[RESOLVED] Bit-patterns.

[ThomasJohnsen]

Hello forum-members.

I'm currently in need of a (very) fast way of determining, if a 14 bit unsigned integer contains 5 consequtive set bits. Only the leftmost, if any, such pattern need be detected. I have ofc implemented the naive approach, but there may be a faster version that can be implemented.

Current code:

private ushort leftmost5bits(ushort v)
        {
            for (ushort p = 0x3e00; p >= 0x001f; p >>= 1) if ((p & v) == p) return p;
 
            return 0;
        }

For instance when counting set bits in a 14-bit number, I came across this excellent little algoritm, that does significantly better than the naive approach:

private byte countbits(ushort v)
        {
            return (byte)((v * 0x200040008001UL & 0x111111111111111UL) 'Modulus-operator' 0xf);
        }

EDIT: Above algorithm Sean Anderson + Bruce Dawson after original idea by Rich Schroeppel.
I'm hoping something similar can be found / coded for the pattern-matching problem above.

Thank you in advance for any input
Thomas