1 /* Initialize a RS codec
3 * Copyright 2002 Phil Karn, KA9Q
4 * May be used under the terms of the GNU General Public License (GPL)
16 #define NULL ((void *)0)
18 void FREE_RS(void *p){
19 struct rs *rs = (struct rs *)p;
27 /* Initialize a Reed-Solomon codec
28 * symsize = symbol size, bits (1-8)
29 * gfpoly = Field generator polynomial coefficients
30 * fcr = first root of RS code generator polynomial, index form
31 * prim = primitive element to generate polynomial roots
32 * nroots = RS code generator polynomial degree (number of roots)
34 void *INIT_RS(unsigned int symsize,unsigned int gfpoly,unsigned fcr,unsigned prim,
37 int i, j, sr,root,iprim;
39 if(symsize > 8*sizeof(DTYPE))
40 return NULL; /* Need version with ints rather than chars */
42 if(fcr >= (1<<symsize))
44 if(prim == 0 || prim >= (1<<symsize))
46 if(nroots >= (1<<symsize))
47 return NULL; /* Can't have more roots than symbol values! */
49 rs = (struct rs *)calloc(1,sizeof(struct rs));
51 rs->nn = (1<<symsize)-1;
53 rs->alpha_to = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
54 if(rs->alpha_to == NULL){
58 rs->index_of = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
59 if(rs->index_of == NULL){
65 /* Generate Galois field lookup tables */
66 rs->index_of[0] = A0; /* log(zero) = -inf */
67 rs->alpha_to[A0] = 0; /* alpha**-inf = 0 */
69 for(i=0;i<rs->nn;i++){
78 /* field generator polynomial is not primitive! */
85 /* Form RS code generator polynomial from its roots */
86 rs->genpoly = (DTYPE *)malloc(sizeof(DTYPE)*(nroots+1));
87 if(rs->genpoly == NULL){
97 /* Find prim-th root of 1, used in decoding */
98 for(iprim=1;(iprim % prim) != 0;iprim += rs->nn)
100 rs->iprim = iprim / prim;
103 for (i = 0,root=fcr*prim; i < nroots; i++,root += prim) {
104 rs->genpoly[i+1] = 1;
106 /* Multiply rs->genpoly[] by @**(root + x) */
107 for (j = i; j > 0; j--){
108 if (rs->genpoly[j] != 0)
109 rs->genpoly[j] = rs->genpoly[j-1] ^ rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[j]] + root)];
111 rs->genpoly[j] = rs->genpoly[j-1];
113 /* rs->genpoly[0] can never be zero */
114 rs->genpoly[0] = rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[0]] + root)];
116 /* convert rs->genpoly[] to index form for quicker encoding */
117 for (i = 0; i <= nroots; i++)
118 rs->genpoly[i] = rs->index_of[rs->genpoly[i]];
121 printf ("genpoly:\n");
122 for (i = nroots; i >= 0; i--){
123 printf (" %3d*X^%d\n", rs->alpha_to[rs->genpoly[i]], i);