GF256Px.cs

using System;
 
namespace Waher.Content.QR.Encoding
{
    public class GF256Px
    {
        private readonly byte[] coefficients;
        private readonly int degree;
 
        public GF256Px(byte[] Coefficients)
        {
            this.coefficients = Coefficients;
            this.degree = this.coefficients.Length - 1;
        }
 
        public byte[] Coefficients => this.coefficients;
 
        public GF256Px Multiply(GF256Px P)
        {
            byte[] C = new byte[this.degree + P.degree + 1];
            int i, j;
 
            for (i = 0; i <= P.degree; i++)
            {
                for (j = 0; j <= this.degree; j++)
                    C[i + j] ^= GF256.Multiply(this.coefficients[j], P.coefficients[i]);
            }
 
            return new GF256Px(C);
        }
 
        public GF256Px Residue(GF256Px Divisor)
        {
            if (this.degree < Divisor.degree)
                return this;
 
            if (Divisor.coefficients[0] != 1)
                throw new NotSupportedException("Leading coefficien must be 1.");   // Will always be one if only generator polynomials are used.
 
            byte[] C = (byte[])this.coefficients.Clone();
            int i, c, j;
            byte b;
 
            c = this.degree - Divisor.degree;
            for (i = 0; i <= c; i++)
            {
                b = C[i];
                if (b == 0)
                    continue;
 
                C[i] = 0;
                for (j = 1; j <= Divisor.degree; j++)
                    C[i + j] ^= GF256.Multiply(b, Divisor.coefficients[j]);
            }
 
            byte[] Residue = new byte[Divisor.degree];
            Buffer.BlockCopy(C, c + 1, Residue, 0, Divisor.degree);
 
            return new GF256Px(Residue);
        }
 
    }
}