1 /* The guts of the Reed-Solomon encoder, meant to be #included 2 * into a function body with the following typedefs, macros and variables supplied 3 * according to the code parameters: 4 5 * data_t - a typedef for the data symbol 6 * data_t data[] - array of NN-NROOTS-PAD and type data_t to be encoded 7 * data_t parity[] - an array of NROOTS and type data_t to be written with parity symbols 8 * NROOTS - the number of roots in the RS code generator polynomial, 9 * which is the same as the number of parity symbols in a block. 10 Integer variable or literal. 11 * 12 * NN - the total number of symbols in a RS block. Integer variable or literal. 13 * PAD - the number of pad symbols in a block. Integer variable or literal. 14 * ALPHA_TO - The address of an array of NN elements to convert Galois field 15 * elements in index (log) form to polynomial form. Read only. 16 * INDEX_OF - The address of an array of NN elements to convert Galois field 17 * elements in polynomial form to index (log) form. Read only. 18 * MODNN - a function to reduce its argument modulo NN. May be inline or a macro. 19 * GENPOLY - an array of NROOTS+1 elements containing the generator polynomial in index form 20 21 * The memset() and memmove() functions are used. The appropriate header 22 * file declaring these functions (usually <string.h>) must be included by the calling 23 * program. 24 25 * Copyright 2004, Phil Karn, KA9Q 26 * May be used under the terms of the GNU Lesser General Public License (LGPL) 27 */ 28 29 30 #undef A0 31 #define A0 (NN) /* Special reserved value encoding zero in index form */ 32 33 { 34 int i, j; 35 data_t feedback; 36 37 memset(parity,0,NROOTS*sizeof(data_t)); 38 39 for(i=0;i<NN-NROOTS-PAD;i++){ 40 feedback = INDEX_OF[data[i] ^ parity[0]]; 41 if(feedback != A0){ /* feedback term is non-zero */ 42 #ifdef UNNORMALIZED 43 /* This line is unnecessary when GENPOLY[NROOTS] is unity, as it must 44 * always be for the polynomials constructed by init_rs() 45 */ 46 feedback = MODNN(NN - GENPOLY[NROOTS] + feedback); 47 #endif 48 for(j=1;j<NROOTS;j++) 49 parity[j] ^= ALPHA_TO[MODNN(feedback + GENPOLY[NROOTS-j])]; 50 } 51 /* Shift */ 52 memmove(&parity[0],&parity[1],sizeof(data_t)*(NROOTS-1)); 53 if(feedback != A0) 54 parity[NROOTS-1] = ALPHA_TO[MODNN(feedback + GENPOLY[0])]; 55 else 56 parity[NROOTS-1] = 0; 57 } 58 } 59
