Generating All Permutations - Fortran 95 Implementation

Note that the following implementation is a faithful reproduction of the NEXPER algorithm in the book Combinatorial Algorithms For Computers and Calculators by Nijenhuis & Wilf. However, the implementation given in the text has been updated to Fortran 95.

SUBROUTINE NEXPER(N, A, MTC, EVEN)INTEGER NINTEGER, DIMENSION(N) :: AINTEGER S, D, NM3, IA, I1, LLOGICAL MTC, EVEN    IF (MTC) GOTO 10    NM3 = N-3    DO 1 I=1,N1   A(I)=I    MTC=.TRUE.5   EVEN=.TRUE.    IF(N .EQ. 1) GOTO 86   IF(A(N) .NE. 1 .OR. A(1) .NE. 2+MOD(N,2)) RETURN    IF(N .LE. 3) GOTO 8    DO 7 I=1,NM3    IF(A(I+1) .NE. A(I)+1) RETURN7   CONTINUE8   MTC=.FALSE.    RETURN10  IF(N .EQ. 1) GOTO 27    IF(.NOT. EVEN) GOTO 20    IA=A(1)    A(1)=A(2)    A(2)=IA    EVEN=.FALSE.    GOTO 620  S=0    DO 26 I1=2,N25  IA=A(I1)    I=I1-1    D=0    DO 30 J=1,I30  IF(A(J) .GT. IA) D=D+1    S=D+S    IF(D .NE. I*MOD(S,2)) GOTO 3526  CONTINUE27  A(1)=0    GOTO 835  M=MOD(S+1,2)*(N+1)    DO 40 J=1,I    IF(ISIGN(1,A(J)-IA) .EQ. ISIGN(1,A(J)-M)) GOTO 40    M=A(J)    L=J40  CONTINUE    A(L)=IA    A(I1)=M    EVEN=.TRUE.    RETURNENDSUBROUTINE TESTNEXPER    INTEGER, DIMENSION(5) :: IN    LOGICAL MTC    LOGICAL EVEN    DO 10 I = 1, 5    IN(I) = I10  CONTINUE    MTC = .FALSE.20  CONTINUE    CALL NEXPER (5, IN, MTC, EVEN)    DO 30 I = 1, 5        WRITE (*,"(I2), ",advance="no") IN(I)30  CONTINUE    PRINT *    IF (MTC) GOTO 20END

A C# implementation of the above algorithm will be published soon.