Experimental C++11 version of sparsehash
This header-only package contains several hash-map implementations, similar in API to SGI's hashmap class, but with different performance characteristics. sparsehashmap uses very little space overhead, 1-2 bits per entry. densehashmap is very fast, particulary on lookup. (sparsehashset and densehash_set are the set versions of these routines.) On the other hand, these classes have requirements that may not make them appropriate for all applications.