java: sparse bit vector

Question

Are there any well-known libraries in Java for sparse bit vectors?

(And are there guidelines for how sparse is useful to use them vs. java.util.BitSet?)

Answer 1

If its really sparse (e.g., less than 1% loading), then using a hash table indexed by bit index is probably pretty good; mere presence or absence of the index in the table is all you need to know if the bit is one or zero respectively.

If the density is upwards of a few percent, you can use a hash table indexed by bit index divided by 64, and store long words in the hash table containing actual bits. Bit N is set if the hash table contains value V for int(N/64) and (V>>(N mod 64))&1 is true.

Both of these answers assume you want to optimize random access to bits. If you want to optimize sequential (or other access) to bits by index, then you may want a sparse matrix structure, using the same kind of low-level bit vector representation depending on expected density. See Sparse Matrices