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/* Initialize a RS codec
*
* Copyright 2002 Phil Karn, KA9Q
* May be used under the terms of the GNU General Public License (GPL)
*/
#include <stdlib.h>
#ifdef CCSDS
#include "ccsds.h"
#elif defined(BIGSYM)
#include "int.h"
#else
#include "char.h"
#endif
#define NULL ((void *)0)
void FREE_RS(void *p){
struct rs *rs = (struct rs *)p;
free(rs->alpha_to);
free(rs->index_of);
free(rs->genpoly);
free(rs);
}
/* Initialize a Reed-Solomon codec
* symsize = symbol size, bits (1-8)
* gfpoly = Field generator polynomial coefficients
* fcr = first root of RS code generator polynomial, index form
* prim = primitive element to generate polynomial roots
* nroots = RS code generator polynomial degree (number of roots)
*/
void *INIT_RS(unsigned int symsize,unsigned int gfpoly,unsigned fcr,unsigned prim,
unsigned int nroots){
struct rs *rs;
int i, j, sr,root,iprim;
if(symsize > 8*sizeof(DTYPE))
return NULL; /* Need version with ints rather than chars */
if(fcr >= (1<<symsize))
return NULL;
if(prim == 0 || prim >= (1<<symsize))
return NULL;
if(nroots >= (1<<symsize))
return NULL; /* Can't have more roots than symbol values! */
rs = (struct rs *)calloc(1,sizeof(struct rs));
rs->mm = symsize;
rs->nn = (1<<symsize)-1;
rs->alpha_to = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
if(rs->alpha_to == NULL){
free(rs);
return NULL;
}
rs->index_of = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
if(rs->index_of == NULL){
free(rs->alpha_to);
free(rs);
return NULL;
}
/* Generate Galois field lookup tables */
rs->index_of[0] = A0; /* log(zero) = -inf */
rs->alpha_to[A0] = 0; /* alpha**-inf = 0 */
sr = 1;
for(i=0;i<rs->nn;i++){
rs->index_of[sr] = i;
rs->alpha_to[i] = sr;
sr <<= 1;
if(sr & (1<<symsize))
sr ^= gfpoly;
sr &= rs->nn;
}
if(sr != 1){
/* field generator polynomial is not primitive! */
free(rs->alpha_to);
free(rs->index_of);
free(rs);
return NULL;
}
/* Form RS code generator polynomial from its roots */
rs->genpoly = (DTYPE *)malloc(sizeof(DTYPE)*(nroots+1));
if(rs->genpoly == NULL){
free(rs->alpha_to);
free(rs->index_of);
free(rs);
return NULL;
}
rs->fcr = fcr;
rs->prim = prim;
rs->nroots = nroots;
/* Find prim-th root of 1, used in decoding */
for(iprim=1;(iprim % prim) != 0;iprim += rs->nn)
;
rs->iprim = iprim / prim;
rs->genpoly[0] = 1;
for (i = 0,root=fcr*prim; i < nroots; i++,root += prim) {
rs->genpoly[i+1] = 1;
/* Multiply rs->genpoly[] by @**(root + x) */
for (j = i; j > 0; j--){
if (rs->genpoly[j] != 0)
rs->genpoly[j] = rs->genpoly[j-1] ^ rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[j]] + root)];
else
rs->genpoly[j] = rs->genpoly[j-1];
}
/* rs->genpoly[0] can never be zero */
rs->genpoly[0] = rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[0]] + root)];
}
/* convert rs->genpoly[] to index form for quicker encoding */
for (i = 0; i <= nroots; i++)
rs->genpoly[i] = rs->index_of[rs->genpoly[i]];
#if 0
printf ("genpoly:\n");
for (i = nroots; i >= 0; i--){
printf (" %3d*X^%d\n", rs->alpha_to[rs->genpoly[i]], i);
}
#endif
return rs;
}
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