Needle in a haystack

NeedleIndex.cs

// find the needle that exactly matches the specified portion of haystack
// will return -1 for count of 0
public static int NeedleIndex(byte[] haystack, int offset, int count, byte[][] needles)
{
	Debug.Assert(needles.Length <= 8, "Can only search for up to 8 needles at once");
	
	ulong mask = 0;
	for (int i = 0; i < needles.Length; i++)
	{
		if (needles[i] != null && needles[i].Length == count)
		{
			mask = mask | ((ulong)1 << i);
		}
	}
	
	// no candidates
	if (mask == 0 || count == 0) return -1;
	
	int match = -1;
	int needleOffset = 0;
	
	do
	{
		match = -1;
		for (int i = 0; i < needles.Length; i++)
		{
			ulong bit = (ulong)1 << i;
			if ((mask & bit) == 0) continue;
			var needle = needles[i];
			
			// check current byte
			if (haystack[offset] != needle[needleOffset])
			{
				// not a match
				mask = mask & ~bit;
				
				// no candidates left
				if (mask == 0) return -1;
			}
			else
			{
				match = i;
			}
		}
		
		needleOffset++;
		offset++;
	} while(needleOffset < count);

	return match;
}