C# FNV Hash

using System;
using System.Collections.Generic;
using System.Text;

namespace SomeNamespace
{
    public class FnvHash
    {
        #region Constants

        private static readonly BigInteger fnv_prime_32 = new BigInteger("16777619", 10);
        private static readonly BigInteger fnv_prime_64 = new BigInteger("1099511628211", 10);
        private static readonly BigInteger fnv_prime_128 = new BigInteger("309485009821345068724781371", 10);
        private static readonly BigInteger fnv_prime_256 = new BigInteger("374144419156711147060143317175368453031918731002211", 10);
        private static readonly BigInteger fnv_prime_512 = new BigInteger("35835915874844867368919076489095108449946327955754392558399825615420669938882575126094039892345713852759", 10);
        private static readonly BigInteger fnv_prime_1024 = new BigInteger("5016456510113118655434598811035278955030765345404790744303017523831112055108147451509157692220295382716162651878526895249385292291816524375083746691371804094271873160484737966720260389217684476157468082573", 10);

        private static readonly BigInteger fnv_offset_32 = new BigInteger("2166136261", 10);
        private static readonly BigInteger fnv_offset_64 = new BigInteger("14695981039346656037", 10);
        private static readonly BigInteger fnv_offset_128 = new BigInteger("275519064689413815358837431229664493455", 10);
        private static readonly BigInteger fnv_offset_256 = new BigInteger("100029257958052580907070968620625704837092796014241193945225284501741471925557", 10);
        private static readonly BigInteger fnv_offset_512 = new BigInteger("9659303129496669498009435400716310466090418745672637896108374329434462657994582932197716438449813051892206539805784495328239340083876191928701583869517785", 10);
        private static readonly BigInteger fnv_offset_1024 = new BigInteger("14197795064947621068722070641403218320880622795441933960878474914617582723252296732303717722150864096521202355549365628174669108571814760471015076148029755969804077320157692458563003215304957150157403644460363550505412711285966361610267868082893823963790439336411086884584107735010676915", 10);

        private static readonly BigInteger fnv_mod_32 = new BigInteger("2", 10).Pow(32);
        private static readonly BigInteger fnv_mod_64 = new BigInteger("2", 10).Pow(64);
        private static readonly BigInteger fnv_mod_128 = new BigInteger("2", 10).Pow(128);
        private static readonly BigInteger fnv_mod_256 = new BigInteger("2", 10).Pow(256);
        private static readonly BigInteger fnv_mod_512 = new BigInteger("2", 10).Pow(512);
        private static readonly BigInteger fnv_mod_1024 = new BigInteger("2", 10).Pow(1024);

        #endregion

        public static BigInteger GetHash(string value, int hashBitSize)
        {
            BigInteger fnv_prime = null;
            BigInteger fnv_offset = null;
            BigInteger fnv_mod = null;
            if (hashBitSize <= 32)
            {
                fnv_prime = fnv_prime_32;
                fnv_offset = fnv_offset_32;
                fnv_mod = fnv_mod_32;
            }
            else if (hashBitSize <= 64)
            {
                fnv_prime = fnv_prime_64;
                fnv_offset = fnv_offset_64;
                fnv_mod = fnv_mod_64;
            }
            else if (hashBitSize <= 128)
            {
                fnv_prime = fnv_prime_128;
                fnv_offset = fnv_offset_128;
                fnv_mod = fnv_mod_128;
            }
            else if (hashBitSize <= 256)
            {
                fnv_prime = fnv_prime_256;
                fnv_offset = fnv_offset_256;
                fnv_mod = fnv_mod_256;
            }
            else if (hashBitSize <= 512)
            {
                fnv_prime = fnv_prime_512;
                fnv_offset = fnv_offset_512;
                fnv_mod = fnv_mod_512;
            }
            else if (hashBitSize <= 1024)
            {
                fnv_prime = fnv_prime_1024;
                fnv_offset = fnv_offset_1024;
                fnv_mod = fnv_mod_1024;
            }
            else
            {
                throw new ArgumentOutOfRangeException("hashBitSize");
            }

            BigInteger hash = fnv_offset;
            for (int i = 0; i < value.Length; i++)
            {
                hash ^= (uint)value[i];
                hash %= fnv_mod;
                hash *= fnv_prime;
            }

            if (!IsPowerOfTwo(hashBitSize))
            {
                BigInteger mask = new BigInteger(new string('f', (hashBitSize / 4) + (hashBitSize % 4 != 0 ? 1 : 0)), 16);
                hash = (hash >> hashBitSize) ^ (mask & hash);
            }
            return hash;
        }

        private static bool IsPowerOfTwo(int number)
        {
            return (number & (number - 1)) == 0;
        }
    }
}