3 * This is a simple Reed-Solomon encoder
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7 * it under the terms of the GNU General Public License as published by
8 * the Free Software Foundation; either version 2 of the License, or
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22 // It is not written with high efficiency in mind, so is probably
23 // not suitable for real-time encoding. The aim was to keep it
24 // simple, general and clear.
26 // <Some notes on the theory and implementation need to be added here>
29 // First call rs_init_gf(poly) to set up the Galois Field parameters.
30 // Then call rs_init_code(size, index) to set the encoding size
31 // Then call rs_encode(datasize, data, out) to encode the data.
33 // These can be called repeatedly as required - but note that
34 // rs_init_code must be called following any rs_init_gf call.
36 // If the parameters are fixed, some of the statics below can be
37 // replaced with constants in the obvious way, and additionally
38 // malloc/free can be avoided by using static arrays of a suitable
41 #include <stdio.h> // only needed for debug (main)
42 #include <stdlib.h> // only needed for malloc/free
45 static int symsize; // in bits
46 static int logmod; // 2**symsize - 1
49 static int *log = NULL, *alog = NULL, *rspoly = NULL;
51 // rs_init_gf(poly) initialises the parameters for the Galois Field.
52 // The symbol size is determined from the highest bit set in poly
53 // This implementation will support sizes up to 30 bits (though that
54 // will result in very large log/antilog tables) - bit sizes of
57 // The poly is the bit pattern representing the GF characteristic
58 // polynomial. e.g. for ECC200 (8-bit symbols) the polynomial is
59 // a**8 + a**5 + a**3 + a**2 + 1, which translates to 0x12d.
61 void rs_init_gf(int poly)
65 // Return storage from previous setup
72 // Find the top bit, and hence the symbol size
73 for (b = 1, m = 0; b <= poly; b <<= 1)
80 // Calculate the log/alog tables
81 logmod = (1 << m) - 1;
82 log = (int *)malloc(sizeof(int) * (logmod + 1));
83 alog = (int *)malloc(sizeof(int) * logmod);
85 for (p = 1, v = 0; v < logmod; v++) {
94 // rs_init_code(nsym, index) initialises the Reed-Solomon encoder
95 // nsym is the number of symbols to be generated (to be appended
96 // to the input data). index is usually 1 - it is the index of
97 // the constant in the first term (i) of the RS generator polynomial:
98 // (x + 2**i)*(x + 2**(i+1))*... [nsym terms]
99 // For ECC200, index is 1.
101 void rs_init_code(int nsym, int index)
107 rspoly = (int *)malloc(sizeof(int) * (nsym + 1));
112 for (i = 1; i <= nsym; i++) {
114 for (k = i - 1; k > 0; k--) {
117 alog[(log[rspoly[k]] + index) % logmod];
118 rspoly[k] ^= rspoly[k - 1];
120 rspoly[0] = alog[(log[rspoly[0]] + index) % logmod];
125 // Note that the following uses byte arrays, so is only suitable for
126 // symbol sizes up to 8 bits. Just change the data type of data and res
127 // to unsigned int * for larger symbols.
129 void rs_encode(int len, unsigned char *data, unsigned char *res)
132 for (i = 0; i < rlen; i++)
134 for (i = 0; i < len; i++) {
135 m = res[rlen - 1] ^ data[i];
136 for (k = rlen - 1; k > 0; k--) {
141 log[rspoly[k]]) % logmod];
146 res[0] = alog[(log[m] + log[rspoly[0]]) % logmod];
153 // The following tests the routines with the ISO/IEC 16022 Annexe R data
158 unsigned char data[9] = { 142, 164, 186 };
159 unsigned char out[5];
164 rs_encode(3, data, out);
166 printf("Result of Annexe R encoding:\n");
167 for (i = 4; i >= 0; i--)
168 printf(" %d\n", out[i]);