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SUBROUTINE SLAPST( ID, N, D, INDX, INFO )
*
* -- ScaLAPACK auxiliary routine (version 1.7) --
* University of Tennessee, Knoxville, Oak Ridge National Laboratory,
* and University of California, Berkeley.
* December 31, 1998
*
* .. Scalar Arguments ..
CHARACTER ID
INTEGER INFO, N
* ..
* .. Array Arguments ..
INTEGER INDX( * )
REAL D( * )
* ..
*
* Purpose
* =======
* SLAPST is a modified version of the LAPACK routine SLASRT.
*
* Define a permutation INDX that sorts the numbers in D
* in increasing order (if ID = 'I') or
* in decreasing order (if ID = 'D' ).
*
* Use Quick Sort, reverting to Insertion sort on arrays of
* size <= 20. Dimension of STACK limits N to about 2**32.
*
* Arguments
* =========
*
* ID (input) CHARACTER*1
* = 'I': sort D in increasing order;
* = 'D': sort D in decreasing order.
*
* N (input) INTEGER
* The length of the array D.
*
* D (input) REAL array, dimension (N)
* The array to be sorted.
*
* INDX (ouput) INTEGER array, dimension (N).
* The permutation which sorts the array D.
*
* INFO (output) INTEGER
* = 0: successful exit
* < 0: if INFO = -i, the i-th argument had an illegal value
*
* =====================================================================
*
* .. Parameters ..
INTEGER SELECT
PARAMETER ( SELECT = 20 )
* ..
* .. Local Scalars ..
INTEGER DIR, ENDD, I, ITMP, J, START, STKPNT
REAL D1, D2, D3, DMNMX
* ..
* .. Local Arrays ..
INTEGER STACK( 2, 32 )
* ..
* .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
* ..
* .. External Subroutines ..
EXTERNAL XERBLA
* ..
* .. Executable Statements ..
*
* Test the input paramters.
*
INFO = 0
DIR = -1
IF( LSAME( ID, 'D' ) ) THEN
DIR = 0
ELSE IF( LSAME( ID, 'I' ) ) THEN
DIR = 1
END IF
IF( DIR.EQ.-1 ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'SLAPST', -INFO )
RETURN
END IF
*
* Quick return if possible
*
IF( N.LE.1 )
$ RETURN
*
DO 10 I = 1, N
INDX( I ) = I
10 CONTINUE
*
STKPNT = 1
STACK( 1, 1 ) = 1
STACK( 2, 1 ) = N
20 CONTINUE
START = STACK( 1, STKPNT )
ENDD = STACK( 2, STKPNT )
STKPNT = STKPNT - 1
IF( ENDD-START.LE.SELECT .AND. ENDD-START.GT.0 ) THEN
*
* Do Insertion sort on D( START:ENDD )
*
IF( DIR.EQ.0 ) THEN
*
* Sort into decreasing order
*
DO 40 I = START + 1, ENDD
DO 30 J = I, START + 1, -1
IF( D( INDX( J ) ).GT.D( INDX( J-1 ) ) ) THEN
ITMP = INDX( J )
INDX( J ) = INDX( J-1 )
INDX( J-1 ) = ITMP
ELSE
GO TO 40
END IF
30 CONTINUE
40 CONTINUE
*
ELSE
*
* Sort into increasing order
*
DO 60 I = START + 1, ENDD
DO 50 J = I, START + 1, -1
IF( D( INDX( J ) ).LT.D( INDX( J-1 ) ) ) THEN
ITMP = INDX( J )
INDX( J ) = INDX( J-1 )
INDX( J-1 ) = ITMP
ELSE
GO TO 60
END IF
50 CONTINUE
60 CONTINUE
*
END IF
*
ELSE IF( ENDD-START.GT.SELECT ) THEN
*
* Partition D( START:ENDD ) and stack parts, largest one first
*
* Choose partition entry as median of 3
*
D1 = D( INDX( START ) )
D2 = D( INDX( ENDD ) )
I = ( START+ENDD ) / 2
D3 = D( INDX( I ) )
IF( D1.LT.D2 ) THEN
IF( D3.LT.D1 ) THEN
DMNMX = D1
ELSE IF( D3.LT.D2 ) THEN
DMNMX = D3
ELSE
DMNMX = D2
END IF
ELSE
IF( D3.LT.D2 ) THEN
DMNMX = D2
ELSE IF( D3.LT.D1 ) THEN
DMNMX = D3
ELSE
DMNMX = D1
END IF
END IF
*
IF( DIR.EQ.0 ) THEN
*
* Sort into decreasing order
*
I = START - 1
J = ENDD + 1
70 CONTINUE
80 CONTINUE
J = J - 1
IF( D( INDX( J ) ).LT.DMNMX )
$ GO TO 80
90 CONTINUE
I = I + 1
IF( D( INDX( I ) ).GT.DMNMX )
$ GO TO 90
IF( I.LT.J ) THEN
ITMP = INDX( I )
INDX( I ) = INDX( J )
INDX( J ) = ITMP
GO TO 70
END IF
IF( J-START.GT.ENDD-J-1 ) THEN
STKPNT = STKPNT + 1
STACK( 1, STKPNT ) = START
STACK( 2, STKPNT ) = J
STKPNT = STKPNT + 1
STACK( 1, STKPNT ) = J + 1
STACK( 2, STKPNT ) = ENDD
ELSE
STKPNT = STKPNT + 1
STACK( 1, STKPNT ) = J + 1
STACK( 2, STKPNT ) = ENDD
STKPNT = STKPNT + 1
STACK( 1, STKPNT ) = START
STACK( 2, STKPNT ) = J
END IF
ELSE
*
* Sort into increasing order
*
I = START - 1
J = ENDD + 1
100 CONTINUE
110 CONTINUE
J = J - 1
IF( D( INDX( J ) ).GT.DMNMX )
$ GO TO 110
120 CONTINUE
I = I + 1
IF( D( INDX( I ) ).LT.DMNMX )
$ GO TO 120
IF( I.LT.J ) THEN
ITMP = INDX( I )
INDX( I ) = INDX( J )
INDX( J ) = ITMP
GO TO 100
END IF
IF( J-START.GT.ENDD-J-1 ) THEN
STKPNT = STKPNT + 1
STACK( 1, STKPNT ) = START
STACK( 2, STKPNT ) = J
STKPNT = STKPNT + 1
STACK( 1, STKPNT ) = J + 1
STACK( 2, STKPNT ) = ENDD
ELSE
STKPNT = STKPNT + 1
STACK( 1, STKPNT ) = J + 1
STACK( 2, STKPNT ) = ENDD
STKPNT = STKPNT + 1
STACK( 1, STKPNT ) = START
STACK( 2, STKPNT ) = J
END IF
END IF
END IF
IF( STKPNT.GT.0 )
$ GO TO 20
RETURN
*
* End of SLAPST
*
END
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