It's the hyperloglog cardinality estimation algorithm implemented in rust.
Specifically, it's the algorithm as presented on page 140 of the journal the original (Flajolet et al.) paper was published in (here). This is unlike the current HLL crate on crates.io, which takes into account the bias correction (as outlined here).
Rust doc: here
See the tests for some examples of usage, but broadly we can use it as follows:
extern crate basichll as hll;
...
// Note the constructor parameter is the desired typical relative
// error, which dictates the size of the HLL instance. The error
// is 1.04 / sqrt(2 ^ bits), so for a 1% error you want about 12 bits
// -- which gives 2^12 size to it's underlying Vec (i.e. 4kb).
let mut log = hll::HLL::new(0.01625);
// you can insert anything that can be hashed
log.insert(&"foo");
log.insert(&"bar");
log.insert(&1);
log.insert(&2);
// you can count the approximate number of elements
println!("I think there are {} elements", log.count().round());
// would print "I think there are 4 elements"
// It implements Add so you can combine HLLs in a way the follows
// the monoid laws.
let first = hll::HLL::new(0.26);
let second = hll::HLL::new(0.26);
let third = &first + &second;