trialdiv.c
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- /* mpn_trialdiv -- find small factors of an mpn number using trial division.
- Contributed to the GNU project by Torbjorn Granlund.
- THE FUNCTION IN THIS FILE IS INTERNAL WITH A MUTABLE INTERFACE. IT IS ONLY
- SAFE TO REACH IT THROUGH DOCUMENTED INTERFACES. IN FACT, IT IS ALMOST
- GUARANTEED THAT IT WILL CHANGE OR DISAPPEAR IN A FUTURE GNU MP RELEASE.
- Copyright 2009 Free Software Foundation, Inc.
- This file is part of the GNU MP Library.
- The GNU MP Library is free software; you can redistribute it and/or modify
- it under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 3 of the License, or (at your
- option) any later version.
- The GNU MP Library is distributed in the hope that it will be useful, but
- WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
- License for more details.
- You should have received a copy of the GNU Lesser General Public License
- along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
- /*
- Fast, division-free trial division for GMP.
- This function will find the first (smallest) factor represented in
- trialdivtab.h. It does not stop the factoring effort just because it has
- reached some sensible limit, such as the square root of the input number.
- The caller can limit the factoring effort by passing NPRIMES. The function
- well then divide to *at least* that limit. A position which only
- mpn_trialdiv can make sense of is returned in the WHERE parameter. It can
- be used for restarting the factoring effort; the first call should pass 0
- here.
- */
- #include "gmp.h"
- #include "gmp-impl.h"
- struct gmp_primes_dtab {
- mp_limb_t binv;
- mp_limb_t lim;
- };
- struct gmp_primes_ptab {
- mp_limb_t ppp; /* primes, multiplied together */
- mp_limb_t cps[7]; /* ppp values pre-computed for mpn_mod_1s_4p */
- unsigned int idx:24; /* index of first primes in dtab */
- unsigned int np :8; /* number of primes related to this entry */
- };
- #define P(p,inv,lim) {inv,lim}
- #include "trialdivtab.h"
- #define PTAB_LINES (sizeof (gmp_primes_ptab) / sizeof (gmp_primes_ptab[0]))
- /* Attempt to find a factor of T using trial division.
- Input: A non-negative number T.
- Output: non-zero if we found a factor, zero otherwise. To get the actual
- prime factor, compute the mod B inverse of the return value. */
- /* FIXME: We could optimize out one of the outer loop conditions if we
- had a final ptab entry with a huge nd field. */
- mp_limb_t
- mpn_trialdiv (mp_srcptr tp, mp_size_t tn, mp_size_t nprimes, int *where)
- {
- mp_limb_t ppp;
- mp_limb_t *cps;
- struct gmp_primes_dtab *dp;
- long i, j, idx, np;
- mp_limb_t r, q;
- ASSERT (tn >= 1);
- for (i = *where; i < PTAB_LINES; i++)
- {
- ppp = gmp_primes_ptab[i].ppp;
- cps = gmp_primes_ptab[i].cps;
- #if __GNU_MP_VERSION == 4 && __GNU_MP_VERSION_MINOR < 4
- if (tn < 4)
- r = mpn_mod_1 (tp, tn, ppp); /* FIXME */
- else
- #endif
- r = mpn_mod_1s_4p (tp, tn, ppp << cps[1], cps);
- idx = gmp_primes_ptab[i].idx;
- np = gmp_primes_ptab[i].np;
- /* Check divisibility by individual primes. */
- dp = &gmp_primes_dtab[idx] + np;
- for (j = -np; j < 0; j++)
- {
- q = r * dp[j].binv;
- if (q <= dp[j].lim)
- {
- *where = i;
- return dp[j].binv;
- }
- }
- nprimes -= np;
- if (nprimes <= 0)
- return 0;
- }
- return 0;
- }