include Stdlib.Hashtbl
Generic interface
val create : ?random:bool -> int -> ('a, 'b) t
Hashtbl.create n
creates a new, empty hash table, with initial sizen
. 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, son
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 ofHashtbl.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 functionHashtbl.seeded_hash
with a seed that is randomly chosen at hash table creation time. In effect, the hash function used is randomly selected among2^{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 usingHashtbl.fold
orHashtbl.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 callingHashtbl.randomize
or by setting theR
flag in theOCAMLRUNPARAM
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 ofclear
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.
- since
- 4.00.0
val add : ('a, 'b) t -> 'a -> 'b -> unit
Hashtbl.add tbl x y
adds a binding ofx
toy
in tabletbl
. Previous bindings forx
are not removed, but simply hidden. That is, after performingHashtbl.remove
tbl x
, the previous binding forx
, 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 ofx
intbl
, or raisesNot_found
if no such binding exists.
val find_opt : ('a, 'b) t -> 'a -> 'b option
Hashtbl.find_opt tbl x
returns the current binding ofx
intbl
, orNone
if no such binding exists.- since
- 4.05
val find_all : ('a, 'b) t -> 'a -> 'b list
Hashtbl.find_all tbl x
returns the list of all data associated withx
intbl
. 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 ifx
is bound intbl
.
val remove : ('a, 'b) t -> 'a -> unit
Hashtbl.remove tbl x
removes the current binding ofx
intbl
, restoring the previous binding if it exists. It does nothing ifx
is not bound intbl
.
val replace : ('a, 'b) t -> 'a -> 'b -> unit
Hashtbl.replace tbl x y
replaces the current binding ofx
intbl
by a binding ofx
toy
. Ifx
is unbound intbl
, a binding ofx
toy
is added totbl
. This is functionally equivalent toHashtbl.remove
tbl x
followed byHashtbl.add
tbl x y
.
val iter : ('a -> 'b -> unit) -> ('a, 'b) t -> unit
Hashtbl.iter f tbl
appliesf
to all bindings in tabletbl
.f
receives the key as first argument, and the associated value as second argument. Each binding is presented exactly once tof
.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 tof
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 -> unit
Hashtbl.filter_map_inplace f tbl
appliesf
to all bindings in tabletbl
and update each binding depending on the result off
. Iff
returnsNone
, the binding is discarded. If it returnsSome new_val
, the binding is update to associate the key tonew_val
.Other comments for
Hashtbl.iter
apply as well.- since
- 4.03.0
val fold : ('a -> 'b -> 'c -> 'c) -> ('a, 'b) t -> 'c -> 'c
Hashtbl.fold f tbl init
computes(f kN dN ... (f k1 d1 init)...)
, wherek1 ... kN
are the keys of all bindings intbl
, andd1 ... dN
are the associated values. Each binding is presented exactly once tof
.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 tof
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 -> int
Hashtbl.length tbl
returns the number of bindings intbl
. It takes constant time. Multiple bindings are counted once each, soHashtbl.length
gives the number of timesHashtbl.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 theR
parameter in theOCAMLRUNPARAM
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
callHashtbl.randomize()
at initialization time.Note that once
Hashtbl.randomize()
was called, there is no way to revert to the non-randomized default behavior ofHashtbl.create
. This is intentional. Non-randomized hash tables can still be created usingHashtbl.create ~random:false
.- since
- 4.00.0
val is_randomized : unit -> bool
return if the tables are currently created in randomized mode by default
- since
- 4.03.0
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 lengthmax_bucket_length + 1
. The value ofhisto.(i)
is the number of buckets whose size isi
.}
- since
- 4.00.0
val stats : ('a, 'b) t -> statistics
Hashtbl.stats tbl
returns statistics about the tabletbl
: number of buckets, size of the biggest bucket, distribution of buckets by size.- since
- 4.00.0
Iterators
val to_seq : ('a, 'b) t -> ('a * 'b) Stdlib.Seq.t
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 defined if the hash table is modified during the iteration.
- since
- 4.07
val to_seq_keys : ('a, _) t -> 'a Stdlib.Seq.t
Same as
Seq.map fst (to_seq m)
- since
- 4.07
val to_seq_values : (_, 'b) t -> 'b Stdlib.Seq.t
Same as
Seq.map snd (to_seq m)
- since
- 4.07
val add_seq : ('a, 'b) t -> ('a * 'b) Stdlib.Seq.t -> unit
Add the given bindings to the table, using
add
- since
- 4.07
val replace_seq : ('a, 'b) t -> ('a * 'b) Stdlib.Seq.t -> unit
Add the given bindings to the table, using
replace
- since
- 4.07
val of_seq : ('a * 'b) Stdlib.Seq.t -> ('a, 'b) t
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
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. The functor
Hashtbl.Make
returns a structure containing a typekey
of keys and a type'a t
of hash tables associating data of type'a
to keys of typekey
. The operations perform similarly to those of the generic interface, but use the hashing and equality functions specified in the functor argumentH
instead of generic equality and hashing. Since the hash function is not seeded, thecreate
operation of the result structure always returns non-randomized hash tables.
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 functor
Hashtbl.MakeSeeded
returns a structure containing a typekey
of keys and a type'a t
of hash tables associating data of type'a
to keys of typekey
. The operations perform similarly to those of the generic interface, but use the seeded hashing and equality functions specified in the functor argumentH
instead of generic equality and hashing. Thecreate
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 (seeHashtbl.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 ifx = y
orStdlib.compare x y = 0
, thenhash 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 forx
, with the same properties as forhash
. The two extra integer parametersmeaningful
andtotal
give more precise control over hashing. Hashing performs a breadth-first, left-to-right traversal of the structurex
, stopping aftermeaningful
meaningful nodes were encountered, ortotal
nodes (meaningful or not) were encountered. Iftotal
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 ofmeaningful
andtotal
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 parametersmeaningful
andtotal
govern the tradeoff between accuracy and speed. As default choices,Hashtbl.hash
andHashtbl.seeded_hash
takemeaningful = 10
andtotal = 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
val sexp_of_t : ('a -> Sexplib0.Sexp.t) -> ('b -> Sexplib0.Sexp.t) -> ('a, 'b) Stdlib.Hashtbl.t -> Sexplib0.Sexp.t
val t_of_sexp : (Sexplib0.Sexp.t -> 'a) -> (Sexplib0.Sexp.t -> 'b) -> Sexplib0.Sexp.t -> ('a, 'b) Stdlib.Hashtbl.t