1  /* gmp_urandomm_ui -- uniform random number 0 to N-1 for ulong N.
       2  
       3  Copyright 2003, 2004 Free Software Foundation, Inc.
       4  
       5  This file is part of the GNU MP Library.
       6  
       7  The GNU MP Library is free software; you can redistribute it and/or modify
       8  it under the terms of either:
       9  
      10    * the GNU Lesser General Public License as published by the Free
      11      Software Foundation; either version 3 of the License, or (at your
      12      option) any later version.
      13  
      14  or
      15  
      16    * the GNU General Public License as published by the Free Software
      17      Foundation; either version 2 of the License, or (at your option) any
      18      later version.
      19  
      20  or both in parallel, as here.
      21  
      22  The GNU MP Library is distributed in the hope that it will be useful, but
      23  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
      24  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
      25  for more details.
      26  
      27  You should have received copies of the GNU General Public License and the
      28  GNU Lesser General Public License along with the GNU MP Library.  If not,
      29  see https://www.gnu.org/licenses/.  */
      30  
      31  #include "gmp-impl.h"
      32  #include "longlong.h"
      33  
      34  
      35  /* If n is a power of 2 then the test ret<n is always true and the loop is
      36     unnecessary, but there's no need to add special code for this.  Just get
      37     the "bits" calculation correct and let it go through normally.
      38  
      39     If n is 1 then will have bits==0 and _gmp_rand will produce no output and
      40     we always return 0.  Again there seems no need for a special case, just
      41     initialize a[0]=0 and let it go through normally.  */
      42  
      43  #define MAX_URANDOMM_ITER  80
      44  
      45  unsigned long
      46  gmp_urandomm_ui (gmp_randstate_ptr rstate, unsigned long n)
      47  {
      48    mp_limb_t      a[LIMBS_PER_ULONG];
      49    unsigned long  ret, bits, leading;
      50    int            i;
      51  
      52    if (UNLIKELY (n == 0))
      53      DIVIDE_BY_ZERO;
      54  
      55    /* start with zeros, since if bits==0 then _gmp_rand will store nothing at
      56       all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it
      57       will store only a[0].  */
      58    a[0] = 0;
      59  #if LIMBS_PER_ULONG > 1
      60    a[1] = 0;
      61  #endif
      62  
      63    count_leading_zeros (leading, (mp_limb_t) n);
      64    bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0);
      65  
      66    for (i = 0; i < MAX_URANDOMM_ITER; i++)
      67      {
      68        _gmp_rand (a, rstate, bits);
      69  #if LIMBS_PER_ULONG == 1
      70        ret = a[0];
      71  #else
      72        ret = a[0] | (a[1] << GMP_NUMB_BITS);
      73  #endif
      74        if (LIKELY (ret < n))   /* usually one iteration suffices */
      75          goto done;
      76      }
      77  
      78    /* Too many iterations, there must be something degenerate about the
      79       rstate algorithm.  Return r%n.  */
      80    ret -= n;
      81    ASSERT (ret < n);
      82  
      83   done:
      84    return ret;
      85  }