package grenier

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package grenier
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Module HllSource

An implementation of HyperLogLog probabilistic cardinality estimator.

Sourcetype t

Type of HyperLogLog counters

Sourceval make : error:float -> t

Create a new counter with error error rate. error should verify 0.0 < error && error < 1.0. 0.05 is a reasonable default.

Use estimate_memory to measure memory consumption and runtime of this function.

Sourceval add : t -> int64 -> unit

add t k counts item k in t.

k should be "random": it should be the output of some cryptographic hashing algorithm like SHA. It is not treated as an integer. This is key to getting proper results. No patterns should appear in the bits of the different items added.

Runtime is O(1).

Sourceval estimate_memory : error:float -> int

Estimate the memory consumed in bytes by a counter with the specified error rate.

This ignores the constant overhead of the OCaml representation, around two words. It is a bytes of estimate_memory ~error + 1 length.

Sourceval card : t -> float

Get the cardinality estimation. Defaults to HyperLogLog++.

Sourceval card_hll : t -> float
Sourceval card_hllpp : t -> float
Sourceval copy : t -> t

Get a copy of a counter.

Sourceval merge : into:t -> t -> unit

merge ~into:t0 t' has the same effect as adding all items added to t' to t0.

t0 and t' must have been constructed with the same error rate!

Sourceval clear : t -> unit

Reset counter to 0.

Sourceval hash_int64 : int64 -> int64

The following algorithm provide a reasonable hashing function for integers, if you want to feed the HLL with "normal" integers.

Serialization

Sourceval to_string : t -> string

Returns a string with the current state stored.

Sourceval of_string : string -> t

Restore a HLL saved with to_string.

of_string (to_string t) is functionnally equivalent to copy t, except a bit more expensive.

It can raise Invalid_argument if the string provided was not saved by to_string.