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LibrePilot/flight/libraries/rscode/rs.c

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/*
* Reed Solomon Encoder/Decoder
*
* Copyright Henry Minsky (hqm@alum.mit.edu) 1991-2009
*
* This software library is licensed under terms of the GNU GENERAL
* PUBLIC LICENSE
*
* RSCODE is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* RSCODE is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with Rscode. If not, see <http://www.gnu.org/licenses/>.
* Commercial licensing is available under a separate license, please
* contact author for details.
*
* Source code is available at http://rscode.sourceforge.net
*/
#include <stdio.h>
#include <ctype.h>
#include "ecc.h"
/* Encoder parity bytes */
int pBytes[MAXDEG];
/* Decoder syndrome bytes */
int synBytes[MAXDEG];
/* generator polynomial */
int genPoly[MAXDEG*2];
//int DEBUG = FALSE;
static void
compute_genpoly (int nbytes, int genpoly[]);
/* Initialize lookup tables, polynomials, etc. */
void
initialize_ecc ()
{
/* Initialize the galois field arithmetic tables */
init_galois_tables();
/* Compute the encoder generator polynomial */
compute_genpoly(RS_ECC_NPARITY, genPoly);
}
void
zero_fill_from (unsigned char buf[], int from, int to)
{
int i;
for (i = from; i < to; i++) buf[i] = 0;
}
/* debugging routines */
void
print_parity (void)
{
#ifdef NEVER
int i;
printf("Parity Bytes: ");
for (i = 0; i < RS_ECC_NPARITY; i++)
printf("[%d]:%x, ",i,pBytes[i]);
printf("\n");
#endif
}
void
print_syndrome (void)
{
#ifdef NEVER
int i;
printf("Syndrome Bytes: ");
for (i = 0; i < RS_ECC_NPARITY; i++)
printf("[%d]:%x, ",i,synBytes[i]);
printf("\n");
#endif
}
/* Append the parity bytes onto the end of the message */
void
build_codeword (unsigned char msg[], int nbytes, unsigned char dst[])
{
int i;
for (i = 0; i < nbytes; i++) dst[i] = msg[i];
for (i = 0; i < RS_ECC_NPARITY; i++) {
dst[i+nbytes] = pBytes[RS_ECC_NPARITY-1-i];
}
}
/**********************************************************
* Reed Solomon Decoder
*
* Computes the syndrome of a codeword. Puts the results
* into the synBytes[] array.
*/
void
decode_data(unsigned char data[], int nbytes)
{
int i, j, sum;
for (j = 0; j < RS_ECC_NPARITY; j++) {
sum = 0;
for (i = 0; i < nbytes; i++) {
sum = data[i] ^ gmult(gexp[j+1], sum);
}
synBytes[j] = sum;
}
}
/* Check if the syndrome is zero */
int
check_syndrome (void)
{
int i, nz = 0;
for (i =0 ; i < RS_ECC_NPARITY; i++) {
if (synBytes[i] != 0) {
nz = 1;
break;
}
}
return nz;
}
void
debug_check_syndrome (void)
{
#ifdef NEVER
int i;
for (i = 0; i < 3; i++) {
printf(" inv log S[%d]/S[%d] = %d\n", i, i+1,
glog[gmult(synBytes[i], ginv(synBytes[i+1]))]);
}
#endif
}
/* Create a generator polynomial for an n byte RS code.
* The coefficients are returned in the genPoly arg.
* Make sure that the genPoly array which is passed in is
* at least n+1 bytes long.
*/
static void
compute_genpoly (int nbytes, int genpoly[])
{
int i, tp[MAXDEG], tp1[MAXDEG];
/* multiply (x + a^n) for n = 1 to nbytes */
zero_poly(tp1);
tp1[0] = 1;
for (i = 1; i <= nbytes; i++) {
zero_poly(tp);
tp[0] = gexp[i]; /* set up x+a^n */
tp[1] = 1;
mult_polys(genpoly, tp, tp1);
copy_poly(tp1, genpoly);
}
}
/* Simulate a LFSR with generator polynomial for n byte RS code.
* Pass in a pointer to the data array, and amount of data.
*
* The parity bytes are deposited into pBytes[], and the whole message
* and parity are copied to dest to make a codeword.
*
*/
void
encode_data (unsigned char msg[], int nbytes, unsigned char dst[])
{
int i, LFSR[RS_ECC_NPARITY+1],dbyte, j;
for(i=0; i < RS_ECC_NPARITY+1; i++) LFSR[i]=0;
for (i = 0; i < nbytes; i++) {
dbyte = msg[i] ^ LFSR[RS_ECC_NPARITY-1];
for (j = RS_ECC_NPARITY-1; j > 0; j--) {
LFSR[j] = LFSR[j-1] ^ gmult(genPoly[j], dbyte);
}
LFSR[0] = gmult(genPoly[0], dbyte);
}
for (i = 0; i < RS_ECC_NPARITY; i++)
pBytes[i] = LFSR[i];
build_codeword(msg, nbytes, dst);
}