
Hashtbl(3) OCaml library Hashtbl(3)
NAME
Hashtbl  Hash tables and hash functions.
Module
Module Hashtbl
Documentation
Module Hashtbl
: sig end
Hash tables and hash functions.
Hash tables are hashed association tables, with inplace 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 function ( Hashtbl.hash
) to distribute keys among buckets. As a consequence, collisions between keys happen
deterministically. In Webfacing applications or other securitysensitive applications,
the deterministic collision patterns can be exploited by a malicious user to create a
denialofservice 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
denialofservice attack described above. However, because of randomization, enumerating
all elements of the hash table using Hashtbl.fold or Hashtbl.iter is no longer determinis
tic: elements are enumerated in different orders at different runs of the program.
If no ~random parameter is given, hash tables are created in nonrandom 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.
Before4.00.0 the random parameter was not present and all hash tables were created in
nonrandomized 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 initial 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 performing Hashtbl.remove tbl x , the pre
vious 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 cur
rent binding is returned first, then the previous bindings, in reverse order of introduc
tion 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 , restoring 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. 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 nonrandomized 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 ran
dom.
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. 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 nonrandomized 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 ran
dom.
val length : ('a, 'b) t > int
Hashtbl.length tbl returns the number of bindings in tbl . It takes constant time. Mul
tiple 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: 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 denialofservice 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 nonran
domized default behavior of Hashtbl.create . This is intentional. Nonrandomized 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 associat
ing 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 nonrandomized 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.Make
Seeded 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 speci
fied in the functor argument H instead of generic equality and hashing. The create opera
tion of the result structure supports the ~random optional parameter and returns random
ized 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 proper
ties as for hash . The two extra integer parameters meaningful and total give more precise
control over hashing. Hashing performs a breadthfirst, lefttoright traversal of the
structure x , stopping after meaningful meaningful nodes were encountered, or total nodes
(meaningful or not) were encountered. Meaningful nodes are: integers; floatingpoint num
bers; strings; characters; booleans; and constant constructors. Larger values of meaning
ful 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 20140609 Hashtbl(3) 
