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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