encode_rs.h

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/* The guts of the Reed-Solomon encoder, meant to be #included
 * into a function body with the following typedefs, macros and variables supplied
 * according to the code parameters:
 
 * data_t - a typedef for the data symbol
 * data_t data[] - array of NN-NROOTS-PAD and type data_t to be encoded
 * data_t parity[] - an array of NROOTS and type data_t to be written with parity symbols
 * NROOTS - the number of roots in the RS code generator polynomial,
 *          which is the same as the number of parity symbols in a block.
            Integer variable or literal.
      *
 * NN - the total number of symbols in a RS block. Integer variable or literal.
 * PAD - the number of pad symbols in a block. Integer variable or literal.
 * ALPHA_TO - The address of an array of NN elements to convert Galois field
 *            elements in index (log) form to polynomial form. Read only.
 * INDEX_OF - The address of an array of NN elements to convert Galois field
 *            elements in polynomial form to index (log) form. Read only.
 * MODNN - a function to reduce its argument modulo NN. May be inline or a macro.
 * GENPOLY - an array of NROOTS+1 elements containing the generator polynomial in index form
 
 * The memset() and memmove() functions are used. The appropriate header
 * file declaring these functions (usually <string.h>) must be included by the calling
 * program.
 
 * Copyright 2004, Phil Karn, KA9Q
 * May be used under the terms of the GNU Lesser General Public License (LGPL)
 */
 
 
#undef A0
#define A0 (NN) /* Special reserved value encoding zero in index form */
 
{
  int i, j;
  data_t feedback;
 
  memset(parity, 0, NROOTS * sizeof(data_t));
 
  for (i = 0; i < NN - NROOTS - PAD; i++)
  {
    feedback = INDEX_OF[data[i] ^ parity[0]];
    if (feedback != A0) {    /* feedback term is non-zero */
#ifdef UNNORMALIZED
      /* This line is unnecessary when GENPOLY[NROOTS] is unity, as it must
       * always be for the polynomials constructed by init_rs()
       */
      feedback = MODNN(NN - GENPOLY[NROOTS] + feedback);
#endif
      for (j = 1; j < NROOTS; j++) {
        parity[j] ^= ALPHA_TO[MODNN(feedback + GENPOLY[NROOTS - j])];
      }
    }
    /* Shift */
    memmove(&parity[0], &parity[1], sizeof(data_t) * (NROOTS - 1));
    if (feedback != A0) {
      parity[NROOTS - 1] = ALPHA_TO[MODNN(feedback + GENPOLY[0])];
    }
    else {
      parity[NROOTS - 1] = 0;
    }
  }
}