Re: Good hash for pointers

Richard Harnden <richard.nospam@gmail.invalid> writes:

On 23/05/2024 12:11, Malcolm McLean wrote:

What is a good hash function for pointers to use in portable ANSI C?

>
All your pointers from malloc are going to be unique.
All of their low bits are going to be zero, because they need to align
on some n-bit boundary.
All of their high bit are going to be the same, because that's just
how it works.
>
So just take the middle:  hash = ((intptr_t) ptr) >> 4 & 0xffff;

I'd use uintptr_t, not intptr_t.  I prefer not to think about how
bitwise operations work on signed values unless I have to.

You're assuming a 16-bit hash.  I have no idea whether that suits
Malcolm's requirements.

Another approach might be to divide the pointer representation into
N-bit chunks and xor them together.

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */