flight/libraries/rscode/example.c

   1 /* Example use of Reed-Solomon library
   2  *
   3  * Copyright Henry Minsky (hqm@alum.mit.edu) 1991-2009
   4  *
   5  * This software library is licensed under terms of the GNU GENERAL
   6  * PUBLIC LICENSE
   7  *
   8  * RSCODE is free software: you can redistribute it and/or modify
   9  * it under the terms of the GNU General Public License as published by
  10  * the Free Software Foundation, either version 3 of the License, or
  11  * (at your option) any later version.
  12  *
  13  * RSCODE is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16  * GNU General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU General Public License
  19  * along with Rscode.  If not, see <http://www.gnu.org/licenses/>.
  20
  21  * Commercial licensing is available under a separate license, please
  22  * contact author for details.
  23  *
  24  * This same code demonstrates the use of the encodier and
  25  * decoder/error-correction routines.
  26  *
  27  * We are assuming we have at least four bytes of parity (NPAR >= 4).
  28  *
  29  * This gives us the ability to correct up to two errors, or
  30  * four erasures.
  31  *
  32  * In general, with E errors, and K erasures, you will need
  33  * 2E + K bytes of parity to be able to correct the codeword
  34  * back to recover the original message data.
  35  *
  36  * You could say that each error 'consumes' two bytes of the parity,
  37  * whereas each erasure 'consumes' one byte.
  38  *
  39  * Thus, as demonstrated below, we can inject one error (location unknown)
  40  * and two erasures (with their locations specified) and the
  41  * error-correction routine will be able to correct the codeword
  42  * back to the original message.
  43  * */
  44
  45 #include <stdio.h>
  46 #include <stdlib.h>
  47 #include "ecc.h"
  48
  49 unsigned char msg[] = "Nervously I loaded the twin ducks aboard the revolving pl\
  50 atform.";
  51 unsigned char codeword[256];
  52
  53 /* Some debugging routines to introduce errors or erasures
  54    into a codeword.
  55    */
  56
  57 /* Introduce a byte error at LOC */
  58 void
  59 byte_err (int err, int loc, unsigned char *dst)
  60 {
  61   printf("Adding Error at loc %d, data %#x\n", loc, dst[loc-1]);
  62   dst[loc-1] ^= err;
  63 }
  64
  65 /* Pass in location of error (first byte position is
  66    labeled starting at 1, not 0), and the codeword.
  67 */
  68 void
  69 byte_erasure (int loc, unsigned char dst[], int cwsize, int erasures[])
  70 {
  71   printf("Erasure at loc %d, data %#x\n", loc, dst[loc-1]);
  72   dst[loc-1] = 0;
  73 }
  74
  75
  76 int
  77 main (int argc, char *argv[])
  78 {
  79
  80   int erasures[16];
  81   int nerasures = 0;
  82
  83   /* Initialization the ECC library */
  84
  85   initialize_ecc ();
  86
  87   /* ************** */
  88
  89   /* Encode data into codeword, adding NPAR parity bytes */
  90   encode_data(msg, sizeof(msg), codeword);
  91
  92   printf("Encoded data is: \"%s\"\n", codeword);
  93
  94 #define ML (sizeof (msg) + NPAR)
  95
  96
  97   /* Add one error and two erasures */
  98   byte_err(0x35, 3, codeword);
  99
 100   byte_err(0x23, 17, codeword);
 101   byte_err(0x34, 19, codeword);
 102
 103
 104   printf("with some errors: \"%s\"\n", codeword);
 105
 106   /* We need to indicate the position of the erasures.  Eraseure
 107      positions are indexed (1 based) from the end of the message... */
 108
 109   erasures[nerasures++] = ML-17;
 110   erasures[nerasures++] = ML-19;
 111
 112
 113   /* Now decode -- encoded codeword size must be passed */
 114   decode_data(codeword, ML);
 115
 116   /* check if syndrome is all zeros */
 117   if (check_syndrome () != 0) {
 118     correct_errors_erasures (codeword,
 119                              ML,
 120                              nerasures,
 121                              erasures);
 122
 123     printf("Corrected codeword: \"%s\"\n", codeword);
 124   }
 125
 126   exit(0);
 127 }
 128