On 2017-10-10 15:22:05 +0000, Steven Schveighoffer said:
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
Not sure this thread fits or a new one would be better. Anyway, here is
an interesting article about a very fast hashtable and maybe it's a
good candidate for the default AA implementation in D. IMO providing a
very performant AA implementation is a big asset:
https://probablydance.com/2018/05/28/a-new-fast-hash-table-in-response-to-googles-new-fast-hash-table/
--
Robert M. Münch
http://www.saphirion.com
smarter | better | faster
