internal GF256Poly Multiply(GF256Poly other)
{
if (!field.Equals(other.field))
{
throw new ArgumentException("GF256Polys do not have same GF256 field");
}
if (Zero || other.Zero)
{
return(field.Zero);
}
int[] aCoefficients = 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] = GF256.AddOrSubtract(product[i + j], field.Multiply(aCoeff, bCoefficients[j]));
}
}
return(new GF256Poly(field, product));
}
/// <summary> evaluation of this polynomial at a given point
/// </summary>
internal int EvaluateAt(int a)
{
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
int result = 0;
for (int i = 0; i < size; i++)
{
result = GF256.AddOrSubtract(result, Coefficients[i]);
}
return(result);
}
int result2 = Coefficients[0];
for (int i = 1; i < size; i++)
{
result2 = GF256.AddOrSubtract(field.Multiply(a, result2), Coefficients[i]);
}
return(result2);
}
internal GF256Poly AddOrSubtract(GF256Poly other)
{
if (!field.Equals(other.field))
{
throw new ArgumentException("GF256Polys do not have same GF256 field");
}
if (Zero)
{
return(other);
}
if (other.Zero)
{
return(this);
}
int[] smallerCoefficients = 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] = GF256.AddOrSubtract(smallerCoefficients[i - lengthDiff], largerCoefficients[i]);
}
return(new GF256Poly(field, sumDiff));
}
