the second half of WORK
00280 *
00281                   DO 160 I = J, N
00282 *
00283                      IF( J.GT.K ) THEN
00284                         WORK( I ) = WORK( I ) + A( I, J-1 )**2
00285                      END IF
00286                      WORK( N+I ) = A( I, I ) - WORK( I )
00287 *
00288   160             CONTINUE
00289 *
00290                   IF( J.GT.1 ) THEN
00291                      ITEMP = MAXLOC( WORK( (N+J):(2*N) ), 1 )
00292                      PVT = ITEMP + J - 1
00293                      AJJ = WORK( N+PVT )
00294                      IF( AJJ.LE.SSTOP.OR.SISNAN( AJJ ) ) THEN
00295                         A( J, J ) = AJJ
00296                         GO TO 190
00297                      END IF
00298                   END IF
00299 *
00300                   IF( J.NE.PVT ) THEN
00301 *
00302 *                    Pivot OK, so can now swap pivot rows and columns
00303 *
00304                      A( PVT, PVT ) = A( J, J )
00305                      CALL SSWAP( J-1, A( J, 1 ), LDA, A( PVT, 1 ), LDA )
00306                      IF( PVT.LT.N )
00307      $                  CALL SSWAP( N-PVT, A( PVT+1, J ), 1,
00308      $                              A( PVT+1, PVT ), 1 )
00309                      CALL SSWAP( PVT-J-1, A( J+1, J ), 1, A( PVT, J+1 ),
00310      $                           LDA )
00311 *
00312 *                    Swap dot products and PIV
00313 *
00314                      STEMP = WORK( J )
00315                      WORK( J ) = WORK( PVT )
00316                      WORK( PVT ) = STEMP
00317                      ITEMP = PIV( PVT )
00318                      PIV( PVT ) = PIV( J )
00319                      PIV( J ) = ITEMP
00320                   END IF
00321 *
00322                   AJJ = SQRT( AJJ )
00323                   A( J, J ) = AJJ
00324 *
00325 *                 Compute elements J+1:N of column J.
00326 *
00327                   IF( J.LT.N ) THEN
00328                      CALL SGEMV( 'No Trans', N-J, J-K, -ONE,
00329      $                           A( J+1, K ), LDA, A( J, K ), LDA, ONE,
00330      $                           A( J+1, J ), 1 )
00331                      CALL SSCAL( N-J, ONE / AJJ, A( J+1, J ), 1 )
00332                   END IF
00333 *
00334   170          CONTINUE
00335 *
00336 *              Update trailing matrix, J already incremented
00337 *
00338                IF( K+JB.LE.N ) THEN
00339                   CALL SSYRK( 'Lower', 'No Trans', N-J+1, JB, -ONE,
00340      $                        A( J, K ), LDA, ONE, A( J, J ), LDA )
00341                END IF
00342 *
00343   180       CONTINUE
00344 *
00345          END IF
00346       END IF
00347 *
00348 *     Ran to completion, A has full rank
00349 *
00350       RANK = N
00351 *
00352       GO TO 200
00353   190 CONTINUE
00354 *
00355 *     Rank is the number of steps completed.  Set INFO = 1 to signal
00356 *     that the factorization cannot be used to solve a system.
00357 *
00358       RANK = J - 1
00359       INFO = 1
00360 *
00361   200 CONTINUE
00362       RETURN
00363 *
00364 *     End of SPSTRF
00365 *
00366       END
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