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/* The guts of the Reed-Solomon encoder, meant to be #included
* into a function body with the following typedefs, macros and variables supplied
* according to the code parameters:
* data_t - a typedef for the data symbol
* data_t data[] - array of NN-NROOTS-PAD and type data_t to be encoded
* data_t parity[] - an array of NROOTS and type data_t to be written with parity symbols
* NROOTS - the number of roots in the RS code generator polynomial,
* which is the same as the number of parity symbols in a block.
Integer variable or literal.
*
* NN - the total number of symbols in a RS block. Integer variable or literal.
* PAD - the number of pad symbols in a block. Integer variable or literal.
* ALPHA_TO - The address of an array of NN elements to convert Galois field
* elements in index (log) form to polynomial form. Read only.
* INDEX_OF - The address of an array of NN elements to convert Galois field
* elements in polynomial form to index (log) form. Read only.
* MODNN - a function to reduce its argument modulo NN. May be inline or a macro.
* GENPOLY - an array of NROOTS+1 elements containing the generator polynomial in index
form
* The memset() and memmove() functions are used. The appropriate header
* file declaring these functions (usually <string.h>) must be included by the calling
* program.
* Copyright 2004, Phil Karn, KA9Q
* May be used under the terms of the GNU Lesser General Public License (LGPL)
*/
#undef A0
#define A0 (NN) /* Special reserved value encoding zero in index form */
{
int i, j;
data_t feedback;
memset(parity, 0, NROOTS * sizeof(data_t));
for (i = 0; i < NN - NROOTS - PAD; i++) {
feedback = INDEX_OF[data[i] ^ parity[0]];
if (feedback != A0) { /* feedback term is non-zero */
#ifdef UNNORMALIZED
/* This line is unnecessary when GENPOLY[NROOTS] is unity, as it must
* always be for the polynomials constructed by init_rs()
*/
feedback = MODNN(NN - GENPOLY[NROOTS] + feedback);
#endif
for (j = 1; j < NROOTS; j++)
parity[j] ^= ALPHA_TO[MODNN(feedback + GENPOLY[NROOTS - j])];
}
/* Shift */
memmove(&parity[0], &parity[1], sizeof(data_t) * (NROOTS - 1));
if (feedback != A0)
parity[NROOTS - 1] = ALPHA_TO[MODNN(feedback + GENPOLY[0])];
else
parity[NROOTS - 1] = 0;
}
}
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