github.com/aergoio/aergo@v1.3.1/libtool/src/gmp-6.1.2/rand/randmui.c (about)

     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.h"
    32  #include "gmp-impl.h"
    33  #include "longlong.h"
    34  
    35  
    36  /* If n is a power of 2 then the test ret<n is always true and the loop is
    37     unnecessary, but there's no need to add special code for this.  Just get
    38     the "bits" calculation correct and let it go through normally.
    39  
    40     If n is 1 then will have bits==0 and _gmp_rand will produce no output and
    41     we always return 0.  Again there seems no need for a special case, just
    42     initialize a[0]=0 and let it go through normally.  */
    43  
    44  #define MAX_URANDOMM_ITER  80
    45  
    46  unsigned long
    47  gmp_urandomm_ui (gmp_randstate_ptr rstate, unsigned long n)
    48  {
    49    mp_limb_t      a[LIMBS_PER_ULONG];
    50    unsigned long  ret, bits, leading;
    51    int            i;
    52  
    53    if (UNLIKELY (n == 0))
    54      DIVIDE_BY_ZERO;
    55  
    56    /* start with zeros, since if bits==0 then _gmp_rand will store nothing at
    57       all (bits==0 arises when n==1), or if bits <= GMP_NUMB_BITS then it
    58       will store only a[0].  */
    59    a[0] = 0;
    60  #if LIMBS_PER_ULONG > 1
    61    a[1] = 0;
    62  #endif
    63  
    64    count_leading_zeros (leading, (mp_limb_t) n);
    65    bits = GMP_LIMB_BITS - leading - (POW2_P(n) != 0);
    66  
    67    for (i = 0; i < MAX_URANDOMM_ITER; i++)
    68      {
    69        _gmp_rand (a, rstate, bits);
    70  #if LIMBS_PER_ULONG == 1
    71        ret = a[0];
    72  #else
    73        ret = a[0] | (a[1] << GMP_NUMB_BITS);
    74  #endif
    75        if (LIKELY (ret < n))   /* usually one iteration suffices */
    76          goto done;
    77      }
    78  
    79    /* Too many iterations, there must be something degenerate about the
    80       rstate algorithm.  Return r%n.  */
    81    ret -= n;
    82    ASSERT (ret < n);
    83  
    84   done:
    85    return ret;
    86  }