/// <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>
/// <returns>false: decoding fails</returns>
public bool Decode(int[] received, int twoS)
{
var poly = new GenericGFPoly(field, received);
var syndromeCoefficients = new int[twoS];
var noError = true;
for (var i = 0; i < twoS; i++)
{
int 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);
GenericGFPoly[] sigmaOmega = RunEuclideanAlgorithm(field.BuildMonomial(twoS, 1), syndrome, twoS);
if (sigmaOmega == null)
{
return(false);
}
GenericGFPoly sigma = sigmaOmega[0];
int[] errorLocations = FindErrorLocations(sigma);
if (errorLocations == null)
{
return(false);
}
GenericGFPoly omega = sigmaOmega[1];
int[] errorMagnitudes = FindErrorMagnitudes(omega, errorLocations);
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");
return(false);
}
received[position] = GenericGF.AddOrSubtract(received[position], errorMagnitudes[i]);
}
return(true);
}
private GenericGFPoly BuildGenerator(int degree)
{
if (degree >= cachedGenerators.Count)
{
GenericGFPoly lastGenerator = cachedGenerators[cachedGenerators.Count - 1];
for (int d = cachedGenerators.Count; d <= degree; d++)
{
var nextGenerator = lastGenerator.Multiply(new GenericGFPoly(field, new int[] { 1, field.Exp(d - 1 + field.GeneratorBase) }));
cachedGenerators.Add(nextGenerator);
lastGenerator = nextGenerator;
}
}
return(cachedGenerators[degree]);
}
