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 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 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.

       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 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
       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  ,  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 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.   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.

       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 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 .

       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 structure 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: 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 -> 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