1/* Common code for intializing a Reed-Solomon control block (char or int symbols)
2 * Copyright 2004 Phil Karn, KA9Q
3 * May be used under the terms of the GNU Lesser General Public License (LGPL)
4 */
5#undef NULL
6#define NULL ((void *)0)
7
8{
9  int i, j, sr,root,iprim;
10
11  rs = NULL;
12  /* Check parameter ranges */
13  if(symsize < 0 || symsize > 8*(int)sizeof(data_t)){
14    goto done;
15  }
16
17  if(fcr < 0 || fcr >= (1<<symsize))
18    goto done;
19  if(prim <= 0 || prim >= (1<<symsize))
20    goto done;
21  if(nroots < 0 || nroots >= (1<<symsize))
22    goto done; /* Can't have more roots than symbol values! */
23  if(pad < 0 || pad >= ((1<<symsize) -1 - nroots))
24    goto done; /* Too much padding */
25
26  rs = (struct rs *)calloc(1,sizeof(struct rs));
27  if(rs == NULL)
28    goto done;
29
30  rs->mm = symsize;
31  rs->nn = (1<<symsize)-1;
32  rs->pad = pad;
33
34  rs->alpha_to = (data_t *)malloc(sizeof(data_t)*(rs->nn+1));
35  if(rs->alpha_to == NULL){
36    free(rs);
37    rs = NULL;
38    goto done;
39  }
40  rs->index_of = (data_t *)malloc(sizeof(data_t)*(rs->nn+1));
41  if(rs->index_of == NULL){
42    free(rs->alpha_to);
43    free(rs);
44    rs = NULL;
45    goto done;
46  }
47
48  /* Generate Galois field lookup tables */
49  rs->index_of[0] = A0; /* log(zero) = -inf */
50  rs->alpha_to[A0] = 0; /* alpha**-inf = 0 */
51  sr = 1;
52  for(i=0;i<rs->nn;i++){
53    rs->index_of[sr] = i;
54    rs->alpha_to[i] = sr;
55    sr <<= 1;
56    if(sr & (1<<symsize))
57      sr ^= gfpoly;
58    sr &= rs->nn;
59  }
60  if(sr != 1){
61    /* field generator polynomial is not primitive! */
62    free(rs->alpha_to);
63    free(rs->index_of);
64    free(rs);
65    rs = NULL;
66    goto done;
67  }
68
69  /* Form RS code generator polynomial from its roots */
70  rs->genpoly = (data_t *)malloc(sizeof(data_t)*(nroots+1));
71  if(rs->genpoly == NULL){
72    free(rs->alpha_to);
73    free(rs->index_of);
74    free(rs);
75    rs = NULL;
76    goto done;
77  }
78  rs->fcr = fcr;
79  rs->prim = prim;
80  rs->nroots = nroots;
81
82  /* Find prim-th root of 1, used in decoding */
83  for(iprim=1;(iprim % prim) != 0;iprim += rs->nn)
84    ;
85  rs->iprim = iprim / prim;
86
87  rs->genpoly[0] = 1;
88  for (i = 0,root=fcr*prim; i < nroots; i++,root += prim) {
89    rs->genpoly[i+1] = 1;
90
91    /* Multiply rs->genpoly[] by  @**(root + x) */
92    for (j = i; j > 0; j--){
93      if (rs->genpoly[j] != 0)
94	rs->genpoly[j] = rs->genpoly[j-1] ^ rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[j]] + root)];
95      else
96	rs->genpoly[j] = rs->genpoly[j-1];
97    }
98    /* rs->genpoly[0] can never be zero */
99    rs->genpoly[0] = rs->alpha_to[modnn(rs,rs->index_of[rs->genpoly[0]] + root)];
100  }
101  /* convert rs->genpoly[] to index form for quicker encoding */
102  for (i = 0; i <= nroots; i++)
103    rs->genpoly[i] = rs->index_of[rs->genpoly[i]];
104 done:;
105
106}
107