On 10/9/17 10:11 AM, RazvanN wrote:
Hi all,
We in the UPB dlang group have been having discussions about the hashing
functions of associative arrays. In particular, we were wondering why is
the AA implementation in druntime is not using the hash function
implemented in druntime/src/core/internal/hash.hashOf for classes that
don't define toHash().
AA uses typeid(Key).getHash. [1]
For objects, this calls the virtual function `toHash`. [2]
Please keep in mind that all you are hashing is the class reference
pointer, as that is the default comparison for `opEquals`. It might make
sense to shuffle those bits a bit, since the bucket algorithm only looks
at the lower bits of the hash, and this basically guarantees keys with
the default hash will be empty in the first few buckets, since class
objects are aligned on a power-of-2 boundary.
But I'm not sure running a full blown hash on the pointer is necessary.
Perhaps just xor-ing the upper bits with the lower bits makes more sense.
Alternatively, you could change the default `opEquals` but that may
break things more than expected.
What you *can't* do is change what the hash is based on without changing
`opEquals`.
Note that I wouldn't recommend using an Object as a key without defining
toHash and opEquals anyway.
-Steve
[1] https://github.com/dlang/druntime/blob/master/src/rt/aaA.d#L310
[2] https://github.com/dlang/druntime/blob/master/src/object.d#L66
