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Hash tables and hash functions.
Hash tables are hashed association tables, with in-place modification.
Unsynchronized accesses
Unsynchronized accesses to a hash table may lead to an invalid hash table state. Thus, concurrent accesses to a hash tables must be synchronized (for instance with a Mutex.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 set to false uses a fixed hash function (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 set to true uses the seeded hash function 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 fold or 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 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.
Hashtbl.add tbl key data adds a binding of key to data in table tbl. Previous bindings for key are not removed, but simply hidden. That is, after performing remove tbl key, the previous binding for key, if any, is restored. (Same behavior as with association lists.)
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.
Hashtbl.replace tbl key data replaces the current binding of key in tbl by a binding of key to data. If key is unbound in tbl, a binding of key to data is added to tbl. This is functionally equivalent to remove tbl key followed by add tbl key data.
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.
The behavior is not specified if the hash table is modified by f during the iteration.
val filter_map_inplace : ('a->'b->'b option)->('a, 'b)t-> unit
Hashtbl.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.
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.
The behavior is not specified if the hash table is modified by f during the iteration.
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: 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 create call Hashtbl.randomize() at initialization time before any domains are created.
Note that once Hashtbl.randomize() was called, there is no way to revert to the non-randomized default behavior of create. This is intentional. Non-randomized hash tables can still be created using Hashtbl.create ~random:false.
since 4.00.0
val is_randomized : unit -> bool
Return true if the tables are currently created in randomized mode by default, false otherwise.
Return a copy of the given hashtable. Unlike copy, rebuild h re-hashes all the (key, value) entries of the original table h. The returned hash table is randomized if h was randomized, or the optional random parameter is true, or if the default is to create randomized hash tables; see create for more information.
rebuild can safely be used to import a hash table built by an old version of the Hashtbl module, then marshaled to persistent storage. After unmarshaling, apply rebuild to produce a hash table for the current version of the Hashtbl module.
since 4.12.0
type statistics = {
num_bindings : int;
(*
Number of bindings present in the table. Same value as returned by 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.
Iterate on the whole table. The order in which the bindings appear in the sequence is unspecified. However, if the table contains several bindings for the same key, they appear in reversed order of introduction, that is, the most recent binding appears first.
The behavior is not specified if the hash table is modified during the iteration.
Build a table from the given bindings. The bindings are added in the same order they appear in the sequence, using replace_seq, which means that if two pairs have the same key, only the latest one will appear in the table.
since 4.07
Functorial interface
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.
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.
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).
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 Stdlib.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 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. 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, hash and seeded_hash take meaningful = 10 and total = 100.
val seeded_hash_param : int ->int ->int ->'a-> int
A variant of hash_param that is further parameterized by an integer seed. Usage: Hashtbl.seeded_hash_param meaningful total seed x.