/* 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) * pad = padding bytes at front of shortened block */void *INIT_RS(int symsize,int gfpoly,int fcr,int prim,	int nroots,int pad){  struct rs *rs;  int i, j, sr,root,iprim;  /* Check parameter ranges */  if(symsize < 0 || symsize > 8*sizeof(DTYPE))    return NULL; /* Need version with ints rather than chars */  if(fcr < 0 || fcr >= (1<<symsize))    return NULL;  if(prim <= 0 || prim >= (1<<symsize))    return NULL;  if(nroots < 0 || nroots >= (1<<symsize))    return NULL; /* Can't have more roots than symbol values! */  if(pad < 0 || pad >= ((1<<symsize) -1 - nroots))    return NULL; /* Too much padding */  rs = (struct rs *)calloc(1,sizeof(struct rs));  rs->mm = symsize;  rs->nn = (1<<symsize)-1;  rs->pad = pad;  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]];  return rs;}