2 * Provides routines for encoding and decoding the extended Golay
5 * This implementation will detect up to 4 errors in a codeword (without
6 * being able to correct them); it will correct up to 3 errors.
8 * Wireshark - Network traffic analyzer
9 * By Gerald Combs <gerald@wireshark.org>
10 * Copyright 1998 Gerald Combs
12 * This program is free software; you can redistribute it and/or
13 * modify it under the terms of the GNU General Public License
14 * as published by the Free Software Foundation; either version 2
15 * of the License, or (at your option) any later version.
17 * This program is distributed in the hope that it will be useful,
18 * but WITHOUT ANY WARRANTY; without even the implied warranty of
19 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
20 * GNU General Public License for more details.
22 * You should have received a copy of the GNU General Public License
23 * along with this program; if not, write to the Free Software
24 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
33 These entries are formed from the matrix specified in H.223/B.3.2.1.3;
34 it's first transposed so we have:
36 [P1 ] [111110010010] [MC1 ]
37 [P2 ] [011111001001] [MC2 ]
38 [P3 ] [110001110110] [MC3 ]
39 [P4 ] [011000111011] [MC4 ]
40 [P5 ] [110010001111] [MPL1]
41 [P6 ] = [100111010101] [MPL2]
42 [P7 ] [101101111000] [MPL3]
43 [P8 ] [010110111100] [MPL4]
44 [P9 ] [001011011110] [MPL5]
45 [P10] [000101101111] [MPL6]
46 [P11] [111100100101] [MPL7]
47 [P12] [101011100011] [MPL8]
49 So according to the equation, P1 = MC1+MC2+MC3+MC4+MPL1+MPL4+MPL7
51 Looking down the first column, we see that if MC1 is set, we toggle bits
52 1,3,5,6,7,11,12 of the parity: in binary, 110001110101 = 0xE3A
54 Similarly, to calculate the inverse, we read across the top of the table and
55 see that P1 is affected by bits MC1,MC2,MC3,MC4,MPL1,MPL4,MPL7: in binary,
58 I've seen cunning implementations of this which only use one table. That
59 technique doesn't seem to work with these numbers though.
62 static const guint golay_encode_matrix[12] = {
77 static const guint golay_decode_matrix[12] = {
94 /* Function to compute the Hamming weight of a 12-bit integer */
95 static guint weight12(guint vector)
105 /* returns the golay coding of the given 12-bit word */
106 static guint golay_coding(guint w)
111 for( i = 0; i<12; i++ ) {
113 out ^= golay_encode_matrix[i];
118 /* encodes a 12-bit word to a 24-bit codeword */
119 guint32 golay_encode(guint w)
121 return ((guint32)w) | ((guint32)golay_coding(w))<<12;
126 /* returns the golay coding of the given 12-bit word */
127 static guint golay_decoding(guint w)
132 for( i = 0; i<12; i++ ) {
134 out ^= golay_decode_matrix[i];
140 /* return a mask showing the bits which are in error in a received
141 * 24-bit codeword, or -1 if 4 errors were detected.
143 gint32 golay_errors(guint32 codeword)
145 guint received_data, received_parity;
148 guint inv_syndrome = 0;
150 received_parity = (guint)(codeword>>12);
151 received_data = (guint)codeword & 0xfff;
153 /* We use the C notation ^ for XOR to represent addition modulo 2.
155 * Model the received codeword (r) as the transmitted codeword (u)
156 * plus an error vector (e).
160 * Then we calculate a syndrome (s):
162 * s = r * H, where H = [ P ], where I12 is the identity matrix
165 * (In other words, we calculate the parity check for the received
166 * data bits, and add them to the received parity bits)
169 syndrome = received_parity ^ (golay_coding(received_data));
170 w = weight12(syndrome);
173 * The properties of the golay code are such that the Hamming distance (ie,
174 * the minimum distance between codewords) is 8; that means that one bit of
175 * error in the data bits will cause 7 errors in the parity bits.
177 * In particular, if we find 3 or fewer errors in the parity bits, either:
178 * - there are no errors in the data bits, or
179 * - there are at least 5 errors in the data bits
180 * we hope for the former (we don't profess to deal with the
184 return ((gint32) syndrome)<<12;
187 /* the next thing to try is one error in the data bits.
188 * we try each bit in turn and see if an error in that bit would have given
189 * us anything like the parity bits we got. At this point, we tolerate two
190 * errors in the parity bits, but three or more errors would give a total
191 * error weight of 4 or more, which means it's actually uncorrectable or
192 * closer to another codeword. */
194 for( i = 0; i<12; i++ ) {
196 guint coding_error = golay_encode_matrix[i];
197 if( weight12(syndrome^coding_error) <= 2 ) {
198 return (gint32)((((guint32)(syndrome^coding_error))<<12) | (guint32)error) ;
202 /* okay then, let's see whether the parity bits are error free, and all the
203 * errors are in the data bits. model this as follows:
205 * [r | pr] = [u | pu] + [e | 0]
208 * pu = H * u => u = H' * pu = H' * pr , where H' is inverse of H
210 * we already have s = H*r + pr, so pr = s - H*r = s ^ H*r
212 * = (H' * ( s ^ H*r )) ^ r
216 * Once again, we accept up to three error bits...
219 inv_syndrome = golay_decoding(syndrome);
220 w = weight12(inv_syndrome);
222 return (gint32)inv_syndrome;
225 /* Final shot: try with 2 errors in the data bits, and 1 in the parity
226 * bits; as before we try each of the bits in the parity in turn */
227 for( i = 0; i<12; i++ ) {
229 guint coding_error = golay_decode_matrix[i];
230 if( weight12(inv_syndrome^coding_error) <= 2 ) {
231 guint32 error_word = ((guint32)(inv_syndrome^coding_error)) | ((guint32)error)<<12;
232 return (gint32)error_word;
236 /* uncorrectable error */
242 /* decode a received codeword. Up to 3 errors are corrected for; 4
243 errors are detected as uncorrectable (return -1); 5 or more errors
244 cause an incorrect correction.
246 gint golay_decode(guint32 w)
248 guint data = (guint)w & 0xfff;
249 gint32 errors = golay_errors(w);
254 data_errors = (guint)errors & 0xfff;
255 return (gint)(data ^ data_errors);
264 * indent-tabs-mode: nil
267 * ex: set shiftwidth=4 tabstop=8 expandtab:
268 * :indentSize=4:tabSize=8:noTabs=true: