1  /* mpz_urandomm (rop, state, n) -- Generate a uniform pseudorandom
       2     integer in the range 0 to N-1, using STATE as the random state
       3     previously initialized by a call to gmp_randinit().
       4  
       5  Copyright 2000, 2002, 2012, 2015 Free Software Foundation, Inc.
       6  
       7  This file is part of the GNU MP Library.
       8  
       9  The GNU MP Library is free software; you can redistribute it and/or modify
      10  it under the terms of either:
      11  
      12    * the GNU Lesser General Public License as published by the Free
      13      Software Foundation; either version 3 of the License, or (at your
      14      option) any later version.
      15  
      16  or
      17  
      18    * the GNU General Public License as published by the Free Software
      19      Foundation; either version 2 of the License, or (at your option) any
      20      later version.
      21  
      22  or both in parallel, as here.
      23  
      24  The GNU MP Library is distributed in the hope that it will be useful, but
      25  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
      26  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
      27  for more details.
      28  
      29  You should have received copies of the GNU General Public License and the
      30  GNU Lesser General Public License along with the GNU MP Library.  If not,
      31  see https://www.gnu.org/licenses/.  */
      32  
      33  #include "gmp-impl.h"
      34  #include "longlong.h" /* for count_leading_zeros */
      35  
      36  
      37  #define MAX_URANDOMM_ITER  80
      38  
      39  void
      40  mpz_urandomm (mpz_ptr rop, gmp_randstate_ptr rstate, mpz_srcptr n)
      41  {
      42    mp_ptr rp, np;
      43    mp_size_t nbits, size;
      44    mp_limb_t nh;
      45    int count;
      46    int pow2;
      47    int cmp;
      48    TMP_DECL;
      49  
      50    size = ABSIZ (n);
      51    if (UNLIKELY (size == 0))
      52      DIVIDE_BY_ZERO;
      53  
      54    np = PTR (n);
      55    nh = np[size - 1];
      56  
      57    /* Detect whether n is a power of 2.  */
      58    pow2 = POW2_P (nh) && (size == 1 || mpn_zero_p (np, size - 1));
      59  
      60    count_leading_zeros (count, nh);
      61    nbits = size * GMP_NUMB_BITS - (count - GMP_NAIL_BITS) - pow2;
      62    if (nbits == 0)		/* nbits == 0 means that n was == 1.  */
      63      {
      64        SIZ (rop) = 0;
      65        return;
      66      }
      67  
      68    TMP_MARK;
      69    if (rop == n)
      70      {
      71        mp_ptr tp;
      72        tp = TMP_ALLOC_LIMBS (size);
      73        MPN_COPY (tp, np, size);
      74        np = tp;
      75      }
      76  
      77    /* Here the allocated size can be one too much if n is a power of
      78       (2^GMP_NUMB_BITS) but it's convenient for using mpn_cmp below.  */
      79    rp = MPZ_NEWALLOC (rop, size);
      80    /* Clear last limb to prevent the case in which size is one too much.  */
      81    rp[size - 1] = 0;
      82  
      83    count = MAX_URANDOMM_ITER;	/* Set iteration count limit.  */
      84    do
      85      {
      86        _gmp_rand (rp, rstate, nbits);
      87        MPN_CMP (cmp, rp, np, size);
      88      }
      89    while (cmp >= 0 && --count != 0);
      90  
      91    if (count == 0)
      92      /* Too many iterations; return result mod n == result - n */
      93      mpn_sub_n (rp, rp, np, size);
      94  
      95    MPN_NORMALIZE (rp, size);
      96    SIZ (rop) = size;
      97    TMP_FREE;
      98  }