Creates permutations of an integer for a fixed number of '1' bits

ComboMaskFactory.cs

public class ComboMaskFactory
{
	private int _current;
	private readonly int _lim;

	public ComboMaskFactory(int maxNumberOfBitsInMask, int numberOfOneBits)
	{
		if (maxNumberOfBitsInMask > 32) throw new Exception("ComboMaskFactory can only be used with maxNumberOfBitsInMask up to 32.");
		_current = GetLowestValueUsingSuppliedNumberOfOneBits(numberOfOneBits);
		_lim = 1 + _current << maxNumberOfBitsInMask - numberOfOneBits;
	}

	private int GetLowestValueUsingSuppliedNumberOfOneBits(int numberOfOneBits)
	{
		var result = 0;
		for (var i = 0; i < numberOfOneBits; i++)
		{
			result += (int)Math.Pow(2, i);
		}
		return result;
	}

	private static int GetLowestBit(int l)
	{
		return l & -l;
	}

	public bool HasNext()
	{
		return _current < _lim;
	}

	public int Next()
	{
		var result = _current;
		MoveNext();
		return result;
	}

	private void MoveNext()
	{
		var lowestBitOfCurrent = GetLowestBit(_current);
		var resultA = _current + lowestBitOfCurrent;
		var lowestBitOfResultA = GetLowestBit(resultA);
		var resultB = ((lowestBitOfResultA / lowestBitOfCurrent) >> 1) - 1;
		_current = resultA | resultB;
	}
}