source/libs/gmp/gmp-src/mpn/generic/trialdiv.c

   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   unsigned int idx:24;  /* index of  first primes in dtab */
  69   unsigned int 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 }