bch.h

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/* BCH.H */

#ifndef _BCH_H_
#define _BCH_H_

typedef enum { BCHNONE, BCH15_5, BCH15_7, BCH15_11, BCH31_6, BCH31_11, BCH63_7, BCH63_10, GOLAY23_12,} bch_type;

/* bch.h
 *
 * Brendan Dowling 9/20/95
 * Max Luttrell 9/96
 */


static 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)
 */


static 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.
 */

void encode_bch(unsigned long bits, byte *block, bch_type type);
/*
 * This function takes the bits and inserts the bch bits into
 *   the (header) block 
 */

int decode_bch(unsigned long *bits, byte *block, bch_type type, int t0);
/*
 * This function takes a header & decodes the bch putting result in bits
 *
 * For detection mode:
 *   Returns 1 if decoding ok.
 *   Returns 0 if decoding not ok.
 *
 * For correction mode,
 *   always returns 1
 *
 * correction is 0 if bch codec used for detection, 1 if for correction
 *   at this point, only correction mode works properly
 */

bch_type strtobch_type(char *str);
/*
 * This function takes a string (such as "BCH15_5") and
 *   returns a bch_type (which looks almost identical, i.e. BCH15_5)
 */

int code_length(bch_type b);
/*
 * This function takes a bch_type (such as the one returned above)
 *   and returns the number of bits total that the bch code will be
 *   i.e. BCH15_5 will return 15
 */

int info_length(bch_type b);
/*
 * This function does the same as code_length() but it returns the
 *   info part instead. i.e. BCH15_5 will return 5
 */

char *bch_type_str(bch_type type);
/*
 * This function take and type and returns the string equivalent,
 *   kind of like a reverse strtobch_type()
 */

#endif

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