ut0rnd.cc

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/** @file ut/ut0rnd.cc
 Random numbers and hashing

 Created 5/11/1994 Heikki Tuuri
 ********************************************************************/

#include "ut0rnd.h"

/** These random numbers are used in ut_find_prime */
/*@{*/
#define UT_RANDOM_1 1.0412321
#define UT_RANDOM_2 1.1131347
#define UT_RANDOM_3 1.0132677
/*@}*/

thread_local ulint ut_rnd_ulint_counter = 0;

/** Looks for a prime number slightly greater than the given argument.
 The prime is chosen so that it is not near any power of 2.
 @return prime */
ulint ut_find_prime(ulint n) /*!< in: positive number > 100 */
{
  ulint pow2;
  ulint i;

  n += 100;

  pow2 = 1;
  while (pow2 * 2 < n) {
    pow2 = 2 * pow2;
  }

  if ((double)n < 1.05 * (double)pow2) {
    n = (ulint)((double)n * UT_RANDOM_1);
  }

  pow2 = 2 * pow2;

  if ((double)n > 0.95 * (double)pow2) {
    n = (ulint)((double)n * UT_RANDOM_2);
  }

  if (n > pow2 - 20) {
    n += 30;
  }

  /* Now we have n far enough from powers of 2. To make
  n more random (especially, if it was not near
  a power of 2), we then multiply it by a random number. */

  n = (ulint)((double)n * UT_RANDOM_3);

  for (;; n++) {
    i = 2;
    while (i * i <= n) {
      if (n % i == 0) {
        goto next_n;
      }
      i++;
    }

    /* Found a prime */
    break;
  next_n:;
  }

  return (n);
}