/// <summary>Encodes the provided data.</summary>
/// <param name="toEncode">data to encode</param>
/// <param name="ecBytes">error correction bytes</param>
public void Encode(int[] toEncode, int ecBytes)
{
if (ecBytes == 0)
{
throw new ArgumentException("No error correction bytes");
}
int dataBytes = toEncode.Length - ecBytes;
if (dataBytes <= 0)
{
throw new ArgumentException("No data bytes provided");
}
GF256Poly generator = BuildGenerator(ecBytes);
int[] infoCoefficients = new int[dataBytes];
Array.Copy(toEncode, 0, infoCoefficients, 0, dataBytes);
GF256Poly info = new GF256Poly(field, infoCoefficients);
info = info.MultiplyByMonomial(ecBytes, 1);
GF256Poly remainder = info.Divide(generator)[1];
int[] coefficients = remainder.GetCoefficients();
int numZeroCoefficients = ecBytes - coefficients.Length;
for (int i = 0; i < numZeroCoefficients; i++)
{
toEncode[dataBytes + i] = 0;
}
Array.Copy(coefficients, 0, toEncode, dataBytes + numZeroCoefficients, coefficients.Length);
}
/// <summary>Create a representation of GF(256) using the given primitive polynomial.</summary>
/// <param name="primitive">
/// irreducible polynomial whose coefficients are represented by
/// the bits of an int, where the least-significant bit represents the constant
/// coefficient
/// </param>
private GF256(int primitive)
{
// x^8 + x^4 + x^3 + x^2 + 1
// x^8 + x^5 + x^3 + x^2 + 1
expTable = new int[256];
logTable = new int[256];
int x = 1;
for (int i = 0; i < 256; i++)
{
expTable[i] = x;
x <<= 1;
// x = x * 2; we're assuming the generator alpha is 2
if (x >= 0x100)
{
x ^= primitive;
}
}
for (int i = 0; i < 255; i++)
{
logTable[expTable[i]] = i;
}
// logTable[0] == 0 but this should never be used
zero = new GF256Poly(this, new int[] { 0 });
one = new GF256Poly(this, new int[] { 1 });
}
private GF256Poly BuildGenerator(int degree)
{
if (degree >= cachedGenerators.Count)
{
GF256Poly lastGenerator = 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(cachedGenerators[degree]);
}
