SRC/sla_syrpvgrw.f

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      REAL FUNCTION SLA_SYRPVGRW( UPLO, N, INFO, A, LDA, AF, LDAF, IPIV,
     $                            WORK )
*
*     -- LAPACK routine (version 3.2.2)                                 --
*     -- Contributed by James Demmel, Deaglan Halligan, Yozo Hida and --
*     -- Jason Riedy of Univ. of California Berkeley.                 --
*     -- June 2010                                                    --
*
*     -- LAPACK is a software package provided by Univ. of Tennessee, --
*     -- Univ. of California Berkeley and NAG Ltd.                    --
*
      IMPLICIT NONE
*     ..
*     .. Scalar Arguments ..
      CHARACTER*1        UPLO
      INTEGER            N, INFO, LDA, LDAF
*     ..
*     .. Array Arguments ..
      INTEGER            IPIV( * )
      REAL               A( LDA, * ), AF( LDAF, * ), WORK( * )
*     ..
*
*  Purpose
*  =======
* 
*  SLA_SYRPVGRW computes the reciprocal pivot growth factor
*  norm(A)/norm(U). The "max absolute element" norm is used. If this is
*  much less than 1, the stability of the LU factorization of the
*  (equilibrated) matrix A could be poor. This also means that the
*  solution X, estimated condition numbers, and error bounds could be
*  unreliable.
*
*  Arguments
*  =========
*
*     UPLO    (input) CHARACTER*1
*       = 'U':  Upper triangle of A is stored;
*       = 'L':  Lower triangle of A is stored.
*
*     N       (input) INTEGER
*     The number of linear equations, i.e., the order of the
*     matrix A.  N >= 0.
*
*     INFO    (input) INTEGER
*     The value of INFO returned from SSYTRF, .i.e., the pivot in
*     column INFO is exactly 0.
*
*     NCOLS   (input) INTEGER
*     The number of columns of the matrix A. NCOLS >= 0.
*
*     A       (input) REAL array, dimension (LDA,N)
*     On entry, the N-by-N matrix A.
*
*     LDA     (input) INTEGER
*     The leading dimension of the array A.  LDA >= max(1,N).
*
*     AF      (input) REAL array, dimension (LDAF,N)
*     The block diagonal matrix D and the multipliers used to
*     obtain the factor U or L as computed by SSYTRF.
*
*     LDAF    (input) INTEGER
*     The leading dimension of the array AF.  LDAF >= max(1,N).
*
*     IPIV    (input) INTEGER array, dimension (N)
*     Details of the interchanges and the block structure of D
*     as determined by SSYTRF.
*
*     WORK    (input) REAL array, dimension (2*N)
*
*  =====================================================================
*
*     .. Local Scalars ..
      INTEGER            NCOLS, I, J, K, KP
      REAL               AMAX, UMAX, RPVGRW, TMP
      LOGICAL            UPPER
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          ABS, MAX, MIN
*     ..
*     .. External Functions ..
      EXTERNAL           LSAME, SLASET
      LOGICAL            LSAME
*     ..
*     .. Executable Statements ..
*
      UPPER = LSAME( 'Upper', UPLO )
      IF ( INFO.EQ.0 ) THEN
         IF ( UPPER ) THEN
            NCOLS = 1
         ELSE
            NCOLS = N
         END IF
      ELSE
         NCOLS = INFO
      END IF

      RPVGRW = 1.0
      DO I = 1, 2*N
         WORK( I ) = 0.0
      END DO
*
*     Find the max magnitude entry of each column of A.  Compute the max
*     for all N columns so we can apply the pivot permutation while
*     looping below.  Assume a full factorization is the common case.
*
      IF ( UPPER ) THEN
         DO J = 1, N
            DO I = 1, J
               WORK( N+I ) = MAX( ABS( A( I, J ) ), WORK( N+I ) )
               WORK( N+J ) = MAX( ABS( A( I, J ) ), WORK( N+J ) )
            END DO
         END DO
      ELSE
         DO J = 1, N
            DO I = J, N
               WORK( N+I ) = MAX( ABS( A( I, J ) ), WORK( N+I ) )
               WORK( N+J ) = MAX( ABS( A( I, J ) ), WORK( N+J ) )
            END DO
         END DO
      END IF
*
*     Now find the max magnitude entry of each column of U or L.  Also
*     permute the magnitudes of A above so they're in the same order as
*     the factor.
*
*     The iteration orders and permutations were copied from ssytrs.
*     Calls to SSWAP would be severe overkill.
*
      IF ( UPPER ) THEN
         K = N
         DO WHILE ( K .LT. NCOLS .AND. K.GT.0 )
            IF ( IPIV( K ).GT.0 ) THEN
!              1x1 pivot
               KP = IPIV( K )
               IF ( KP .NE. K ) THEN
                  TMP = WORK( N+K )
                  WORK( N+K ) = WORK( N+KP )
                  WORK( N+KP ) = TMP
               END IF
               DO I = 1, K
                  WORK( K ) = MAX( ABS( AF( I, K ) ), WORK( K ) )
               END DO
               K = K - 1
            ELSE
!              2x2 pivot
               KP = -IPIV( K )
               TMP = WORK( N+K-1 )
               WORK( N+K-1 ) = WORK( N+KP )
               WORK( N+KP ) = TMP
               DO I = 1, K-1
                  WORK( K ) = MAX( ABS( AF( I, K ) ), WORK( K ) )
                  WORK( K-1 ) = MAX( ABS( AF( I, K-1 ) ), WORK( K-1 ) )
               END DO
               WORK( K ) = MAX( ABS( AF( K, K ) ), WORK( K ) )
               K = K - 2
            END IF
         END DO
         K = NCOLS
         DO WHILE ( K .LE. N )
            IF ( IPIV( K ).GT.0 ) THEN
               KP = IPIV( K )
               IF ( KP .NE. K ) THEN
                  TMP = WORK( N+K )
                  WORK( N+K ) = WORK( N+KP )
                  WORK( N+KP ) = TMP
               END IF
               K = K + 1
            ELSE
               KP = -IPIV( K )
               TMP = WORK( N+K )
               WORK( N+K ) = WORK( N+KP )
               WORK( N+KP ) = TMP
               K = K + 2
            END IF
         END DO
      ELSE
         K = 1
         DO WHILE ( K .LE. NCOLS )
            IF ( IPIV( K ).GT.0 ) THEN
!              1x1 pivot
               KP = IPIV( K )
               IF ( KP .NE. K ) THEN
                  TMP = WORK( N+K )
                  WORK( N+K ) = WORK( N+KP )
                  WORK( N+KP ) = TMP
               END IF
               DO I = K, N
                  WORK( K ) = MAX( ABS( AF( I, K ) ), WORK( K ) )
               END DO
               K = K + 1
            ELSE
!              2x2 pivot
               KP = -IPIV( K )
               TMP = WORK( N+K+1 )
               WORK( N+K+1 ) = WORK( N+KP )
               WORK( N+KP ) = TMP
               DO I = K+1, N
                  WORK( K ) = MAX( ABS( AF( I, K ) ), WORK( K ) )
                  WORK( K+1 ) = MAX( ABS( AF(I, K+1 ) ), WORK( K+1 ) )
               END DO
               WORK( K ) = MAX( ABS( AF( K, K ) ), WORK( K ) )
               K = K + 2
            END IF
         END DO
         K = NCOLS
         DO WHILE ( K .GE. 1 )
            IF ( IPIV( K ).GT.0 ) THEN
               KP = IPIV( K )
               IF ( KP .NE. K ) THEN
                  TMP = WORK( N+K )
                  WORK( N+K ) = WORK( N+KP )
                  WORK( N+KP ) = TMP
               END IF
               K = K - 1
            ELSE
               KP = -IPIV( K )
               TMP = WORK( N+K )
               WORK( N+K ) = WORK( N+KP )
               WORK( N+KP ) = TMP
               K = K - 2
            ENDIF
         END DO
      END IF
*
*     Compute the *inverse* of the max element growth factor.  Dividing
*     by zero would imply the largest entry of the factor's column is
*     zero.  Than can happen when either the column of A is zero or
*     massive pivots made the factor underflow to zero.  Neither counts
*     as growth in itself, so simply ignore terms with zero
*     denominators.
*
      IF ( UPPER ) THEN
         DO I = NCOLS, N
            UMAX = WORK( I )
            AMAX = WORK( N+I )
            IF ( UMAX /= 0.0 ) THEN
               RPVGRW = MIN( AMAX / UMAX, RPVGRW )
            END IF
         END DO
      ELSE
         DO I = 1, NCOLS
            UMAX = WORK( I )
            AMAX = WORK( N+I )
            IF ( UMAX /= 0.0 ) THEN
               RPVGRW = MIN( AMAX / UMAX, RPVGRW )
            END IF
         END DO
      END IF

      SLA_SYRPVGRW = RPVGRW
      END