On Thursday, July 16, 2015 at 12:19:25 PM UTC-4, milktrader wrote: > > Also, back to the OP question, is the correct solution to simply define > > Base.hash(f::Foo) = f.x >
No, I'd define Base.hash(f::Foo) = hash(f.x, 0x64c74221932dea5b), where I chose the constant by rand(UInt). This way it won't collide with other types. I really need to spend a bit more time with my interfaces chapter and add this info there. On Thursday, July 16, 2015 at 12:07:50 PM UTC-4, Seth wrote: > If I'm reading between the lines correctly, the default/existing hash > function is based on the last byte of the ID? Is there a reason we don't > make the table wider to reduce the chances of collisions (or would this > have bad effects on memory utilization)? > It's not really a hash collision, but rather a hashtable index collision. A new dict only has 16 slots for its keys, and the index for an element is simply chosen by the last 8 bits of its hash. If those last 8 bits match the hash of an object already in the table, there's a collision, and the Dict checks to see if the existing element `isequal` to the new one (and doesn't look at the hash again). So the hashes are different, but isequal says the objects are the same. This is why things are wonky.
