What is the main implementation idea behind sparse hash table?

Question

Why does Google sparsehash open-source library has two implementations: a dense hashtable and a sparse one?

Answer 1

sparsehash are a memory-efficient way of mapping keys to values (1-2 bits per key). Bloom filters can give you even fewer bits per key, but they don't attach values to keys other than outside/probably-inside, which is slightly less than a bit of information.