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/**
*
* This is a simple Reed-Solomon encoder
* (C) Cliff Hones 2004
*
* This program 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 2 of the License, or
* (at your option) any later version.
*
* This program 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 this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*
*/
// It is not written with high efficiency in mind, so is probably
// not suitable for real-time encoding. The aim was to keep it
// simple, general and clear.
//
// <Some notes on the theory and implementation need to be added here>
// Usage:
// First call rs_init_gf(poly) to set up the Galois Field parameters.
// Then call rs_init_code(size, index) to set the encoding size
// Then call rs_encode(datasize, data, out) to encode the data.
//
// These can be called repeatedly as required - but note that
// rs_init_code must be called following any rs_init_gf call.
//
// If the parameters are fixed, some of the statics below can be
// replaced with constants in the obvious way, and additionally
// malloc/free can be avoided by using static arrays of a suitable
// size.
#include <stdio.h> // only needed for debug (main)
#include <stdlib.h> // only needed for malloc/free
static int gfpoly;
static int symsize; // in bits
static int logmod; // 2**symsize - 1
static int rlen;
static int *log = NULL, *alog = NULL, *rspoly = NULL;
// rs_init_gf(poly) initialises the parameters for the Galois Field.
// The symbol size is determined from the highest bit set in poly
// This implementation will support sizes up to 30 bits (though that
// will result in very large log/antilog tables) - bit sizes of
// 8 or 4 are typical
//
// The poly is the bit pattern representing the GF characteristic
// polynomial. e.g. for ECC200 (8-bit symbols) the polynomial is
// a**8 + a**5 + a**3 + a**2 + 1, which translates to 0x12d.
void rs_init_gf(int poly)
{
int m, b, p, v;
// Return storage from previous setup
if (log) {
free(log);
free(alog);
free(rspoly);
rspoly = NULL;
}
// Find the top bit, and hence the symbol size
for (b = 1, m = 0; b <= poly; b <<= 1)
m++;
b >>= 1;
m--;
gfpoly = poly;
symsize = m;
// Calculate the log/alog tables
logmod = (1 << m) - 1;
log = (int *)malloc(sizeof(int) * (logmod + 1));
alog = (int *)malloc(sizeof(int) * logmod);
for (p = 1, v = 0; v < logmod; v++) {
alog[v] = p;
log[p] = v;
p <<= 1;
if (p & b)
p ^= poly;
}
}
// rs_init_code(nsym, index) initialises the Reed-Solomon encoder
// nsym is the number of symbols to be generated (to be appended
// to the input data). index is usually 1 - it is the index of
// the constant in the first term (i) of the RS generator polynomial:
// (x + 2**i)*(x + 2**(i+1))*... [nsym terms]
// For ECC200, index is 1.
void rs_init_code(int nsym, int index)
{
int i, k;
if (rspoly)
free(rspoly);
rspoly = (int *)malloc(sizeof(int) * (nsym + 1));
rlen = nsym;
rspoly[0] = 1;
for (i = 1; i <= nsym; i++) {
rspoly[i] = 1;
for (k = i - 1; k > 0; k--) {
if (rspoly[k])
rspoly[k] =
alog[(log[rspoly[k]] + index) % logmod];
rspoly[k] ^= rspoly[k - 1];
}
rspoly[0] = alog[(log[rspoly[0]] + index) % logmod];
index++;
}
}
// Note that the following uses byte arrays, so is only suitable for
// symbol sizes up to 8 bits. Just change the data type of data and res
// to unsigned int * for larger symbols.
void rs_encode(int len, unsigned char *data, unsigned char *res)
{
int i, k, m;
for (i = 0; i < rlen; i++)
res[i] = 0;
for (i = 0; i < len; i++) {
m = res[rlen - 1] ^ data[i];
for (k = rlen - 1; k > 0; k--) {
if (m && rspoly[k])
res[k] =
res[k -
1] ^ alog[(log[m] +
log[rspoly[k]]) % logmod];
else
res[k] = res[k - 1];
}
if (m && rspoly[0])
res[0] = alog[(log[m] + log[rspoly[0]]) % logmod];
else
res[0] = 0;
}
}
#ifndef LIB
// The following tests the routines with the ISO/IEC 16022 Annexe R data
int main(void)
{
register int i;
unsigned char data[9] = { 142, 164, 186 };
unsigned char out[5];
rs_init_gf(0x12d);
rs_init_code(5, 1);
rs_encode(3, data, out);
printf("Result of Annexe R encoding:\n");
for (i = 4; i >= 0; i--)
printf(" %d\n", out[i]);
return 0;
}
#endif
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