github.com/aergoio/aergo@v1.3.1/libtool/src/gmp-6.1.2/mpn/generic/trialdiv.c (about)

     1  /* mpn_trialdiv -- find small factors of an mpn number using trial division.
     2  
     3     Contributed to the GNU project by Torbjorn Granlund.
     4  
     5     THE FUNCTION IN THIS FILE IS INTERNAL WITH A MUTABLE INTERFACE.  IT IS ONLY
     6     SAFE TO REACH IT THROUGH DOCUMENTED INTERFACES.  IN FACT, IT IS ALMOST
     7     GUARANTEED THAT IT WILL CHANGE OR DISAPPEAR IN A FUTURE GNU MP RELEASE.
     8  
     9  Copyright 2009, 2010, 2012, 2013 Free Software Foundation, Inc.
    10  
    11  This file is part of the GNU MP Library.
    12  
    13  The GNU MP Library is free software; you can redistribute it and/or modify
    14  it under the terms of either:
    15  
    16    * the GNU Lesser General Public License as published by the Free
    17      Software Foundation; either version 3 of the License, or (at your
    18      option) any later version.
    19  
    20  or
    21  
    22    * the GNU General Public License as published by the Free Software
    23      Foundation; either version 2 of the License, or (at your option) any
    24      later version.
    25  
    26  or both in parallel, as here.
    27  
    28  The GNU MP Library is distributed in the hope that it will be useful, but
    29  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
    30  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
    31  for more details.
    32  
    33  You should have received copies of the GNU General Public License and the
    34  GNU Lesser General Public License along with the GNU MP Library.  If not,
    35  see https://www.gnu.org/licenses/.  */
    36  
    37  /*
    38     This function finds the first (smallest) factor represented in
    39     trialdivtab.h.  It does not stop the factoring effort just because it has
    40     reached some sensible limit, such as the square root of the input number.
    41  
    42     The caller can limit the factoring effort by passing NPRIMES.  The function
    43     will then divide until that limit, or perhaps a few primes more.  A position
    44     which only mpn_trialdiv can make sense of is returned in the WHERE
    45     parameter.  It can be used for restarting the factoring effort; the first
    46     call should pass 0 here.
    47  
    48     Input:        1. A non-negative number T = {tp,tn}
    49                   2. NPRIMES as described above,
    50                   3. *WHERE as described above.
    51     Output:       1. *WHERE updated as described above.
    52                   2. Return value is non-zero if we found a factor, else zero
    53                      To get the actual prime factor, compute the mod B inverse
    54                      of the return value.
    55  */
    56  
    57  #include "gmp.h"
    58  #include "gmp-impl.h"
    59  
    60  struct gmp_primes_dtab {
    61    mp_limb_t binv;
    62    mp_limb_t lim;
    63  };
    64  
    65  struct gmp_primes_ptab {
    66    mp_limb_t ppp;	/* primes, multiplied together */
    67    mp_limb_t cps[7];	/* ppp values pre-computed for mpn_mod_1s_4p */
    68    gmp_uint_least32_t idx:24;	/* index of  first primes in dtab */
    69    gmp_uint_least32_t np :8;	/* number of primes related to this entry */
    70  };
    71  
    72  
    73  static const struct gmp_primes_dtab gmp_primes_dtab[] =
    74  {
    75  #define WANT_dtab
    76  #define P(p,inv,lim) {inv,lim}
    77  #include "trialdivtab.h"
    78  #undef WANT_dtab
    79  #undef P
    80    {0,0}
    81  };
    82  
    83  static const struct gmp_primes_ptab gmp_primes_ptab[] =
    84  {
    85  #define WANT_ptab
    86  #include "trialdivtab.h"
    87  #undef WANT_ptab
    88  };
    89  
    90  #define PTAB_LINES (sizeof (gmp_primes_ptab) / sizeof (gmp_primes_ptab[0]))
    91  
    92  /* FIXME: We could optimize out one of the outer loop conditions if we
    93     had a final ptab entry with a huge np field.  */
    94  mp_limb_t
    95  mpn_trialdiv (mp_srcptr tp, mp_size_t tn, mp_size_t nprimes, int *where)
    96  {
    97    mp_limb_t ppp;
    98    const mp_limb_t *cps;
    99    const struct gmp_primes_dtab *dp;
   100    long i, j, idx, np;
   101    mp_limb_t r, q;
   102  
   103    ASSERT (tn >= 1);
   104  
   105    for (i = *where; i < PTAB_LINES; i++)
   106      {
   107        ppp = gmp_primes_ptab[i].ppp;
   108        cps = gmp_primes_ptab[i].cps;
   109  
   110        r = mpn_mod_1s_4p (tp, tn, ppp << cps[1], cps);
   111  
   112        idx = gmp_primes_ptab[i].idx;
   113        np = gmp_primes_ptab[i].np;
   114  
   115        /* Check divisibility by individual primes.  */
   116        dp = &gmp_primes_dtab[idx] + np;
   117        for (j = -np; j < 0; j++)
   118  	{
   119  	  q = r * dp[j].binv;
   120  	  if (q <= dp[j].lim)
   121  	    {
   122  	      *where = i;
   123  	      return dp[j].binv;
   124  	    }
   125  	}
   126  
   127        nprimes -= np;
   128        if (nprimes <= 0)
   129  	return 0;
   130      }
   131    return 0;
   132  }