init_rs.h

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/* Common code for intializing a Reed-Solomon control block (char or int symbols)
 * Copyright 2004 Phil Karn, KA9Q
 * May be used under the terms of the GNU Lesser General Public License (LGPL)
 */
#undef NULL
#define NULL ((void *)0)
 
{
  int i, j, sr, root, iprim;
 
  rs = NULL;
  /* Check parameter ranges */
  if (symsize < 0 || symsize > 8 * sizeof(data_t))
  {
    goto done;
  }
 
  if (fcr < 0 || fcr >= (1 << symsize))
  {
    goto done;
  }
  if (prim <= 0 || prim >= (1 << symsize))
  {
    goto done;
  }
  if (nroots < 0 || nroots >= (1 << symsize))
  {
    goto done;  /* Can't have more roots than symbol values! */
  }
  if (pad < 0 || pad >= ((1 << symsize) - 1 - nroots))
  {
    goto done;  /* Too much padding */
  }
 
  rs = (struct rs *)calloc(1, sizeof(struct rs));
  if (rs == NULL)
  {
    goto done;
  }
 
  rs->mm = symsize;
  rs->nn = (1 << symsize) - 1;
  rs->pad = pad;
 
  rs->alpha_to = (data_t *)malloc(sizeof(data_t) * (rs->nn + 1));
  if (rs->alpha_to == NULL)
  {
    free(rs);
    rs = NULL;
    goto done;
  }
  rs->index_of = (data_t *)malloc(sizeof(data_t) * (rs->nn + 1));
  if (rs->index_of == NULL)
  {
    free(rs->alpha_to);
    free(rs);
    rs = NULL;
    goto done;
  }
 
  /* 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);
    rs = NULL;
    goto done;
  }
 
  /* Form RS code generator polynomial from its roots */
  rs->genpoly = (data_t *)malloc(sizeof(data_t) * (nroots + 1));
  if (rs->genpoly == NULL)
  {
    free(rs->alpha_to);
    free(rs->index_of);
    free(rs);
    rs = NULL;
    goto done;
  }
  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]];
  }
done:
  ;
 
}