Bloom Filter
A Bloom Filter is a space-efficient probabilistic data structure for testing set membership. It uses a bit array and multiple hash functions. It can produce false positives, meaning it may report that an absent item exists, but a correctly implemented Bloom Filter never produces false negatives for inserted items.
False positive: possible.
False negative: not possible under standard insert/query semantics.
m controls memory usage.
k controls the number of hash functions.
Optimal k is approximately (m/n) ln(2).
Common uses include caches, databases, and distributed systems.