120 lines
3.0 KiB
Plaintext
120 lines
3.0 KiB
Plaintext
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/* Initialize a RS codec
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*
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* Copyright 2002 Phil Karn, KA9Q
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* May be used under the terms of the GNU General Public License (GPL)
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*/
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#include <stdlib.h>
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#ifdef CCSDS
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#include "ccsds.h"
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#elif defined(BIGSYM)
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#include "int.h"
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#else
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#include "char.h"
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#endif
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void FREE_RS(void *p){
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struct rs *rs = (struct rs *)p;
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free(rs->alpha_to);
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free(rs->index_of);
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free(rs->genpoly);
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free(rs);
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}
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/* Initialize a Reed-Solomon codec
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* symsize = symbol size, bits (1-8)
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* gfpoly = Field generator polynomial coefficients
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* fcr = first root of RS code generator polynomial, index form
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* prim = primitive element to generate polynomial roots
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* nroots = RS code generator polynomial degree (number of roots)
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*/
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void *INIT_RS(unsigned int symsize,unsigned int gfpoly,unsigned fcr,unsigned prim,
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unsigned int nroots){
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struct rs *rs;
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int i, j, sr,root,iprim;
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if(symsize > 8*sizeof(DTYPE))
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return NULL; /* Need version with ints rather than chars */
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if(fcr >= (1<<symsize))
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return NULL;
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if(prim == 0 || prim >= (1<<symsize))
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return NULL;
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if(nroots >= (1<<symsize))
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return NULL; /* Can't have more roots than symbol values! */
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rs = (struct rs *)calloc(1,sizeof(struct rs));
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rs->mm = symsize;
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rs->nn = (1<<symsize)-1;
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rs->alpha_to = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
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if(rs->alpha_to == NULL){
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free(rs);
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return NULL;
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}
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rs->index_of = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
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if(rs->index_of == NULL){
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free(rs->alpha_to);
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free(rs);
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return NULL;
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}
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/* Generate Galois field lookup tables */
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rs->index_of[0] = A0; /* log(zero) = -inf */
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rs->alpha_to[A0] = 0; /* alpha**-inf = 0 */
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sr = 1;
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for(i=0;i<rs->nn;i++){
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rs->index_of[sr] = i;
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rs->alpha_to[i] = sr;
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sr <<= 1;
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if(sr & (1<<symsize))
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sr ^= gfpoly;
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sr &= rs->nn;
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}
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if(sr != 1){
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/* field generator polynomial is not primitive! */
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free(rs->alpha_to);
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free(rs->index_of);
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free(rs);
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return NULL;
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}
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/* Form RS code generator polynomial from its roots */
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rs->genpoly = (DTYPE *)malloc(sizeof(DTYPE)*(nroots+1));
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if(rs->genpoly == NULL){
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free(rs->alpha_to);
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free(rs->index_of);
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free(rs);
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return NULL;
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}
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rs->fcr = fcr;
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rs->prim = prim;
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rs->nroots = nroots;
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/* Find prim-th root of 1, used in decoding */
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for(iprim=1;(iprim % prim) != 0;iprim += rs->nn)
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;
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rs->iprim = iprim / prim;
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rs->genpoly[0] = 1;
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for (i = 0,root=fcr*prim; i < nroots; i++,root += prim) {
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rs->genpoly[i+1] = 1;
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/* Multiply rs->genpoly[] by @**(root + x) */
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for (j = i; j > 0; j--){
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if (rs->genpoly[j] != 0)
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rs->genpoly[j] = rs->genpoly[j-1] ^ rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[j]] + root)];
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else
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rs->genpoly[j] = rs->genpoly[j-1];
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}
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/* rs->genpoly[0] can never be zero */
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rs->genpoly[0] = rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[0]] + root)];
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}
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/* convert rs->genpoly[] to index form for quicker encoding */
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for (i = 0; i <= nroots; i++)
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rs->genpoly[i] = rs->index_of[rs->genpoly[i]];
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return rs;
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}
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