Hash table element existence check

[Basile B. via Digitalmars-d-learn]

On Friday, 2 September 2016 at 19:38:34 UTC, Illuminati wrote:
> I am trying to create a hash table and would like an efficient 
> way to be able to know if an element exists to test for 
> collisions.
>
> I could keep a bitarray, but wasting around 12% space. I could 
> use pointers(null check) to elements but this creates 
> fragmentation. It is not terrible, just curious if anyone has a 
> better way?

fragmentation is a consequence of the hash function. You should 
set the hasher as a template parameter so that, according to the 
value type, the best hash fun (the one that creates less 
clustering) can be supplied.

But otherwise the buckets is almost always an array of 
ReturnType!hashFun with the max value wrapped around the next 
power of two value following entry count.