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

   1 /* -*-c-*-
   2  *
   3  * $Id: pgen-gcd.c,v 1.3 2004/04/08 01:36:15 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 /*----- Header files ------------------------------------------------------*/
  31
  32 #include "mp.h"
  33 #include "pgen.h"
  34
  35 /*----- Main code ---------------------------------------------------------*/
  36
  37 int pgen_gcdstep(int rq, pgen_event *ev, void *p)
  38 {
  39   pgen_gcdstepctx *g = p;
  40   int rc = PGEN_ABORT;
  41
  42   switch (rq) {
  43
  44     /* --- Set everything up --- *
  45      *
  46      * Call things off if @p@ and @jp@ have common factors, or if @q@, @r@
  47      * and @jq@ have common factors greater than @max@.
  48      */
  49
  50     case PGEN_BEGIN: {
  51       mp *p = ev->m;
  52       mp_gcd(&g->g, 0, 0, p, g->jp.m);
  53       if (MP_CMP(g->g, >, MP_ONE))
  54         return (PGEN_ABORT);
  55       g->q = mp_lsr(MP_NEW, p, 1);
  56       g->jq = mp_lsr(MP_NEW, g->jp.m, 1);
  57       mp_gcd(&g->g, 0, 0, g->q, g->jq);
  58       mp_gcd(&g->g, 0, 0, g->g, g->r);
  59       if (MP_CMP(g->g, >, g->max)) {
  60         mp_drop(g->q);
  61         mp_drop(g->jq);
  62         return (PGEN_ABORT);
  63       }
  64       rc = pfilt_create(&g->p, p);
  65       mp_drop(p);
  66     } break;
  67
  68     /* --- Grind through another iteration --- */
  69
  70     case PGEN_TRY:
  71       mp_drop(ev->m);
  72       rc = pfilt_jump(&g->p, &g->jp);
  73       g->q = mp_add(g->q, g->q, g->jq);
  74       break;
  75
  76     /* --- Finished --- */
  77
  78     case PGEN_DONE:
  79       pfilt_destroy(&g->p);
  80       mp_drop(g->q);
  81       mp_drop(g->jq);
  82       return (PGEN_DONE);
  83   }
  84
  85   /* --- Step on until everything is OK --- */
  86
  87   for (;;) {
  88     if (rc != PGEN_FAIL) {
  89       mp_gcd(&g->g, 0, 0, g->r, g->q);
  90       if (MP_CMP(g->g, >, g->max))
  91         rc = PGEN_FAIL;
  92     }
  93     if (rc != PGEN_FAIL)
  94       break;
  95     rc = pfilt_jump(&g->p, &g->jp);
  96     g->q = mp_add(g->q, g->q, g->jq);
  97   }
  98
  99   ev->m = MP_COPY(g->p.m);
 100   return (rc);
 101 }
 102
 103 /*----- That's all, folks -------------------------------------------------*/