module Hashtbl:sig..end
Hash tables and hash functions.
Hash tables are hashed association tables, with in-place modification.
type ('a, 'b) t
The type of hash tables from type 'a to type 'b.
val create : ?random:bool -> int -> ('a, 'b) 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.
random parameter was not present and all
hash tables were created in non-randomized mode.val clear : ('a, 'b) 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) t -> unitEmpty a hash table and shrink the size of the bucket table to its initial size.
val copy : ('a, 'b) t -> ('a, 'b) tReturn a copy of the given hashtable.
val add : ('a, 'b) 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) 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) 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) 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) t -> 'a -> boolHashtbl.mem tbl x checks if x is bound in tbl.
val remove : ('a, 'b) 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) 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) 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) 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) 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) 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.
val randomize : unit -> unitAfter 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.
val is_randomized : unit -> boolreturn if the tables are currently created in randomized mode by default
type statistics = {
|
num_bindings : |
(* | Number of bindings present in the table.
Same value as returned by | *) |
|
num_buckets : |
(* | Number of buckets in the table. | *) |
|
max_bucket_length : |
(* | Maximal number of bindings per bucket. | *) |
|
bucket_histogram : |
(* | Histogram of bucket sizes. This array | *) |
}
val stats : ('a, 'b) t -> statisticsHashtbl.stats tbl returns statistics about the table tbl:
number of buckets, size of the biggest bucket, distribution of
buckets by size.
val to_seq : ('a, 'b) t -> ('a * 'b) Seq.tIterate 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 defined if the hash table is modified during the iteration.
val to_seq_keys : ('a, 'b) t -> 'a Seq.tSame as Seq.map fst (to_seq m)
val to_seq_values : ('a, 'b) t -> 'b Seq.tSame as Seq.map snd (to_seq m)
val add_seq : ('a, 'b) t -> ('a * 'b) Seq.t -> unitAdd the given bindings to the table, using Hashtbl.add
val replace_seq : ('a, 'b) t -> ('a * 'b) Seq.t -> unitAdd the given bindings to the table, using Hashtbl.replace
val of_seq : ('a * 'b) Seq.t -> ('a, 'b) tBuild a table from the given bindings. The bindings are added
in the same order they appear in the sequence, using Hashtbl.replace_seq,
which means that if two pairs have the same key, only the latest one
will appear in the table.
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 of tables from
IntHashtbl.tint 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..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 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 building an implementation of the hashtable structure.
val hash : 'a -> intHashtbl.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 -> intA variant of Hashtbl.hash that is further parameterized by
an integer seed.
val hash_param : int -> int -> 'a -> intHashtbl.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.
val seeded_hash_param : int -> int -> int -> 'a -> intA variant of Hashtbl.hash_param that is further parameterized by
an integer seed. Usage:
Hashtbl.seeded_hash_param meaningful total seed x.