#include <math.h>
#include <stdio.h>
#define mm  4           /* RS code over GF(2**4) - change to suit */
#define nn  15          /* nn=2**mm -1   length of codeword */
#define tt  3           /* number of errors that can be corrected */
#define kk  9           /* kk = nn-2*tt  */
int pp  = { 1, 1, 0, 0, 1} ; /* specify irreducible polynomial coeffts */
int alpha_to , index_of , gg  ;
int recd , data , bb  ;

void generate_gf()
/* generate GF(2**mm) from the irreducible polynomial p(X) in pp..pp
   lookup tables:  index->polynomial form   alpha_to contains j=alpha**i;
                   polynomial form -> index form  index_of = i
   alpha=2 is the primitive element of GF(2**mm)
*/
 {
   register int i, mask ;
  mask = 1 ;
  alpha_to = 0 ;
  for (i=0; i<mm; i++)
   { alpha_to = mask ;
     index_of] = i ;
     if (pp!=0)
       alpha_to ^= mask ;
     mask <<= 1 ;
   }
  index_of] = mm ;
  mask >>= 1 ;
  for (i=mm+1; i<nn; i++)
   { if (alpha_to >= mask)
        alpha_to = alpha_to ^ ((alpha_to^mask)<<1) ;
     else alpha_to = alpha_to<<1 ;
     index_of] = i ;
   }
  index_of = -1 ;
 }