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