An extremely memory-efficient hash_map implementation. 2 bits/entry overhead!
The SparseHash library contains several hash-map implementations, including
implementations that optimize for space or speed.
These hashtable implementations are similar in API to SGI's hash_map class and
the tr1 unordered_map class, but with different performance characteristics.
It's easy to replace hash_map or unordered_map by sparse_hash_map or
dense_hash_map in C++ code.
