114 lines
2.6 KiB
C
114 lines
2.6 KiB
C
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/*****************************
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* Copyright Henry Minsky (hqm@alum.mit.edu) 1991-2009
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*
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* This software library is licensed under terms of the GNU GENERAL
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* PUBLIC LICENSE
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*
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* RSCODE is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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* RSCODE is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with Rscode. If not, see <http://www.gnu.org/licenses/>.
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* Commercial licensing is available under a separate license, please
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* contact author for details.
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*
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* Source code is available at http://rscode.sourceforge.net
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*
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*
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* Multiplication and Arithmetic on Galois Field GF(256)
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*
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* From Mee, Daniel, "Magnetic Recording, Volume III", Ch. 5 by Patel.
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*
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*
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******************************/
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#include <stdio.h>
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#include <stdlib.h>
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#include "ecc.h"
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/* This is one of 14 irreducible polynomials
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* of degree 8 and cycle length 255. (Ch 5, pp. 275, Magnetic Recording)
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* The high order 1 bit is implicit */
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/* x^8 + x^4 + x^3 + x^2 + 1 */
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#define PPOLY 0x1D
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int gexp[512];
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int glog[256];
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static void init_exp_table (void);
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void
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init_galois_tables (void)
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{
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/* initialize the table of powers of alpha */
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init_exp_table();
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}
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static void
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init_exp_table (void)
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{
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int i, z;
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int pinit,p1,p2,p3,p4,p5,p6,p7,p8;
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pinit = p2 = p3 = p4 = p5 = p6 = p7 = p8 = 0;
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p1 = 1;
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gexp[0] = 1;
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gexp[255] = gexp[0];
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glog[0] = 0; /* shouldn't log[0] be an error? */
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// Private pp8() As Integer = {1, 0, 1, 1, 1, 0, 0, 0, 1} 'specify irreducible polynomial coeffts */
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for (i = 1; i < 256; i++) {
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pinit = p8;
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p8 = p7;
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p7 = p6;
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p6 = p5;
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p5 = p4 ^ pinit;
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p4 = p3 ^ pinit;
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p3 = p2 ^ pinit;
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p2 = p1;
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p1 = pinit;
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gexp[i] = p1 + p2*2 + p3*4 + p4*8 + p5*16 + p6*32 + p7*64 + p8*128;
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gexp[i+255] = gexp[i];
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}
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for (i = 1; i < 256; i++) {
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for (z = 0; z < 256; z++) {
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if (gexp[z] == i) {
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glog[i] = z;
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break;
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}
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}
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}
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}
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/* multiplication using logarithms */
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int gmult(int a, int b)
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{
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int i,j;
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if (a==0 || b == 0) return (0);
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i = glog[a];
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j = glog[b];
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return (gexp[i+j]);
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}
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int ginv (int elt)
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{
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return (gexp[255-glog[elt]]);
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}
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