Module Base.Hash

module type S = Base__.Hash_intf.S
module F : functor (Hash : S) -> Full with type hash_value = Hash.hash_value and type state = Hash.state and type seed = Hash.seed

The code of ppx_hash is agnostic to the choice of hash algorithm that is used. However, it is not currently possible to mix various choices of hash algorithms in a given code base.

We experimented with:

Our findings were as follows:

Hence, we are here making the choice of using this Internalhash (that is, Murmur3, the OCaml hash algorithm as of 4.03) as our hash algorithm. It means that the state of the hash function does not need to be preallocated, and makes for simpler use in hash tables and other structures.

include Full with type Full.state = private int and type Full.seed = int and type Full.hash_value = int
val description : string

Name of the hash-function, e.g., "internalhash", "siphash"

type state = private int
val fold_int : state ‑> int ‑> state

fold_<T> state v incorporates a value v of type <T> into the hash-state, returning a modified hash-state. Implementations of the fold_<T> functions may mutate the state argument in place, and return a reference to it. Implementations of the fold_<T> functions should not allocate.

val fold_int64 : state ‑> int64 ‑> state
val fold_float : state ‑> float ‑> state
val fold_string : state ‑> string ‑> state
type seed = int
val alloc : unit ‑> state

alloc () returns a fresh uninitialized hash-state. May allocate.

val reset : ?⁠seed:seed ‑> state ‑> state

reset ?seed state initializes/resets a hash-state with the given seed, or else a default-seed. Argument state may be mutated. Should not allocate.

type hash_value = int
val get_hash_value : state ‑> hash_value

get_hash_value extracts a hash-value from the hash-state.

module For_tests : sig ... end
type 'a folder = state ‑> 'a ‑> state
val create : ?⁠seed:seed ‑> unit ‑> state

create ?seed () is a convenience. Equivalent to reset ?seed (alloc ()).

val of_fold : (state ‑> 'a ‑> state) ‑> 'a ‑> hash_value

of_fold fold constructs a standard hash function from an existing fold function.

module Builtin : sig ... end
val run : ?⁠seed:seed ‑> 'a folder ‑> 'a ‑> hash_value

run ?seed folder x runs folder on x in a newly allocated hash-state, initialized using optional seed or a default-seed.

The following identity exists: run [%hash_fold: T] == [%hash: T]

run can be used if we wish to run a hash-folder with a non-default seed.