counting bits

bits.cs

using System;

namespace Bits
{
    class Program
    {
        static void Main(string[] args)
        {
            var lookup = BuildLookup();

            var input = Convert.ToUInt32("00000000000011110000100010001111", 2);

            // how many bits in the input?
            int count = 0;
            int iterations = 0;

            while (input != 0)
            {
                count += lookup[(byte)(input & 15)];
                input = input >> 4;
                iterations++;
            }

            Console.WriteLine("found {0} bits in {1} iterations", count, iterations);
        }

        private static byte[] BuildLookup()
        {
            const int Size = 2 << 3;
            var lookup = new byte[Size];

            for (int i = 0; i < Size; i++)
            {
                int count = 0;
                int num = i;
                while (num != 0)
                {
                    count += (1 & num);
                    num = num >> 1;
                }
                lookup[i] = (byte)count;
            }

            return lookup;
        }
    }
}