git.distorted.org.uk Git - u/mdw/catacomb/blob - math/pgen-gcd.c

   1 /* -*-c-*-
   2  *
   3  * Prime search stepper ensuring a low GCD for %$(p - 1)/2$%
   4  *
   5  * (c) 2000 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of Catacomb.
  11  *
  12  * Catacomb is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU Library General Public License as
  14  * published by the Free Software Foundation; either version 2 of the
  15  * License, or (at your option) any later version.
  16  *
  17  * Catacomb is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Catacomb; if not, write to the Free
  24  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  25  * MA 02111-1307, USA.
  26  */
  27
  28 /*----- Header files ------------------------------------------------------*/
  29
  30 #include "mp.h"
  31 #include "pgen.h"
  32
  33 /*----- Main code ---------------------------------------------------------*/
  34
  35 int pgen_gcdstep(int rq, pgen_event *ev, void *p)
  36 {
  37   pgen_gcdstepctx *g = p;
  38   int rc = PGEN_ABORT;
  39
  40   switch (rq) {
  41
  42     /* --- Set everything up --- *
  43      *
  44      * Call things off if @p@ and @jp@ have common factors, or if @q@, @r@
  45      * and @jq@ have common factors greater than @max@.
  46      */
  47
  48     case PGEN_BEGIN: {
  49       mp *p = ev->m;
  50       mp_gcd(&g->g, 0, 0, p, g->jp.m);
  51       if (MP_CMP(g->g, >, MP_ONE))
  52         return (PGEN_ABORT);
  53       g->q = mp_lsr(MP_NEW, p, 1);
  54       g->jq = mp_lsr(MP_NEW, g->jp.m, 1);
  55       mp_gcd(&g->g, 0, 0, g->q, g->jq);
  56       mp_gcd(&g->g, 0, 0, g->g, g->r);
  57       if (MP_CMP(g->g, >, g->max)) {
  58         mp_drop(g->q);
  59         mp_drop(g->jq);
  60         return (PGEN_ABORT);
  61       }
  62       rc = pfilt_create(&g->p, p);
  63       mp_drop(p);
  64     } break;
  65
  66     /* --- Grind through another iteration --- */
  67
  68     case PGEN_TRY:
  69       mp_drop(ev->m);
  70       rc = pfilt_jump(&g->p, &g->jp);
  71       g->q = mp_add(g->q, g->q, g->jq);
  72       break;
  73
  74     /* --- Finished --- */
  75
  76     case PGEN_DONE:
  77       pfilt_destroy(&g->p);
  78       mp_drop(g->q);
  79       mp_drop(g->jq);
  80       return (PGEN_DONE);
  81   }
  82
  83   /* --- Step on until everything is OK --- */
  84
  85   for (;;) {
  86     if (rc != PGEN_FAIL) {
  87       mp_gcd(&g->g, 0, 0, g->r, g->q);
  88       if (MP_CMP(g->g, >, g->max))
  89         rc = PGEN_FAIL;
  90     }
  91     if (rc != PGEN_FAIL)
  92       break;
  93     rc = pfilt_jump(&g->p, &g->jp);
  94     g->q = mp_add(g->q, g->q, g->jq);
  95   }
  96
  97   ev->m = MP_COPY(g->p.m);
  98   return (rc);
  99 }
 100
 101 /*----- That's all, folks -------------------------------------------------*/