This library contains several hash-map implementations, similar in API to SGI's hash_map class, but with different performance characteristics. sparse_hash_map uses very little space overhead, 1-2 bits per entry. dense_hash_map is very fast, particularly on lookup. sparse_hash_set and dense_hash_set are the set versions of these routines. All these implementation use a hashtable with internal quadratic probing. This method is space-efficient -- there is no pointer overhead -- and time-efficient for good hash functions.