/* Example use of Reed-Solomon library * * (C) Universal Access Inc. 1996 * hqm@alum.mit.edu * * This same code demonstrates the use of the encodier and * decoder/error-correction routines. * * We are assuming we have at least four bytes of parity (NPAR >= 4). * * This gives us the ability to correct up to two errors, or * four erasures. * * In general, with E errors, and K erasures, you will need * 2E + K bytes of parity to be able to correct the codeword * back to recover the original message data. * * You could say that each error 'consumes' two bytes of the parity, * whereas each erasure 'consumes' one byte. * * Thus, as demonstrated below, we can inject one error (location unknown) * and two erasures (with their locations specified) and the * error-correction routine will be able to correct the codeword * back to the original message. * */ #include #include #include "ecc.h" unsigned char msg[] = "Nervously I loaded the twin ducks aboard the revolving pl\ atform."; unsigned char codeword[256]; /* Some debugging routines to introduce errors or erasures into a codeword. */ /* Introduce a byte error at LOC */ void byte_err (int err, int loc, unsigned char *dst) { printf("Adding Error at loc %d, data %#x\n", loc, dst[loc-1]); dst[loc-1] ^= err; } /* Pass in location of error (first byte position is labeled starting at 1, not 0), and the codeword. */ void byte_erasure (int loc, unsigned char dst[], int cwsize, int erasures[]) { printf("Erasure at loc %d, data %#x\n", loc, dst[loc-1]); dst[loc-1] = 0; } void main (int argc, char *argv[]) { int erasures[16]; int nerasures = 0; /* Initialization the ECC library */ initialize_ecc (); /* ************** */ /* Encode data into codeword, adding NPAR parity bytes */ encode_data(msg, sizeof(msg), codeword); printf("Encoded data is: \"%s\"\n", codeword); #define ML (sizeof (msg) + NPAR) /* Add one error and two erasures */ byte_err(0x35, 3, codeword); byte_err(0x23, 17, codeword); byte_err(0x34, 19, codeword); printf("with some errors: \"%s\"\n", codeword); /* We need to indicate the position of the erasures. Eraseure positions are indexed (1 based) from the end of the message... */ erasures[nerasures++] = ML-17; erasures[nerasures++] = ML-19; /* Now decode -- encoded codeword size must be passed */ decode_data(codeword, ML); /* check if syndrome is all zeros */ if (check_syndrome () != 0) { correct_errors_erasures (codeword, ML, nerasures, erasures); printf("Corrected codeword: \"%s\"\n", codeword); } exit(0); }