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

   1 /* -*-c-*-
   2  *
   3  * $Id: pgen-gcd.c,v 1.2 2000/07/01 11:09:20 mdw Exp $
   4  *
   5  * Prime search stepper ensuring a low GCD for %$(p - 1)/2$%
   6  *
   7  * (c) 2000 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of Catacomb.
  13  *
  14  * Catacomb is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU Library General Public License as
  16  * published by the Free Software Foundation; either version 2 of the
  17  * License, or (at your option) any later version.
  18  *
  19  * Catacomb is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  22  * GNU Library General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU Library General Public
  25  * License along with Catacomb; if not, write to the Free
  26  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  27  * MA 02111-1307, USA.
  28  */
  29
  30 /*----- Revision history --------------------------------------------------*
  31  *
  32  * $Log: pgen-gcd.c,v $
  33  * Revision 1.2  2000/07/01 11:09:20  mdw
  34  * (pgen_gcd): Bug fix -- check the GCDs of the right things when deciding
  35  * whether to abort.
  36  *
  37  * Revision 1.1  2000/06/17 11:51:53  mdw
  38  * Filter which imposes additional restrictions on GCD of %$(p - 1)/2$%
  39  * with a given integer.
  40  *
  41  */
  42
  43 /*----- Header files ------------------------------------------------------*/
  44
  45 #include "mp.h"
  46 #include "pgen.h"
  47
  48 /*----- Main code ---------------------------------------------------------*/
  49
  50 int pgen_gcdstep(int rq, pgen_event *ev, void *p)
  51 {
  52   pgen_gcdstepctx *g = p;
  53   int rc = PGEN_ABORT;
  54
  55   switch (rq) {
  56
  57     /* --- Set everything up --- *
  58      *
  59      * Call things off if @p@ and @jp@ have common factors, or if @q@, @r@
  60      * and @jq@ have common factors greater than @max@.
  61      */
  62
  63     case PGEN_BEGIN: {
  64       mp *p = ev->m;
  65       mp_gcd(&g->g, 0, 0, p, g->jp.m);
  66       if (MP_CMP(g->g, >, MP_ONE))
  67         return (PGEN_ABORT);
  68       g->q = mp_lsr(MP_NEW, p, 1);
  69       g->jq = mp_lsr(MP_NEW, g->jp.m, 1);
  70       mp_gcd(&g->g, 0, 0, g->q, g->jq);
  71       mp_gcd(&g->g, 0, 0, g->g, g->r);
  72       if (MP_CMP(g->g, >, g->max)) {
  73         mp_drop(g->q);
  74         mp_drop(g->jq);
  75         return (PGEN_ABORT);
  76       }
  77       rc = pfilt_create(&g->p, p);
  78       mp_drop(p);
  79     } break;
  80
  81     /* --- Grind through another iteration --- */
  82
  83     case PGEN_TRY:
  84       mp_drop(ev->m);
  85       rc = pfilt_jump(&g->p, &g->jp);
  86       g->q = mp_add(g->q, g->q, g->jq);
  87       break;
  88
  89     /* --- Finished --- */
  90
  91     case PGEN_DONE:
  92       pfilt_destroy(&g->p);
  93       mp_drop(g->q);
  94       mp_drop(g->jq);
  95       return (PGEN_DONE);
  96   }
  97
  98   /* --- Step on until everything is OK --- */
  99
 100   for (;;) {
 101     if (rc != PGEN_FAIL) {
 102       mp_gcd(&g->g, 0, 0, g->r, g->q);
 103       if (MP_CMP(g->g, >, g->max))
 104         rc = PGEN_FAIL;
 105     }
 106     if (rc != PGEN_FAIL)
 107       break;
 108     rc = pfilt_jump(&g->p, &g->jp);
 109     g->q = mp_add(g->q, g->q, g->jq);
 110   }
 111
 112   ev->m = MP_COPY(g->p.m);
 113   return (rc);
 114 }
 115
 116 /*----- That's all, folks -------------------------------------------------*/