third_party/gmp/mpz/urandomm.c

/* mpz_urandomm (rop, state, n) -- Generate a uniform pseudorandom
   integer in the range 0 to N-1, using STATE as the random state
   previously initialized by a call to gmp_randinit().

Copyright 2000, 2002, 2012, 2015 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 either:

  * 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.

or

  * the GNU General Public License as published by the Free Software
    Foundation; either version 2 of the License, or (at your option) any
    later version.

or both in parallel, as here.

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 General Public License
for more details.

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

#include "gmp-impl.h"
#include "longlong.h" /* for count_leading_zeros */


#define MAX_URANDOMM_ITER  80

void
mpz_urandomm (mpz_ptr rop, gmp_randstate_t rstate, mpz_srcptr n)
{
  mp_ptr rp, np;
  mp_size_t nbits, size;
  mp_limb_t nh;
  int count;
  int pow2;
  int cmp;
  TMP_DECL;

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

  np = PTR (n);
  nh = np[size - 1];

  /* Detect whether n is a power of 2.  */
  pow2 = POW2_P (nh) && (size == 1 || mpn_zero_p (np, size - 1));

  count_leading_zeros (count, nh);
  nbits = size * GMP_NUMB_BITS - (count - GMP_NAIL_BITS) - pow2;
  if (nbits == 0)		/* nbits == 0 means that n was == 1.  */
    {
      SIZ (rop) = 0;
      return;
    }

  TMP_MARK;
  if (rop == n)
    {
      mp_ptr tp;
      tp = TMP_ALLOC_LIMBS (size);
      MPN_COPY (tp, np, size);
      np = tp;
    }

  /* Here the allocated size can be one too much if n is a power of
     (2^GMP_NUMB_BITS) but it's convenient for using mpn_cmp below.  */
  rp = MPZ_NEWALLOC (rop, size);
  /* Clear last limb to prevent the case in which size is one too much.  */
  rp[size - 1] = 0;

  count = MAX_URANDOMM_ITER;	/* Set iteration count limit.  */
  do
    {
      _gmp_rand (rp, rstate, nbits);
      MPN_CMP (cmp, rp, np, size);
    }
  while (cmp >= 0 && --count != 0);

  if (count == 0)
    /* Too many iterations; return result mod n == result - n */
    mpn_sub_n (rp, rp, np, size);

  MPN_NORMALIZE (rp, size);
  SIZ (rop) = size;
  TMP_FREE;
}