/** * <p>Decodes given set of received codewords, which include both data and error-correction * codewords. Really, this means it uses Reed-Solomon to detect and correct errors, in-place, * in the input.</p> * * @param received data and error-correction codewords * @param twoS number of error-correction codewords available * @throws ReedSolomonException if decoding fails for any reason */ public void decode(int[] received, int twoS) { try{ GF256Poly poly = new GF256Poly(field, received); int[] syndromeCoefficients = new int[twoS]; bool dataMatrix = field.Equals(GF256.DATA_MATRIX_FIELD); bool noError = true; for (int i = 0; i < twoS; i++) { // Thanks to sanfordsquires for this fix: int eval = poly.evaluateAt(field.exp(dataMatrix ? i + 1 : i)); syndromeCoefficients[syndromeCoefficients.Length - 1 - i] = eval; if (eval != 0) { noError = false; } } if (noError) { return; } GF256Poly syndrome = new GF256Poly(field, syndromeCoefficients); GF256Poly[] sigmaOmega = runEuclideanAlgorithm(field.buildMonomial(twoS, 1), syndrome, twoS); GF256Poly sigma = sigmaOmega[0]; GF256Poly omega = sigmaOmega[1]; int[] errorLocations = findErrorLocations(sigma); int[] errorMagnitudes = findErrorMagnitudes(omega, errorLocations, dataMatrix); for (int i = 0; i < errorLocations.Length; i++) { int position = received.Length - 1 - field.log(errorLocations[i]); if (position < 0) { throw new ReedSolomonException("Bad error location"); } received[position] = GF256.addOrSubtract(received[position], errorMagnitudes[i]); } }catch (ReedSolomonException e) { throw new ReedSolomonException(e.Message); } }
private GF256Poly buildGenerator(int degree) { if (degree >= cachedGenerators.Count) { GF256Poly lastGenerator = (GF256Poly)cachedGenerators[cachedGenerators.Count - 1]; for (int d = cachedGenerators.Count; d <= degree; d++) { GF256Poly nextGenerator = lastGenerator.multiply(new GF256Poly(field, new int[] { 1, field.exp(d - 1) })); cachedGenerators.Add(nextGenerator); lastGenerator = nextGenerator; } } return((GF256Poly)cachedGenerators[degree]); }