Hash tables and hash functions.
Hash tables are hashed association tables, with in-place modification.
val create : ?random:bool -> int -> ('a, 'b) Hashtbl.tHashtbl.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.
val clear : ('a, 'b) Hashtbl.t -> unitEmpty a hash table. Use reset instead of clear to shrink the size of the bucket table to its initial size.
val reset : ('a, 'b) Hashtbl.t -> unitEmpty a hash table and shrink the size of the bucket table to its initial size.
val add : ('a, 'b) Hashtbl.t -> 'a -> 'b -> unitHashtbl.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) Hashtbl.t -> 'a -> 'bHashtbl.find tbl x returns the current binding of x in tbl, or raises Not_found if no such binding exists.
val find_opt : ('a, 'b) Hashtbl.t -> 'a -> 'b optionHashtbl.find_opt tbl x returns the current binding of x in tbl, or None if no such binding exists.
val find_all : ('a, 'b) Hashtbl.t -> 'a -> 'b listHashtbl.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) Hashtbl.t -> 'a -> boolHashtbl.mem tbl x checks if x is bound in tbl.
val remove : ('a, 'b) Hashtbl.t -> 'a -> unitHashtbl.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) Hashtbl.t -> 'a -> 'b -> unitHashtbl.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) Hashtbl.t -> unitHashtbl.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.
The behavior is not defined if the hash table is modified by f during the iteration.
val filter_map_inplace : ('a -> 'b -> 'b option) -> ('a, 'b) Hashtbl.t -> unitHashtbl.filter_map_inplace f tbl applies f to all bindings in table tbl and update each binding depending on the result of f. If f returns None, the binding is discarded. If it returns Some new_val, the binding is update to associate the key to new_val.
Other comments for Hashtbl.iter apply as well.
val fold : ('a -> 'b -> 'c -> 'c) -> ('a, 'b) Hashtbl.t -> 'c -> 'cHashtbl.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.
The behavior is not defined if the hash table is modified by f during the iteration.
val length : ('a, 'b) Hashtbl.t -> intHashtbl.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.
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.
return if the tables are currently created in randomized mode by default
val stats : ('a, 'b) Hashtbl.t -> Hashtbl.statisticsHashtbl.stats tbl returns statistics about the table tbl: number of buckets, size of the biggest bucket, distribution of buckets by size.
The functorial interface allows the use of specific comparison and hash functions, either for performance/security concerns, or because keys are not hashable/comparable with the polymorphic builtins.
For instance, one might want to specialize a table for integer keys:
module IntHash =
struct
type t = int
let equal i j = i=j
let hash i = i land max_int
end
module IntHashtbl = Hashtbl.Make(IntHash)
let h = IntHashtbl.create 17 in
IntHashtbl.add h 12 "hello"This creates a new module IntHashtbl, with a new type 'a
IntHashtbl.t of tables from int to 'a. In this example, h contains string values so its type is string IntHashtbl.t.
Note that the new type 'a IntHashtbl.t is not compatible with the type ('a,'b) Hashtbl.t of the generic interface. For example, Hashtbl.length h would not type-check, you must use IntHashtbl.length.
module type HashedType = sig ... endThe input signature of the functor Hashtbl.Make.
module type S = sig ... endThe output signature of the functor Hashtbl.Make.
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 ... endThe input signature of the functor Hashtbl.MakeSeeded.
module type SeededS = sig ... endThe output signature of the functor Hashtbl.MakeSeeded.
module MakeSeeded (H : SeededHashedType) : SeededS with type key = H.tFunctor 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).
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.
A variant of Hashtbl.hash that is further parameterized by an integer seed.
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. If total as specified by the user exceeds a certain value, currently 256, then it is capped to that value. 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.
A variant of Hashtbl.hash_param that is further parameterized by an integer seed. Usage: Hashtbl.seeded_hash_param meaningful total seed x.