BitSet iteration

BitSet.cs

public class BitSet
{
	public ulong[] LowBitSet = Array.Empty<ulong>();
	public ulong[] HighBitSet = Array.Empty<ulong>();

	public void Add(int id)
	{
		int lowIndex = id >> 6;
		int highIndex = id >> 12;

		if (highIndex >= HighBitSet.Length)
		{
			Array.Resize(ref HighBitSet, (highIndex + 1) << 2);
			Array.Resize(ref LowBitSet, HighBitSet.Length << 6);
		}

		ulong lowMask = 1UL << (id & 63);
		ulong highMask = 1UL << (lowIndex & 63);

		if (LowBitSet[lowIndex] == 0UL)
		{
			HighBitSet[highIndex] |= highMask;
		}
		LowBitSet[lowIndex] |= lowMask;
	}

	public void Remove(int id)
	{
		int lowIndex = id >> 6;
		int highIndex = id >> 12;

		if (lowIndex >= LowBitSet.Length)
		{
			return;
		}

		ulong lowMask = 1UL << (id & 63);
		ulong highMask = 1UL << (lowIndex & 63);

		LowBitSet[lowIndex] &= ~lowMask;
		if (LowBitSet[lowIndex] == 0UL)
		{
			HighBitSet[highIndex] &= ~highMask;
		}
	}

	public bool Has(int id)
	{
		int lowIndex = id >> 6;

		if (lowIndex >= LowBitSet.Length)
		{
			return false;
		}

		ulong lowMask = 1UL << (id & 63);
		return (LowBitSet[lowIndex] & lowMask) != 0UL;
	}
}

Query.cs

public struct Query
{
	public static readonly byte[] DeBruijn =
	{
		0, 1, 17, 2, 18, 50, 3, 57, 47, 19, 22, 51, 29, 4, 33, 58,
		15, 48, 20, 27, 25, 23, 52, 41, 54, 30, 38, 5, 43, 34, 59, 8,
		63, 16, 49, 56, 46, 21, 28, 32, 14, 26, 24, 40, 53, 37, 42, 7,
		62, 55, 45, 31, 13, 39, 36, 6, 61, 44, 12, 35, 60, 11, 10, 9,
	};

	public BitSet Bitset1 { get; }
	public BitSet Bitset2 { get; }
	public BitSet Bitset3 { get; }
	public BitSet Bitset4 { get; }

	public delegate void Iterator(int id);

	public Query(BitSet bitset1, BitSet bitset2, BitSet bitset3, BitSet bitset4)
	{
		Bitset1 = bitset1;
		Bitset2 = bitset2;
		Bitset3 = bitset3;
		Bitset4 = bitset4;
	}

	public void For(Iterator iterator)
	{
		int minHighBitSetLength =
			Math.Min(Bitset1.HighBitSet.Length,
					Math.Min(Bitset2.HighBitSet.Length,
							Math.Min(Bitset3.HighBitSet.Length, Bitset4.HighBitSet.Length)));

		byte[] deBruijn = DeBruijn;
		for (var highIndex = 0; highIndex < minHighBitSetLength; highIndex++)
		{
			ulong highMask = Bitset1.HighBitSet[highIndex]
							& Bitset2.HighBitSet[highIndex]
							& Bitset3.HighBitSet[highIndex]
							& Bitset4.HighBitSet[highIndex];
			int lowOffset = highIndex << 6;

			while (highMask != 0)
			{
				ulong highBit = highMask & (ulong) -(long) highMask;
				byte highBitIndex = deBruijn[(uint) ((highBit * 0x37E84A99DAE458FUL) >> 58)];
				int lowIndex = lowOffset + highBitIndex;

				do
				{
					ulong lowMask = Bitset1.LowBitSet[lowIndex]
									& Bitset2.LowBitSet[lowIndex]
									& Bitset3.LowBitSet[lowIndex]
									& Bitset4.LowBitSet[lowIndex];
					int idOffset = lowIndex << 6;

					do
					{
						ulong lowBit = lowMask & (ulong) -(long) lowMask;
						byte lowBitIndex = deBruijn[(uint) ((lowBit * 0x37E84A99DAE458FUL) >> 58)];
						int id = idOffset + lowBitIndex;

						do
						{
							iterator.Invoke(id);

							lowBit <<= 1;
							id++;
						} while ((lowMask & lowBit) != 0);

						lowMask &= ~(lowBit - 1);
					} while (lowMask != 0);

					highBit <<= 1;
					lowIndex++;
				} while ((highMask & highBit) != 0);

				highMask &= ~(highBit - 1);
			}
		}
	}
}