Ubuntu Manpage: Hashtbl - Hash tables and hash functions.

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NAME

       Hashtbl - Hash tables and hash functions.

Module

       Module   Hashtbl

Documentation

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.

Before4.00.0 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 .

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) -> sig end

Functor building an implementation of the hashtable structure. The functor Hashtbl.Make returns a
structure containing a type key of keys and a type 'a t of hash tables associating data of type 'a to
keys of type key . The operations perform similarly to those of the generic interface, but use the
hashing and equality functions specified in the functor argument H instead of generic equality and
hashing. Since the hash function is not seeded, the create operation of the result structure always
returns non-randomized hash tables.

module type SeededHashedType = sig end

The input signature of the functor Hashtbl.MakeSeeded .

Since 4.00.0

module type SeededS = sig end

The output signature of the functor Hashtbl.MakeSeeded .

Since 4.00.0

module MakeSeeded : functor (H : SeededHashedType) -> sig end

Functor building an implementation of the hashtable structure. The functor Hashtbl.MakeSeeded returns a
structure containing a type key of keys and a type 'a t of hash tables associating data of type 'a to
keys of type key . The operations perform similarly to those of the generic interface, but use the
seeded hashing and equality functions specified in the functor argument H instead of generic equality and
hashing. The create operation of the result structure supports the ~random optional parameter and
returns randomized hash tables if ~random:true is passed or if randomization is globally on (see
Hashtbl.randomize ).

Since 4.00.0

=== 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