Opposite of a bloom filter... reports "not in set" whenever multiset reports 0 or >1.

GloomFilter.groovy

/**
 * The opposite of a bloom filter
 */
class GloomFilter<T> {
    Multiset<Long> coordinateFilter
    Funnel<T> funnel
    long expectedSize

    static create(int expectedSize, Funnel<T> funnel) {
        return new GloomFilter<T>(expectedSize: expectedSize,
            coordinateFilter: HashMultiset.create(expectedSize),
            funnel: funnel)
    }

    GloomFilter<T> put(T t) {
        coordinateFilter.add(hash(t))
        return this
    }

    boolean mightNotContain(T t) {
        coordinateFilter.count(hash(t)) != 1
    }

    private Long hash(T t) {
        return Hashing.murmur3_128().hashObject(t, funnel).asLong() % expectedSize
    }
}

GloomFilterSpec.groovy

class GloomFilterSpec extends Specification {
    def "gloom filter reports 'might not contain' iff the underlying multiset's cardinality != 1"() {
        when:
        def filter = GloomFilter.create(10, new Funnel<Integer>() {
            @Override
            void funnel(Integer integer, PrimitiveSink into) {
                into.putInt(integer % 10)
            }
        })

        then:
        filter.mightNotContain(9)         // cardinality of 9 == 0
        !filter.put(9).mightNotContain(9) // cardinality of 9 == 1
        filter.put(19).mightNotContain(9) // cardinality of 9 == 2
    }
}