1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
|
/*
* 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(NPAR, 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)
{
int i;
printf("Parity Bytes: ");
for (i = 0; i < NPAR; i++)
printf("[%d]:%x, ",i,pBytes[i]);
printf("\n");
}
void
print_syndrome (void)
{
int i;
printf("Syndrome Bytes: ");
for (i = 0; i < NPAR; i++)
printf("[%d]:%x, ",i,synBytes[i]);
printf("\n");
}
/* 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 < NPAR; i++) {
dst[i+nbytes] = pBytes[NPAR-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 < NPAR; 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 < NPAR; i++) {
if (synBytes[i] != 0) {
nz = 1;
break;
}
}
return nz;
}
void
debug_check_syndrome (void)
{
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]))]);
}
}
/* 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[256], tp1[256];
/* 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[NPAR+1],dbyte, j;
for(i=0; i < NPAR+1; i++) LFSR[i]=0;
for (i = 0; i < nbytes; i++) {
dbyte = msg[i] ^ LFSR[NPAR-1];
for (j = NPAR-1; j > 0; j--) {
LFSR[j] = LFSR[j-1] ^ gmult(genPoly[j], dbyte);
}
LFSR[0] = gmult(genPoly[0], dbyte);
}
for (i = 0; i < NPAR; i++)
pBytes[i] = LFSR[i];
build_codeword(msg, nbytes, dst);
}
|
