# Module Hashtbl

    module Hashtbl: sig .. end

Hash tables and hash functions.

Hash tables are hashed association tables, with in-place modification.

###### Generic interface

    type ('a, 'b) t

The type of hash tables from type `'a` to type `'b`.

    val create : ?random:bool -> int -> ('a, 'b) t

`Hashtbl.create n` creates a new, empty hash table, with initial size `n`. For best results, `n` should be on the order of the expected number of elements that will be in the table. The table grows as needed, so `n` is just an initial guess.

The optional `random` parameter (a boolean) controls whether the internal organization of the hash table is randomized at each execution of `Hashtbl.create` or deterministic over all executions.

A hash table that is created with `~random:false` uses a fixed hash function (`Hashtbl.hash`) to distribute keys among buckets. As a consequence, collisions between keys happen deterministically. In Web-facing applications or other security-sensitive applications, the deterministic collision patterns can be exploited by a malicious user to create a denial-of-service attack: the attacker sends input crafted to create many collisions in the table, slowing the application down.

A hash table that is created with `~random:true` uses the seeded hash function `Hashtbl.seeded_hash` with a seed that is randomly chosen at hash table creation time. In effect, the hash function used is randomly selected among `2^{30}` different hash functions. All these hash functions have different collision patterns, rendering ineffective the denial-of-service attack described above. However, because of randomization, enumerating all elements of the hash table using `Hashtbl.fold` or `Hashtbl.iter` is no longer deterministic: elements are enumerated in different orders at different runs of the program.

If no `~random` parameter is given, hash tables are created in non-random mode by default. This default can be changed either programmatically by calling `Hashtbl.randomize` or by setting the `R` flag in the `OCAMLRUNPARAM` environment variable.
Before 4.00.0 the `random` parameter was not present and all hash tables were created in non-randomized mode.

    val clear : ('a, 'b) t -> unit

Empty a hash table. Use `reset` instead of `clear` to shrink the size of the bucket table to its initial size.

    val reset : ('a, 'b) t -> unit

Empty a hash table and shrink the size of the bucket table to its initial size.

    val copy : ('a, 'b) t -> ('a, 'b) t

Return a copy of the given hashtable.

    val add : ('a, 'b) t -> 'a -> 'b -> unit

`Hashtbl.add tbl x y` adds a binding of `x` to `y` in table `tbl`. Previous bindings for `x` are not removed, but simply hidden. That is, after performing `Hashtbl.remove` `tbl x`, the previous binding for `x`, if any, is restored. (Same behavior as with association lists.)

    val find : ('a, 'b) t -> 'a -> 'b

`Hashtbl.find tbl x` returns the current binding of `x` in `tbl`, or raises `Not_found` if no such binding exists.

    val find_all : ('a, 'b) t -> 'a -> 'b list

`Hashtbl.find_all tbl x` returns the list of all data associated with `x` in `tbl`. The current binding is returned first, then the previous bindings, in reverse order of introduction in the table.

    val mem : ('a, 'b) t -> 'a -> bool

`Hashtbl.mem tbl x` checks if `x` is bound in `tbl`.

    val remove : ('a, 'b) t -> 'a -> unit

`Hashtbl.remove tbl x` removes the current binding of `x` in `tbl`, restoring the previous binding if it exists. It does nothing if `x` is not bound in `tbl`.

    val replace : ('a, 'b) t -> 'a -> 'b -> unit

`Hashtbl.replace tbl x y` replaces the current binding of `x` in `tbl` by a binding of `x` to `y`. If `x` is unbound in `tbl`, a binding of `x` to `y` is added to `tbl`. This is functionally equivalent to `Hashtbl.remove` `tbl x` followed by `Hashtbl.add` `tbl x y`.

    val iter : ('a -> 'b -> unit) -> ('a, 'b) t -> unit

`Hashtbl.iter f tbl` applies `f` to all bindings in table `tbl`. `f` receives the key as first argument, and the associated value as second argument. Each binding is presented exactly once to `f`.

The order in which the bindings are passed to `f` is unspecified. However, if the table contains several bindings for the same key, they are passed to `f` in reverse order of introduction, that is, the most recent binding is passed first.

If the hash table was created in non-randomized mode, the order in which the bindings are enumerated is reproducible between successive runs of the program, and even between minor versions of OCaml. For randomized hash tables, the order of enumeration is entirely random.

    val fold : ('a -> 'b -> 'c -> 'c) -> ('a, 'b) t -> 'c -> 'c

`Hashtbl.fold f tbl init` computes `(f kN dN ... (f k1 d1 init)...)`, where `k1 ... kN` are the keys of all bindings in `tbl`, and `d1 ... dN` are the associated values. Each binding is presented exactly once to `f`.

The order in which the bindings are passed to `f` is unspecified. However, if the table contains several bindings for the same key, they are passed to `f` in reverse order of introduction, that is, the most recent binding is passed first.

If the hash table was created in non-randomized mode, the order in which the bindings are enumerated is reproducible between successive runs of the program, and even between minor versions of OCaml. For randomized hash tables, the order of enumeration is entirely random.

    val length : ('a, 'b) t -> int

`Hashtbl.length tbl` returns the number of bindings in `tbl`. It takes constant time. Multiple bindings are counted once each, so `Hashtbl.length` gives the number of times `Hashtbl.iter` calls its first argument.

    val randomize : unit -> unit

After a call to `Hashtbl.randomize()`, hash tables are created in randomized mode by default: `Hashtbl.create` returns randomized hash tables, unless the `~random:false` optional parameter is given. The same effect can be achieved by setting the `R` parameter in the `OCAMLRUNPARAM` environment variable.

It is recommended that applications or Web frameworks that need to protect themselves against the denial-of-service attack described in `Hashtbl.create` call `Hashtbl.randomize()` at initialization time.

Note that once `Hashtbl.randomize()` was called, there is no way to revert to the non-randomized default behavior of `Hashtbl.create`. This is intentional. Non-randomized hash tables can still be created using `Hashtbl.create ~random:false`.
Since 4.00.0

    type statistics = {

`` | `num_bindings :`int`;` | `(*` | Number of bindings present in the table. Same value as returned by `Hashtbl.length`. | `*)`
`` | `num_buckets :`int`;` | `(*` | Number of buckets in the table. | `*)`
`` | `max_bucket_length :`int`;` | `(*` | Maximal number of bindings per bucket. | `*)`
`` | `bucket_histogram :`int array`;` | `(*` | Histogram of bucket sizes. This array `histo` has length `max_bucket_length + 1`. The value of `histo.(i)` is the number of buckets whose size is `i`. | `*)`

}

    val stats : ('a, 'b) t -> statistics

`Hashtbl.stats tbl` returns statistics about the table `tbl`: number of buckets, size of the biggest bucket, distribution of buckets by size.
Since 4.00.0

###### Functorial interface

    module type HashedType = sig .. end

The input signature of the functor `Hashtbl.Make`.

    module type S = sig .. end

The output signature of the functor `Hashtbl.Make`.

    module Make: functor (H : HashedType) -> S  with type key = H.t

Functor building an implementation of the hashtable structure.

    module type SeededHashedType = sig .. end

The input signature of the functor `Hashtbl.MakeSeeded`.

    module type SeededS = sig .. end

The output signature of the functor `Hashtbl.MakeSeeded`.

    module MakeSeeded: functor (H : SeededHashedType) -> SeededS  with type key = H.t

Functor building an implementation of the hashtable structure.

###### The polymorphic hash functions

    val hash : 'a -> int

`Hashtbl.hash x` associates a nonnegative integer to any value of any type. It is guaranteed that if `x = y` or `Pervasives.compare x y = 0`, then `hash x = hash y`. Moreover, `hash` always terminates, even on cyclic structures.

    val seeded_hash : int -> 'a -> int

A variant of `Hashtbl.hash` that is further parameterized by an integer seed.
Since 4.00.0

    val hash_param : int -> int -> 'a -> int

`Hashtbl.hash_param meaningful total x` computes a hash value for `x`, with the same properties as for `hash`. The two extra integer parameters `meaningful` and `total` give more precise control over hashing. Hashing performs a breadth-first, left-to-right traversal of the structure `x`, stopping after `meaningful` meaningful nodes were encountered, or `total` nodes (meaningful or not) were encountered. Meaningful nodes are: integers; floating-point numbers; strings; characters; booleans; and constant constructors. Larger values of `meaningful` and `total` means that more nodes are taken into account to compute the final hash value, and therefore collisions are less likely to happen. However, hashing takes longer. The parameters `meaningful` and `total` govern the tradeoff between accuracy and speed. As default choices, `Hashtbl.hash` and `Hashtbl.seeded_hash` take `meaningful = 10` and `total = 100`.

    val seeded_hash_param : int -> int -> int -> 'a -> int

A variant of `Hashtbl.hash_param` that is further parameterized by an integer seed. Usage: `Hashtbl.seeded_hash_param meaningful total seed x`.
Since 4.00.0