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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 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.


       Since 4.00.0



       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 ===





       === The functorial interface allows the use of specific comparison  and
       hash  functions,  either  for performance/security concerns, or because
       keys are not hashable/comparable with the  polymorphic  builtins.   For
       instance, one might want to specialize a table for integer keys: module
       IntHash = struct type t = int let equal i j = i=j let hash i =  i  land
       max_int   end   module  IntHashtbl  =  Hashtbl.Make(IntHash)  let  h  =
       IntHashtbl.create 17 in IntHashtbl.add h 12 hello ;; This creates a new
       module  IntHashtbl,  with a new type 'a IntHashtbl.t of tables from int
       to 'a. In this example, h contains string values so its type is  string
       IntHashtbl.t.  Note that the new type 'a IntHashtbl.t is not compatible
       with the type ('a,'b) Hashtbl.t of the generic interface. For  example,
       Hashtbl.length  h would not type-check, you must use IntHashtbl.length.
       ===


       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                          2014-10-18                        Hashtbl(3)

ocaml 4.02.1 - Generated Sun Oct 19 09:10:41 CDT 2014
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