[containers] Addition of a dense map implementation

From: Jeremy Junac (jeremy.junac_at_[hidden])
Date: 2020-06-29 20:30:34

Hi,

While working on different projects in which performance is key, I came to realize that the efficiency of the 
associative containers that are available in the STL can be improved.

For instance, it is well know that neither std::unordered_map nor std::map are cache-friendly.

It isn't that hard to find implementations that are faster, for instance flat_map, already in Boost.Containers.
However, I would be interested in another implementation, a "Dense Map", similar to the one we can find in SparseHash 
(https://github.com/sparsehash/sparsehash) or in LLVM's ADT (https://llvm.org/doxygen/DenseMap_8h.html).
For sure I am not the only one attracted to such implementation, as we can find several ones in the internet, although, 
it really lacks of "one true generic implementation". SparseHash doesn't seem that maintained anymore and, well, LLVM is 
LLVM.

Hence, instead of having yet another implementation of my own, I would rather have one in Boost, to ensure long-term 
maintainability.

I know that this particular subject was kind of already discussed in this thread from 2016
(https://lists.boost.org/Archives/boost//2016/01/227076.php) and some very good points were made.
But in the end, years have passed, and we still don't have an alternative, so in my opinion an implementation in Boost 
would actually be something to consider.

Please let me know what you think about that.
I am open to any other ideas, and in case we agree on the addition of a new implementation, I would actually be pleased 
to contribute.

Best regards,
Jeremy Junac