// Method modified by Sonic-The-Hedgehog-LNK1123 (github.com/Sonic-The-Hedgehog-LNK1123) // added missing "if (denominator == 0)" check private int[] findErrorMagnitudes(GenericGFPoly errorEvaluator, int[] errorLocations) { // This is directly applying Forney's Formula int s = errorLocations.Length; int[] result = new int[s]; for (int i = 0; i < s; i++) { int xiInverse = field.inverse(errorLocations[i]); int denominator = 1; for (int j = 0; j < s; j++) { if (i != j) { denominator = field.multiply(denominator, GenericGF.addOrSubtract(1, field.multiply(errorLocations[j], xiInverse))); } } if (denominator == 0) { return(null); } result[i] = field.multiply(errorEvaluator.evaluateAt(xiInverse), field.inverse(denominator)); if (field.GeneratorBase != 0) { result[i] = field.multiply(result[i], xiInverse); } } return(result); }
// Method added by Sonic-The-Hedgehog-LNK1123 (github.com/Sonic-The-Hedgehog-LNK1123) internal GenericGFPoly calculateForneySyndromes(GenericGFPoly syndromes, int[] positions, int messageLength) { int[] positionsReversed = new int[positions.Length]; for (int i = 0; i < positions.Length; i++) { positionsReversed[i] = messageLength - 1 - positions[i]; } int forneySyndromesLength = syndromes.Coefficients.Length; int[] syndromeCoefficients = new int[syndromes.Coefficients.Length]; Array.Copy(syndromes.Coefficients, 0, syndromeCoefficients, 0, syndromes.Coefficients.Length); GenericGFPoly forneySyndromes = new GenericGFPoly(field, syndromeCoefficients, false); for (int i = 0; i < positions.Length; i++) { int x = field.exp(positionsReversed[i]); for (int j = 0; j < forneySyndromes.Coefficients.Length - 1; j++) { forneySyndromes.Coefficients[forneySyndromesLength - j - 1] = GenericGF.addOrSubtract(field.multiply(forneySyndromes.getCoefficient(j), x), forneySyndromes.getCoefficient(j + 1)); } } return(forneySyndromes); }
internal GenericGFPoly multiply(GenericGFPoly other) { if (!field.Equals(other.field)) { throw new ArgumentException("GenericGFPolys do not have same GenericGF field"); } if (isZero || other.isZero) { return(new GenericGFPoly(field, new int[] { 0 }, encoding)); } int[] aCoefficients = this.coefficients; int aLength = aCoefficients.Length; int[] bCoefficients = other.coefficients; int bLength = bCoefficients.Length; int[] product = new int[aLength + bLength - 1]; for (int i = 0; i < aLength; i++) { int aCoeff = aCoefficients[i]; for (int j = 0; j < bLength; j++) { product[i + j] = GenericGF.addOrSubtract(product[i + j], field.multiply(aCoeff, bCoefficients[j])); } } return(new GenericGFPoly(field, product, encoding)); }
/// <summary> /// evaluation of this polynomial at a given point /// </summary> /// <param name="a">A.</param> /// <returns>evaluation of this polynomial at a given point</returns> internal int evaluateAt(int a) { int result = 0; if (a == 0) { // Just return the x^0 coefficient return(getCoefficient(0)); } int size = coefficients.Length; if (a == 1) { // Just the sum of the coefficients foreach (var coefficient in coefficients) { result = GenericGF.addOrSubtract(result, coefficient); } return(result); } result = coefficients[0]; for (int i = 1; i < size; i++) { result = GenericGF.addOrSubtract(field.multiply(a, result), coefficients[i]); } return(result); }
internal GenericGFPoly addOrSubtract(GenericGFPoly other) { if (!field.Equals(other.field)) { throw new ArgumentException("GenericGFPolys do not have same GenericGF field"); } if (isZero) { return(other); } if (other.isZero) { return(this); } int[] smallerCoefficients = this.coefficients; int[] largerCoefficients = other.coefficients; if (smallerCoefficients.Length > largerCoefficients.Length) { int[] temp = smallerCoefficients; smallerCoefficients = largerCoefficients; largerCoefficients = temp; } int[] sumDiff = new int[largerCoefficients.Length]; int lengthDiff = largerCoefficients.Length - smallerCoefficients.Length; // Copy high-order terms only found in higher-degree polynomial's coefficients Array.Copy(largerCoefficients, 0, sumDiff, 0, lengthDiff); for (int i = lengthDiff; i < largerCoefficients.Length; i++) { sumDiff[i] = GenericGF.addOrSubtract(smallerCoefficients[i - lengthDiff], largerCoefficients[i]); } return(new GenericGFPoly(field, sumDiff, encoding)); }
/// <summary> /// <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> /// </summary> /// <param name="received">data and error-correction codewords</param> /// <param name="twoS">number of error-correction codewords available</param> /// <param name="erasurePos">array of zero-based erasure indices</param> /// <returns>false: decoding fails</returns> public bool Decode(int[] received, int twoS, int[] erasurePos) { // Method modified by Sonic-The-Hedgehog-LNK1123 (github.com/Sonic-The-Hedgehog-LNK1123) // to add support for erasure and errata correction // most code ported to C# from the python code at http://en.wikiversity.org/wiki/Reed–Solomon_codes_for_coders if (received.Length >= field.Size) { throw new ArgumentException("Message is too long for this field", "received"); } if (twoS <= 0) { throw new ArgumentException("No error correction bytes provided", "twoS"); } var dataBytes = received.Length - twoS; if (dataBytes <= 0) { throw new ArgumentException("No data bytes provided", "twoS"); } var syndromeCoefficients = new int[twoS]; var noError = true; if (erasurePos == null) { erasurePos = new int[] { }; } else { for (var i = 0; i < erasurePos.Length; i++) { received[erasurePos[i]] = 0; } } if (erasurePos.Length > twoS) { return(false); } var poly = new GenericGFPoly(field, received, false); for (var i = 0; i < twoS; i++) { var eval = poly.evaluateAt(field.exp(i + field.GeneratorBase)); syndromeCoefficients[syndromeCoefficients.Length - 1 - i] = eval; if (eval != 0) { noError = false; } } if (noError) { return(true); } var syndrome = new GenericGFPoly(field, syndromeCoefficients, false); var forneySyndrome = calculateForneySyndromes(syndrome, erasurePos, received.Length); var sigma = runBerlekampMasseyAlgorithm(forneySyndrome, erasurePos.Length); if (sigma == null) { return(false); } var errorLocations = findErrorLocations(sigma); if (errorLocations == null) { return(false); } // Prepare errors int[] errorPositions = new int[errorLocations.Length]; for (int i = 0; i < errorLocations.Length; i++) { errorPositions[i] = field.log(errorLocations[i]); } // Prepare erasures int[] erasurePositions = new int[erasurePos.Length]; for (int i = 0; i < erasurePos.Length; i++) { erasurePositions[i] = received.Length - 1 - erasurePos[i]; } // Combine errors and erasures int[] errataPositions = new int[errorPositions.Length + erasurePositions.Length]; Array.Copy(errorPositions, 0, errataPositions, 0, errorPositions.Length); Array.Copy(erasurePositions, 0, errataPositions, errorPositions.Length, erasurePositions.Length); var errataLocator = findErrataLocator(errataPositions); var omega = findErrorEvaluator(syndrome, errataLocator); if (omega == null) { return(false); } int[] errata = new int[errataPositions.Length]; for (int i = 0; i < errataPositions.Length; i++) { errata[i] = field.exp(errataPositions[i]); } var errorMagnitudes = findErrorMagnitudes(omega, errata); if (errorMagnitudes == null) { return(false); } for (var i = 0; i < errata.Length; i++) { var position = received.Length - 1 - field.log(errata[i]); if (position < 0) { // throw new ReedSolomonException("Bad error location"); return(false); } received[position] = GenericGF.addOrSubtract(received[position], errorMagnitudes[i]); } var checkPoly = new GenericGFPoly(field, received, false); var error = false; for (var i = 0; i < twoS; i++) { var eval = checkPoly.evaluateAt(field.exp(i + field.GeneratorBase)); if (eval != 0) { error = true; } } if (error) { return(false); } return(true); }