PARI sourcecode for demo in A055881

This demonstrative program should be used for either documentation or instructive purposes only,
and goes without any warranty whatsoever.

This is part of: The On-Line Encyclopedia Of Integer Sequences OEIS(TM)

Visit: http://www.oeis.org/A055881

Author: Joerg Arndt (www.jjj.de), Dec 12 2012
Pastebin by: R. J. Cano (remy@ula.ve), Dec 17 2012

perm(n)=
/* Compute the first n! terms of A055881 by counting
  in rising factorial base.
  Generate all permutations of n along the way.
*/
{
    my( fc=vector(n) ); /* mixed radix numbers (rising factorial base) */
    my( ct=0, j, a=0 );
    my( p=vector(n,k,k-1) ); /* permutation */
    my( k, t );  /* for permutations */
    while ( 1,
        print1(ct, ":  ", p); /* permutation */
        print1("  ", a );  /* A055881(ct): position of rightmost change in fc[] */
        print1("    ", fc);  /* factorial number */
        print();
        ct += 1;

        /* increment factorial number fc[]: */
        j = 1;
        while ( fc[j] == j,  fc[j]=0;  j+=1; );  /* scan over nines */
        if ( j==n,  return() );  /* current is last */
        fc[j] += 1;
        /* update permutation p[], reverse prefix of length j+1: */
        a = j;  /* next term of A055881 */
        j += 1;  k = 1;
        while ( k < j,
            t=p[j]; p[j]=p[k]; p[k]=t;
            k+=1; j-=1;
        );
    );
}

print("Demonstration: Reproduces the example given in the comments section for A055881");
perm(4);
print(" ");
print("You are free to call perm() and experiment with it. When finished, type \"quit\" to exit.");
print(" ");