log2 algorithm

log2.cs

        private static uint[] MultiplyDeBruijnBitPosition = new uint[]
        {
            0, 9, 1, 10, 13, 21, 2, 29, 11, 14, 16, 18, 22, 25, 3, 30,
            8, 12, 20, 28, 15, 17, 24, 7, 19, 27, 23, 6, 26, 5, 4, 31,
        };

        /// <summary>
        /// Find the log base 2 of an N-bit integer in O(lg(N)) operations.
        /// </summary>
        /// <remarks>
        /// http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
        /// </remarks>
        public static uint Log2(uint v)
        {
            v |= v >> 1; // first round down to one less than a power of 2
            v |= v >> 2;
            v |= v >> 4;
            v |= v >> 8;
            v |= v >> 16;

            return MultiplyDeBruijnBitPosition[(uint)(v * 0x07C4ACDDU) >> 27];
        }

        private static uint[] MultiplyDeBruijnBitPosition2 = new uint[]
        {
            0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
            31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9,
        };

        /// <summary>
        /// Find the log base 2 of an N-bit integer in O(lg(N)) operations. Value must be a power of 2.
        /// </summary>
        /// <param name="value">Must be a power of 2</param>
        /// <remarks>
        /// http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
        /// </remarks>
        public static uint Log2OfPowerOf2(uint value)
        {
            return MultiplyDeBruijnBitPosition2[(uint)(value * 0x077CB531U) >> 27];
        }