Besides being a compact and pretty speedy HyperLogLog implementation for cardinality counting, this modified HyperLogLog allows intersection and similarity estimation of different HyperLogLogs.

A simple implementation of HyperLogLog (LogLog-Beta to be specific):

• 16 bit registers instead of 6 bit, the new 10 bit are for b-bit signatures
• Similarity function estimates Jaccard indices (a number between 0-1) of 0.01 for set cardinalities on the order of 1e9 with accuracy around 5%
• Intersection applies the Jaccard index on the union of the sets to return the intersecting set cardinality

The work is based on "HyperMinHash: Jaccard index sketching in LogLog space - Yun William Yu, Griffin M. Weber"

sk1 := hyperminhash.New()
sk2 := hyperminhash.New()

for i := 0; i < 10000; i++ {
    sk1.Add([]byte(strconv.Itoa(i)))
}

sk1.Cardinality() // 10001 (should be 10000)

for i := 3333; i < 23333; i++ {
    sk2.Add([]byte(strconv.Itoa(i)))
}

sk2.Cardinality()     // 19977 (should be 20000)
sk1.Similarity(sk2)   // 0.284589082 (should be 0.2857326533)
sk1.Intersection(sk2) // 6623 (should be 6667)

sk1.Merge(sk2)
sk1.Cardinality() // 23271 (should be 23333)