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CentOS 7.0 - man page for hashtbl (centos section 3)

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 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 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 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 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 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  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 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 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 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-ran-
       domized 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 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  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.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 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  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				    2014-06-09				       Hashtbl(3)


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