GF256.cs

using System.Collections.Generic;
 
namespace Waher.Content.QR.Encoding
{
    public static class GF256
    {
        private readonly static byte[] powersOfTwo;
        private readonly static byte[] log2;
 
        static GF256()
        {
            SortedDictionary<byte, byte> Inv = new SortedDictionary<byte, byte>();
            int i, j;
 
            powersOfTwo = new byte[256];
 
            for (i = 0, j = 1; i < 256; i++)
            {
                powersOfTwo[i] = (byte)j;
 
                if (i != 255)
                    Inv[(byte)j] = (byte)i;
 
                j <<= 1;
                if (j >= 256)
                    j ^= 285;    // equal to j-=poly = rest of j/poly as polynomial division
            }
 
            log2 = new byte[256];
            Inv.Values.CopyTo(log2, 1);
        }
 
        public static byte[] PowerOf2Table => powersOfTwo;
 
        public static byte[] Log2Table => log2;
 
        public static byte Add(byte x, byte y) => (byte)(x ^ y);
 
        public static byte Subtract(byte x, byte y) => (byte)(x ^ y);
 
        public static byte Multiply(byte x, byte y) => powersOfTwo[(log2[x] + log2[y]) % 255];
    }
}