Question

I’m trying to generate random permutations of an 80-character fixed string in C. Much to my dismay, the system I’m working on lacks strfry(). What’s the best way for me to generate a random permutation of this string? Since this will be looped over approx. 100,000 times, performance is an issue.

Answer 1

Just use the Open Source GLIBC implementation, as found by Google Code.

char * strfry (char *string) {   static int init;   static struct random_data rdata;   size_t len, i;    if (!init)     {       static int state[8] = { 1, 2, 3, 4, 5, 6, 7, 8 };       rdata.state = NULL;       __initstate_r (time ((time_t *) NULL), state, 8, &rdata);       init = 1;     }    len = strlen (string);   for (i = 0; i < len; ++i)     {       int32_t j;       char c;        __random_r (&rdata, &j);       j %= len;        c = string[i];       string[i] = string[j];       string[j] = c;     }    return string; } 

You might want to change the GLIBC specific data types to something more generic.

This code uses the Fisher-Yates shuffle which is actually quite easy to implement by yourself, and very efficient.