git.distorted.org.uk Git - u/mdw/catacomb/blob - math/field-exp.c

   1 /* -*-c-*-
   2  *
   3  * Exponentiation in finite fields
   4  *
   5  * (c) 2004 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 "field.h"
  31 #include "field-exp.h"
  32
  33 /*----- Main code ---------------------------------------------------------*/
  34
  35 /* --- @field_exp@ --- *
  36  *
  37  * Arguments:   @field *f@ = pointer to field
  38  *              @mp *d@ = fake destination
  39  *              @mp *a@ = base
  40  *              @mp *e@ = exponent
  41  *
  42  * Returns:     Result, %$a^e$%.
  43  *
  44  * Use:         Exponentiation in a finite field.  Note that all quantities
  45  *              are in internal format.
  46  */
  47
  48 mp *field_exp(field *f, mp *d, mp *a, mp *e)
  49 {
  50   mp *x = MP_COPY(f->one);
  51   mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
  52
  53   MP_COPY(a);
  54   MP_SHRINK(e);
  55   if (MP_ZEROP(e))
  56     ;
  57   else {
  58     if (MP_NEGP(e))
  59       a = F_INV(f, a, a);
  60     if (MP_LEN(e) < EXP_THRESH)
  61       EXP_SIMPLE(x, a, e);
  62     else
  63       EXP_WINDOW(x, a, e);
  64   }
  65   mp_drop(d);
  66   mp_drop(spare);
  67   mp_drop(a);
  68   return (x);
  69 }
  70
  71 /*----- That's all, folks -------------------------------------------------*/