Hashtbl(3) OCaml library Hashtbl(3)NAMEHashtbl - Hash tables and hash functions.
Module
Module HashtblDocumentation
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 func‐
tion ( Hashtbl.hash ) to distribute keys among buckets. As a conse‐
quence, collisions between keys happen deterministically. In Web-fac‐
ing applications or other security-sensitive applications, the deter‐
ministic 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 ran‐
domly 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 random‐
ization, 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 OCAMLRUN‐
PARAM 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 ini‐
tial 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 per‐
forming 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 , restor‐
ing 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. How‐
ever, 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 .
The order in which the bindings are passed to f is unspecified. How‐
ever, 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 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 ran‐
domized 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 envi‐
ronment variable.
It is recommended that applications or Web frameworks that need to pro‐
tect 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 struc‐
ture 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: inte‐
gers; floating-point numbers; strings; characters; booleans; and con‐
stant 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
OCamldoc 2013-10-24 Hashtbl(3)
