gcc/gmp/randmui.c

/* gmp_urandomm_ui -- uniform random number 0 to N-1 for ulong N.

Copyright 2003, 2004 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */

#include "gmp.h"
#include "gmp-impl.h"
#include "longlong.h"


/* If n is a power of 2 then the test ret<n is always true and the loop is
   unnecessary, but there's no need to add special code for this.  Just get
   the "bits" calculation correct and let it go through normally.

   If n is 1 then will have bits==0 and _gmp_rand will produce no output and
   we always return 0.  Again there seems no need for a special case, just
   initialize a[0]=0 and let it go through normally.  */

#define MAX_URANDOMM_ITER  80

unsigned long
gmp_urandomm_ui (gmp_randstate_ptr rstate, unsigned long n)
{
  mp_limb_t      a[LIMBS_PER_ULONG];
  unsigned long  ret, bits, leading;
  int            i;

  if (UNLIKELY (n == 0))
    DIVIDE_BY_ZERO;

  /* start with zeros, since if bits==0 then _gmp_rand will store nothing at
     all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it
     will store only a[0].  */
  a[0] = 0;
#if LIMBS_PER_ULONG > 1
  a[1] = 0;
#endif

  count_leading_zeros (leading, (mp_limb_t) n);
  bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0);

  for (i = 0; i < MAX_URANDOMM_ITER; i++)
    {
      _gmp_rand (a, rstate, bits);
#if LIMBS_PER_ULONG == 1
      ret = a[0];
#else
      ret = a[0] | (a[1] << GMP_NUMB_BITS);
#endif
      if (LIKELY (ret < n))   /* usually one iteration suffices */
        goto done;
    }

  /* Too many iterations, there must be something degenerate about the
     rstate algorithm.  Return r%n.  */
  ret -= n;
  ASSERT (ret < n);

 done:
  return ret;
}