cs_randperm.c

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#include "cs.h"
/* return a random permutation vector, the identity perm, or p = n-1:-1:0.
 * seed = -1 means p = n-1:-1:0.  seed = 0 means p = identity.  otherwise
 * p = random permutation.  */
int *cs_randperm (int n, int seed)
{
    int *p, k, j, t ;
    if (seed == 0) return (NULL) ;      /* return p = NULL (identity) */
    p = (int*)cs_malloc (n, sizeof (int)) ;   /* allocate result */
    if (!p) return (NULL) ;             /* out of memory */
    for (k = 0 ; k < n ; k++) p [k] = n-k-1 ;
    if (seed == -1) return (p) ;        /* return reverse permutation */
    srand (seed) ;                      /* get new random number seed */
    for (k = 0 ; k < n ; k++)
    {
        j = k + (rand ( ) % (n-k)) ;    /* j = rand int in range k to n-1 */
        t = p [j] ;                     /* swap p[k] and p[j] */
        p [j] = p [k] ;
        p [k] = t ;
    }
    return (p) ;
}