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160 lines
4.5 KiB
Fortran
160 lines
4.5 KiB
Fortran
SUBROUTINE sla_PERMUT ( N, ISTATE, IORDER, J )
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*+
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* - - - - - - -
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* P E R M U T
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* - - - - - - -
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*
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* Generate the next permutation of a specified number of items.
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*
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* Given:
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* N i number of items: there will be N! permutations
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*
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* Given and returned:
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* ISTATE i(N) state, ISTATE(1)=-1 to initialize
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*
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* Returned:
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* IORDER i(N) next permutation of numbers 1,2,...,N
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* J i status: -1 = illegal N (zero or less is illegal)
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* 0 = OK
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* +1 = no more permutations available
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*
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* Notes:
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*
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* 1) This routine returns, in the IORDER array, the integers 1 to N
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* inclusive, in an order that depends on the current contents of
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* the ISTATE array. Before calling the routine for the first
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* time, the caller must set the first element of the ISTATE array
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* to -1 (any negative number will do) to cause the ISTATE array
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* to be fully initialized.
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*
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* 2) The first permutation to be generated is:
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*
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* IORDER(1)=N, IORDER(2)=N-1, ..., IORDER(N)=1
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*
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* This is also the permutation returned for the "finished"
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* (J=1) case.
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*
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* The final permutation to be generated is:
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*
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* IORDER(1)=1, IORDER(2)=2, ..., IORDER(N)=N
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*
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* 3) If the "finished" (J=1) status is ignored, the routine continues
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* to deliver permutations, the pattern repeating every N! calls.
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*
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* Last revision: 19 February 2005
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*
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* Copyright P.T.Wallace. All rights reserved.
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*
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* License:
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program (see SLA_CONDITIONS); if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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* Boston, MA 02111-1307 USA
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*
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*-
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IMPLICIT NONE
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INTEGER N,IORDER(N),ISTATE(N),J
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INTEGER I,IP1,ISLOT,ISKIP
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* -------------
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* Preliminaries
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* -------------
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* Validate, and set status.
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IF (N.LT.1) THEN
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J = -1
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GO TO 9999
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ELSE
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J = 0
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END IF
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* If just starting, initialize state array
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IF (ISTATE(1).LT.0) THEN
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ISTATE(1) = -1
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DO I=2,N
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ISTATE(I) = 0
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END DO
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END IF
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* --------------------------
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* Increment the state number
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* --------------------------
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* The state number, maintained in the ISTATE array, is a mixed-radix
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* number with N! states. The least significant digit, with a radix of
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* 1, is in ISTATE(1). The next digit, in ISTATE(2), has a radix of 2,
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* and so on.
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* Increment the least-significant digit of the state number.
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ISTATE(1) = ISTATE(1)+1
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* Digit by digit starting with the least significant.
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DO I=1,N
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* Carry?
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IF (ISTATE(I).GE.I) THEN
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* Yes: reset the current digit.
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ISTATE(I) = 0
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* Overflow?
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IF (I.GE.N) THEN
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* Yes: there are no more permutations.
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J = 1
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ELSE
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* No: carry.
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IP1 = I+1
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ISTATE(IP1) = ISTATE(IP1)+1
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END IF
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END IF
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END DO
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* -------------------------------------------------------------------
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* Translate the state number into the corresponding permutation order
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* -------------------------------------------------------------------
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* Initialize the order array. All but one element will be overwritten.
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DO I=1,N
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IORDER(I) = 1
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END DO
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* Look at each state number digit, starting with the most significant.
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DO I=N,2,-1
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* Initialize the position where the new number will go.
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ISLOT = 0
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* The state number digit says which unfilled slot is to be used.
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DO ISKIP=0,ISTATE(I)
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* Increment the slot number until an unused slot is found.
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ISLOT = ISLOT+1
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DO WHILE (IORDER(ISLOT).GT.1)
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ISLOT = ISLOT+1
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END DO
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END DO
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* Store the number in the permutation order array.
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IORDER(ISLOT) = I
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END DO
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9999 CONTINUE
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END
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