1 /* Initialize a RS codec
2 *
3 * Copyright 2002 Phil Karn, KA9Q
4 * May be used under the terms of the GNU General Public License (GPL)
5 */
6 #include <stdlib.h>
7
8 #ifdef CCSDS
9 #include "ccsds.h"
10 #elif defined(BIGSYM)
11 #include "int.h"
12 #else
13 #include "char.h"
14 #endif
15
16 #define NULL ((void *)0)
17
FREE_RS(void * p)18 void FREE_RS(void *p){
19 struct rs *rs = (struct rs *)p;
20
21 free(rs->alpha_to);
22 free(rs->index_of);
23 free(rs->genpoly);
24 free(rs);
25 }
26
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)
33 * pad = padding bytes at front of shortened block
34 */
INIT_RS(int symsize,int gfpoly,int fcr,int prim,int nroots,int pad)35 void *INIT_RS(int symsize,int gfpoly,int fcr,int prim,
36 int nroots,int pad){
37 struct rs *rs;
38 int i, j, sr,root,iprim;
39
40 /* Check parameter ranges */
41 if(symsize < 0 || symsize > 8*sizeof(DTYPE))
42 return NULL; /* Need version with ints rather than chars */
43
44 if(fcr < 0 || fcr >= (1<<symsize))
45 return NULL;
46 if(prim <= 0 || prim >= (1<<symsize))
47 return NULL;
48 if(nroots < 0 || nroots >= (1<<symsize))
49 return NULL; /* Can't have more roots than symbol values! */
50 if(pad < 0 || pad >= ((1<<symsize) -1 - nroots))
51 return NULL; /* Too much padding */
52
53 rs = (struct rs *)calloc(1,sizeof(struct rs));
54 rs->mm = symsize;
55 rs->nn = (1<<symsize)-1;
56 rs->pad = pad;
57
58 rs->alpha_to = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
59 if(rs->alpha_to == NULL){
60 free(rs);
61 return NULL;
62 }
63 rs->index_of = (DTYPE *)malloc(sizeof(DTYPE)*(rs->nn+1));
64 if(rs->index_of == NULL){
65 free(rs->alpha_to);
66 free(rs);
67 return NULL;
68 }
69
70 /* Generate Galois field lookup tables */
71 rs->index_of[0] = A0; /* log(zero) = -inf */
72 rs->alpha_to[A0] = 0; /* alpha**-inf = 0 */
73 sr = 1;
74 for(i=0;i<rs->nn;i++){
75 rs->index_of[sr] = i;
76 rs->alpha_to[i] = sr;
77 sr <<= 1;
78 if(sr & (1<<symsize))
79 sr ^= gfpoly;
80 sr &= rs->nn;
81 }
82 if(sr != 1){
83 /* field generator polynomial is not primitive! */
84 free(rs->alpha_to);
85 free(rs->index_of);
86 free(rs);
87 return NULL;
88 }
89
90 /* Form RS code generator polynomial from its roots */
91 rs->genpoly = (DTYPE *)malloc(sizeof(DTYPE)*(nroots+1));
92 if(rs->genpoly == NULL){
93 free(rs->alpha_to);
94 free(rs->index_of);
95 free(rs);
96 return NULL;
97 }
98 rs->fcr = fcr;
99 rs->prim = prim;
100 rs->nroots = nroots;
101
102 /* Find prim-th root of 1, used in decoding */
103 for(iprim=1;(iprim % prim) != 0;iprim += rs->nn)
104 ;
105 rs->iprim = iprim / prim;
106
107 rs->genpoly[0] = 1;
108 for (i = 0,root=fcr*prim; i < nroots; i++,root += prim) {
109 rs->genpoly[i+1] = 1;
110
111 /* Multiply rs->genpoly[] by @**(root + x) */
112 for (j = i; j > 0; j--){
113 if (rs->genpoly[j] != 0)
114 rs->genpoly[j] = rs->genpoly[j-1] ^ rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[j]] + root)];
115 else
116 rs->genpoly[j] = rs->genpoly[j-1];
117 }
118 /* rs->genpoly[0] can never be zero */
119 rs->genpoly[0] = rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[0]] + root)];
120 }
121 /* convert rs->genpoly[] to index form for quicker encoding */
122 for (i = 0; i <= nroots; i++)
123 rs->genpoly[i] = rs->index_of[rs->genpoly[i]];
124
125 return rs;
126 }
127