Randomize a string in C

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.

Answer 2

create a 80-line array, put a character and a random number into each line of the array, then sort the array on the random numbers.

Rebuild string from sorted array.

Answer 3

void gcry_randomize (unsigned char *buffer, size_t length, enum gcry_random_level level)

Fill buffer with length random bytes using a random quality as defined by level.

http://www.g10code.com/p-libgcrypt.html