bch3.c

this is bch coder in C

/*
 * File:    bch3.c
 * Title:   Encoder/decoder for binary BCH codes in C (Version 3.1)
 * Author:  Robert Morelos-Zaragoza
 * Date:    August 1994
 * Revised: June 13, 1997
 *
 * ===============  Encoder/Decoder for binary BCH codes in C =================
 *
 * Version 1:   Original program. The user provides the generator polynomial
 *              of the code (cumbersome!).
 * Version 2:   Computes the generator polynomial of the code.
 * Version 3:   No need to input the coefficients of a primitive polynomial of
 *              degree m, used to construct the Galois Field GF(2**m). The
 *              program now works for any binary BCH code of length such that:
 *              2**(m-1) - 1 < length <= 2**m - 1
 *
 * Note:        You may have to change the size of the arrays to make it work.
 *
 * The encoding and decoding methods used in this program are based on the
 * book "Error Control Coding: Fundamentals and Applications", by Lin and
 * Costello, Prentice Hall, 1983.
 *
 * Thanks to Patrick Boyle ([e-mail]) for his observation that 'bch2.c'
 * did not work for lengths other than 2**m-1 which led to this new version.
 * Portions of this program are from 'rs.c', a Reed-Solomon encoder/decoder
 * in C, written by Simon Rockliff ([e-mail]) on 21/9/89. The
 * previous version of the BCH encoder/decoder in C, 'bch2.c', was written by
 * Robert Morelos-Zaragoza ([e-mail]) on 5/19/92.
 *
 * NOTE:    
 *          The author is not responsible for any malfunctioning of
 *          this program, nor for any damage caused by it. Please include the
 *          original program along with these comments in any redistribution.
 *
 *  For more information, suggestions, or other ideas on implementing error
 *  correcting codes, please contact me at:
 *
 *                           Robert Morelos-Zaragoza
 *                           5120 Woodway, Suite 7036
 *                           Houston, Texas 77056
 *
 *                    email: [e-mail]
 *
 * COPYRIGHT NOTICE: This computer program is free for non-commercial purposes.
 * You may implement this program for any non-commercial application. You may 
 * also implement this program for commercial purposes, provided that you
 * obtain my written permission. Any modification of this program is covered
 * by this copyright.
 *
 * == Copyright (c) 1994-7,  Robert Morelos-Zaragoza. All rights reserved.  ==
 *
 * m = order of the Galois field GF(2**m) 
 * n = 2**m - 1 = size of the multiplicative group of GF(2**m)
 * length = length of the BCH code
 * t = error correcting capability (max. no. of errors the code corrects)
 * d = 2*t + 1 = designed min. distance = no. of consecutive roots of g(x) + 1
 * k = n - deg(g(x)) = dimension (no. of information bits/codeword) of the code
 * p[] = coefficients of a primitive polynomial used to generate GF(2**m)
 * g[] = coefficients of the generator polynomial, g(x)
 * alpha_to [] = log table of GF(2**m) 
 * index_of[] = antilog table of GF(2**m)
 * data[] = information bits = coefficients of data polynomial, i(x)
 * bb[] = coefficients of redundancy polynomial x^(length-k) i(x) modulo g(x)
 * numerr = number of errors 
 * errpos[] = error positions 
 * recd[] = coefficients of the received polynomial 
 * decerror = number of decoding errors (in _message_ positions) 
 *
 */

#include <math.h>
#include <stdio.h>

int             m, n, length, k, t, d;
int             p[21];
int             alpha_to[1048576], index_of[1048576], g[548576];
int             recd[1048576], data[1048576], bb[548576];
int             seed;
int             numerr, errpos[1024], decerror = 0;


void 
read_p()
/*
 *	Read m, the degree of a primitive polynomial p(x) used to compute the
 *	Galois field GF(2**m). Get precomputed coefficients p[] of p(x). Read
 *	the code length.
 */
{
	int			i, ninf;

	printf("bch3: An encoder/decoder for binary BCH codes\n");
	printf("Copyright (c) 1994-7. Robert Morelos-Zaragoza.\n");
	printf("This program is free, please read first the copyright notice.\n");
	printf("\nFirst, enter a value of m such that the code length is\n");
	printf("2**(m-1) - 1 < length <= 2**m - 1\n\n");
    do {
	   printf("Enter m (between 2 and 20): ");
	   scanf("%d", &m);
    } while ( !(m>1) || !(m<21) );
	for (i=1; i<m; i++)
		p[i] = 0;
	p[0] = p[m] = 1;
	if (m == 2)			p[1] = 1;
	else if (m == 3)	p[1] = 1;
	else if (m == 4)	p[1] = 1;
	else if (m == 5)	p[2] = 1;
	else if (m == 6)	p[1] = 1;
	else if (m == 7)	p[1] = 1;
	else if (m == 8)	p[4] = p[5] = p[6] = 1;
	else if (m == 9)	p[4] = 1;
	else if (m == 10)	p[3] = 1;
	else if (m == 11)	p[2] = 1;
	else if (m == 12)	p[3] = p[4] = p[7] = 1;
	else if (m == 13)	p[1] = p[3] = p[4] = 1;
	else if (m == 14)	p[1] = p[11] = p[12] = 1;
	else if (m == 15)	p[1] = 1;
	else if (m == 16)	p[2] = p[3] = p[5] = 1;
	else if (m == 17)	p[3] = 1;
	else if (m == 18)	p[7] = 1;
	else if (m == 19)	p[1] = p[5] = p[6] = 1;
	else if (m == 20)	p[3] = 1;
	printf("p(x) = ");
    n = 1;
	for (i = 0; i <= m; i++) {
        n *= 2;
		printf("%1d", p[i]);
        }
	printf("\n");
	n = n / 2 - 1;
	ninf = (n + 1) / 2 - 1;
	do  {
		printf("Enter code length (%d < length <= %d): ", ninf, n);
		scanf("%d", &length);
	} while ( !((length <= n)&&(length>ninf)) );
}


void 
generate_gf()
/*
 * Generate field GF(2**m) from the irreducible polynomial p(X) with
 * coefficients in p[0]..p[m].
 *
 * Lookup tables:
 *   index->polynomial form: alpha_to[] contains j=alpha^i;
 *   polynomial form -> index form:	index_of[j=alpha^i] = i
 *
 * alpha=2 is the primitive element of GF(2**m) 
 */
{
	register int    i, mask;

	mask = 1;
	alpha_to[m] = 0;
	for (i = 0; i < m; i++) {
		alpha_to[i] = mask;
		index_of[alpha_to[i]] = i;
		if (p[i] != 0)
			alpha_to[m] ^= mask;
		mask <<= 1;
	}
	index_of[alpha_to[m]] = m;
	mask >>= 1;
	for (i = m + 1; i < n; i++) {
		if (alpha_to[i - 1] >= mask)
		  alpha_to[i] = alpha_to[m] ^ ((alpha_to[i - 1] ^ mask) << 1);
		else
		  alpha_to[i] = alpha_to[i - 1] << 1;
		index_of[alpha_to[i]] = i;
	}
	index_of[0] = -1;
}


void 
gen_poly()
/*
 * Compute the generator polynomial of a binary BCH code. Fist generate the
 * cycle sets modulo 2**m - 1, cycle[][] =  (i, 2*i, 4*i, ..., 2^l*i). Then
 * determine those cycle sets that contain integers in the set of (d-1)
 * consecutive integers {1..(d-1)}. The generator polynomial is calculated
 * as the product of linear factors of the form (x+alpha^i), for every i in
 * the above cycle sets.
 */
{
	register int	ii, jj, ll, kaux;
	register int	test, aux, nocycles, root, noterms, rdncy;
	int             cycle[1024][21], size[1024], min[1024], zeros[1024];

	/* Generate cycle sets modulo n, n = 2**m - 1 */
	cycle[0][0] = 0;
	size[0] = 1;
	cycle[1][0] = 1;
	size[1] = 1;
	jj = 1;			/* cycle set index */
	if (m > 9)  {
		printf("Computing cycle sets modulo %d\n", n);
		printf("(This may take some time)...\n");
	}
	do {
		/* Generate the jj-th cycle set */
		ii = 0;
		do {
			ii++;
			cycle[jj][ii] = (cycle[jj][ii - 1] * 2) % n;
			size[jj]++;
			aux = (cycle[jj][ii] * 2) % n;
		} while (aux != cycle[jj][0]);
		/* Next cycle set representative */
		ll = 0;
		do {
			ll++;
			test = 0;
			for (ii = 1; ((ii <= jj) && (!test)); ii++)	
			/* Examine previous cycle sets */
			  for (kaux = 0; ((kaux < size[ii]) && (!test)); kaux++)
			     if (ll == cycle[ii][kaux])
			        test = 1;
		} while ((test) && (ll < (n - 1)));
		if (!(test)) {
			jj++;	/* next cycle set index */
			cycle[jj][0] = ll;
			size[jj] = 1;
		}
	} while (ll < (n - 1));
	nocycles = jj;		/* number of cycle sets modulo n */

	printf("Enter the error correcting capability, t: ");
	scanf("%d", &t);

	d = 2 * t + 1;

	/* Search for roots 1, 2, ..., d-1 in cycle sets */
	kaux = 0;
	rdncy = 0;
	for (ii = 1; ii <= nocycles; ii++) {
		min[kaux] = 0;
		test = 0;
		for (jj = 0; ((jj < size[ii]) && (!test)); jj++)
			for (root = 1; ((root < d) && (!test)); root++)
				if (root == cycle[ii][jj])  {
					test = 1;
					min[kaux] = ii;
				}
		if (min[kaux]) {
			rdncy += size[min[kaux]];
			kaux++;
		}
	}
	noterms = kaux;
	kaux = 1;
	for (ii = 0; ii < noterms; ii++)
		for (jj = 0; jj < size[min[ii]]; jj++) {
			zeros[kaux] = cycle[min[ii]][jj];
			kaux++;
		}

	k = length - rdncy;

    if (k<0)
      {
         printf("Parameters invalid!\n");
         exit(0);
      }

	printf("This is a (%d, %d, %d) binary BCH code\n", length, k, d);

	/* Compute the generator polynomial */
	g[0] = alpha_to[zeros[1]];
	g[1] = 1;		/* g(x) = (X + zeros[1]) initially */
	for (ii = 2; ii <= rdncy; ii++) {
	  g[ii] = 1;
	  for (jj = ii - 1; jj > 0; jj--)
	    if (g[jj] != 0)
	      g[jj] = g[jj - 1] ^ alpha_to[(index_of[g[jj]] + zeros[ii]) % n];
	    else
	      g[jj] = g[jj - 1];
	  g[0] = alpha_to[(index_of[g[0]] + zeros[ii]) % n];
	}
	printf("Generator polynomial:\ng(x) = ");
	for (ii = 0; ii <= rdncy; ii++) {
	  printf("%d", g[ii]);
	  if (ii && ((ii % 50) == 0))
	    printf("\n");
	}
	printf("\n");
}


void 
encode_bch()
/*
 * Compute redundacy bb[], the coefficients of b(x). The redundancy
 * polynomial b(x) is the remainder after dividing x^(length-k)*data(x)