# 1 "hashtbl.mli"
(**************************************************************************)
(*                                                                        *)
(*                                 OCaml                                  *)
(*                                                                        *)
                , projet Cristal , INRIA Rocquencourt           
(*                                                                        *)
    Copyright 1996 Institut National de Recherche en Informatique et     
(*     en Automatique.                                                    *)
(*                                                                        *)
(*   All rights reserved.  This file is distributed under the terms of    *)
    the GNU Lesser General Public License version 2.1 , with the          
(*   special exception on linking described in the file LICENSE.          *)
(*                                                                        *)
(**************************************************************************)

open! Stdlib

(* NOTE: If this file is hashtbl.mli, do not edit it directly! Instead,
   edit templates/hashtbl.template.mli and run tools/sync_stdlib_docs *)

(** Hash tables and hash functions.

   Hash tables are hashed association tables, with in-place modification.
*)


* { 1 Generic interface }


type (!'a, !'b) t
(** The type of hash tables from type ['a] to type ['b]. *)

val create : ?random: (* thwart tools/sync_stdlib_docs *) 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 ] set to [ false ] uses a 
    fixed hash function ( { ! 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 ] set to [ true ] uses the seeded 
    hash function { ! 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 { ! fold } 
    or { ! 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 { ! randomize } or by 
    setting the [ R ] flag in the [ OCAMLRUNPARAM ] environment variable . 

    @before 4.00.0 the [ ~][random ] parameter was not present and all 
    hash tables were created in non - randomized mode .
   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] set to [false] uses a
   fixed hash function ({!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] set to [true] uses the seeded
   hash function {!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 {!fold}
   or {!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 {!randomize} or by
   setting the [R] flag in the [OCAMLRUNPARAM] environment variable.

   @before 4.00.0 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 . 
     @since 4.00.0
    to its initial 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 key data ] adds a binding of [ key ] to [ data ] 
    in table [ tbl ] . 
    Previous bindings for [ key ] are not removed , but simply 
    hidden . That is , after performing { ! remove } [  key ] , 
    the previous binding for [ key ] , if any , is restored . 
    ( Same behavior as with association lists . )
   in table [tbl].
   Previous bindings for [key] are not removed, but simply
   hidden. That is, after performing {!remove}[ tbl key],
   the previous binding for [key], 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_opt : ('a, 'b) t -> 'a -> 'b option
* [ Hashtbl.find_opt tbl x ] returns the current binding of [ x ] in [ tbl ] , 
     or [ None ] if no such binding exists . 
     @since 4.05
    or [None] if no such binding exists.
    @since 4.05 *)

val find_all : ('a, 'b) t -> 'a -> 'b list
* [ Hashtbl.find_all  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 .
   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
* [  tbl x ] checks if [ x ] is bound in [ tbl ] .

val remove : ('a, 'b) t -> 'a -> unit
* [ Hashtbl.remove  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 ] .
   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 key data ] replaces the current binding of [ key ] 
    in [ tbl ] by a binding of [ key ] to [ data ] .   If [ key ] is unbound in [ tbl ] , 
    a binding of [ key ] to [ data ] is added to [ tbl ] . 
    This is functionally equivalent to { ! remove } [  key ] 
    followed by { ! add } [ tbl key data ] .
   in [tbl] by a binding of [key] to [data].  If [key] is unbound in [tbl],
   a binding of [key] to [data] is added to [tbl].
   This is functionally equivalent to {!remove}[ tbl key]
   followed by {!add}[ tbl key data]. *)

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 . 

    The behavior is not specified if the hash table is modified 
    by [ f ] during the iteration . 

   [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.

   The behavior is not specified if the hash table is modified
   by [f] during the iteration.
*)

val filter_map_inplace: ('a -> 'b -> 'b option) -> ('a, 'b) t ->
    unit
* [ Hashtbl.filter_map_inplace f tbl ] applies [ f ] to all bindings in 
     table [ tbl ] and update each binding depending on the result of 
     [ f ] .   If [ f ] returns [ None ] , the binding is discarded .   If it 
     returns [ Some new_val ] , the binding is update to associate the key 
     to [ new_val ] . 

     Other comments for { ! iter } apply as well . 
     @since 4.03.0
    table [tbl] and update each binding depending on the result of
    [f].  If [f] returns [None], the binding is discarded.  If it
    returns [Some new_val], the binding is update to associate the key
    to [new_val].

    Other comments for {!iter} apply as well.
    @since 4.03.0 *)

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 . 

    The behavior is not specified if the hash table is modified 
    by [ f ] during the iteration . 

   [(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.

   The behavior is not specified if the hash table is modified
   by [f] during the iteration.
*)

val length : ('a, 'b) t -> int
* [ Hashtbl.length  ] 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 .
   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 : { ! 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 { ! 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 { ! create } . 
     This is intentional .   Non - randomized hash tables can still be 
     created using [ Hashtbl.create ~random : false ] . 

     @since 4.00.0
    randomized mode by default: {!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 {!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 {!create}.
    This is intentional.  Non-randomized hash tables can still be
    created using [Hashtbl.create ~random:false].

    @since 4.00.0 *)

val is_randomized : unit -> bool
* Return [ true ] if the tables are currently created in randomized mode 
     by default , [ false ] otherwise . 
     @since 4.03.0
    by default, [false] otherwise.
    @since 4.03.0 *)

val rebuild : ?random (* thwart tools/sync_stdlib_docs *) :bool ->
    ('a, 'b) t -> ('a, 'b) t
* Return a copy of the given hashtable .   Unlike { ! copy } , 
     { ! rebuild } [ h ] re - hashes all the ( key , value ) entries of 
     the original table [ h ] .   The returned hash table is randomized if 
     [ h ] was randomized , or the optional [ random ] parameter is true , or 
     if the default is to create randomized hash tables ; see 
     { ! create } for more information . 

     { ! rebuild } can safely be used to import a hash table built 
     by an old version of the { !  } module , then marshaled to 
     persistent storage .   After unmarshaling , apply { ! rebuild } 
     to produce a hash table for the current version of the { !  } 
     module . 

     @since 4.12.0
    {!rebuild}[ h] re-hashes all the (key, value) entries of
    the original table [h].  The returned hash table is randomized if
    [h] was randomized, or the optional [random] parameter is true, or
    if the default is to create randomized hash tables; see
    {!create} for more information.

    {!rebuild} can safely be used to import a hash table built
    by an old version of the {!Hashtbl} module, then marshaled to
    persistent storage.  After unmarshaling, apply {!rebuild}
    to produce a hash table for the current version of the {!Hashtbl}
    module.

    @since 4.12.0 *)

* @since 4.00.0
type statistics = {
  num_bindings: int;
    (** Number of bindings present in the table.
        Same value as returned by {!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  ] returns statistics about the table [ tbl ] : 
    number of buckets , size of the biggest bucket , distribution of 
    buckets by size . 
    @since 4.00.0
   number of buckets, size of the biggest bucket, distribution of
   buckets by size.
   @since 4.00.0 *)

* { 1 Hash tables and Sequences }

val to_seq : ('a,'b) t -> ('a * 'b) Seq.t
* Iterate on the whole table .   The order in which the bindings 
     appear in the sequence is unspecified . However , if the table contains 
     several bindings for the same key , they appear in reversed order of 
     introduction , that is , the most recent binding appears first . 

     The behavior is not specified if the hash table is modified 
     during the iteration . 

     @since 4.07
    appear in the sequence is unspecified. However, if the table contains
    several bindings for the same key, they appear in reversed order of
    introduction, that is, the most recent binding appears first.

    The behavior is not specified if the hash table is modified
    during the iteration.

    @since 4.07 *)

val to_seq_keys : ('a,_) t -> 'a Seq.t
* Same as [ Seq.map fst ( to_seq m ) ] 
     @since 4.07
    @since 4.07 *)

val to_seq_values : (_,'b) t -> 'b Seq.t
* Same as [ Seq.map snd ( to_seq m ) ] 
     @since 4.07
    @since 4.07 *)

val add_seq : ('a,'b) t -> ('a * 'b) Seq.t -> unit
* Add the given bindings to the table , using { ! add } 
     @since 4.07
    @since 4.07 *)

val replace_seq : ('a,'b) t -> ('a * 'b) Seq.t -> unit
* Add the given bindings to the table , using { ! replace } 
     @since 4.07
    @since 4.07 *)

val of_seq : ('a * 'b) Seq.t -> ('a, 'b) t
* Build a table from the given bindings . The bindings are added 
     in the same order they appear in the sequence , using { ! replace_seq } , 
     which means that if two pairs have the same key , only the latest one 
     will appear in the table . 
     @since 4.07
    in the same order they appear in the sequence, using {!replace_seq},
    which means that if two pairs have the same key, only the latest one
    will appear in the table.
    @since 4.07 *)

* { 1 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  = 
         struct 
           type t = int 
           let equal i j = i = j 
           let hash i = i land max_int 
         end 

       module IntHashtbl =  .  ) 

       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 ] . 

    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
    type t
    (** The type of the hashtable keys. *)

    val equal : t -> t -> bool
    (** The equality predicate used to compare keys. *)

    val hash : t -> int
* A hashing function on keys . It must be such that if two keys are 
           equal according to [ equal ] , then they have identical hash values 
           as computed by [ hash ] . 
           Examples : suitable ( [ equal ] , [ hash ] ) pairs for arbitrary key 
           types include 
 -          ( [ ( =) ] , { ! hash } ) for comparing objects by structure 
               ( provided objects do not contain floats ) 
 -          ( [ ( fun x y - > compare x y = 0 ) ] , { ! hash } ) 
               for comparing objects by structure 
               and handling { !  } correctly 
 -          ( [ (= =) ] , { ! hash } ) for comparing objects by physical 
               equality ( e.g. for mutable or cyclic objects ) .
          equal according to [equal], then they have identical hash values
          as computed by [hash].
          Examples: suitable ([equal], [hash]) pairs for arbitrary key
          types include
-         ([(=)], {!hash}) for comparing objects by structure
              (provided objects do not contain floats)
-         ([(fun x y -> compare x y = 0)], {!hash})
              for comparing objects by structure
              and handling {!Stdlib.nan} correctly
-         ([(==)], {!hash}) for comparing objects by physical
              equality (e.g. for mutable or cyclic objects). *)
   end
(** The input signature of the functor {!Make}. *)

module type S =
  sig
    type key
    type !'a t
    val create : int -> 'a t
    val clear : 'a t -> unit
* @since 4.00.0

    val copy : 'a t -> 'a t
    val add : 'a t -> key -> 'a -> unit
    val remove : 'a t -> key -> unit
    val find : 'a t -> key -> 'a
    val find_opt : 'a t -> key -> 'a option
* @since 4.05.0

    val find_all : 'a t -> key -> 'a list
    val replace : 'a t -> key -> 'a -> unit
    val mem : 'a t -> key -> bool
    val iter : (key -> 'a -> unit) -> 'a t -> unit
    val filter_map_inplace: (key -> 'a -> 'a option) -> 'a t ->
      unit
* @since 4.03.0

    val fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
    val length : 'a t -> int
* @since 4.00.0

    val to_seq : 'a t -> (key * 'a) Seq.t
* @since 4.07

    val to_seq_keys : _ t -> key Seq.t
* @since 4.07

    val to_seq_values : 'a t -> 'a Seq.t
* @since 4.07

    val add_seq : 'a t -> (key * 'a) Seq.t -> unit
* @since 4.07

    val replace_seq : 'a t -> (key * 'a) Seq.t -> unit
* @since 4.07

    val of_seq : (key * 'a) Seq.t -> 'a t
* @since 4.07
  end
(** The output signature of the functor {!Make}. *)

module Make (H : HashedType) : S with type key = H.t
* Functor building an implementation of the hashtable structure . 
     The functor [  . 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 .
    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
    type t
    (** The type of the hashtable keys. *)

    val equal: t -> t -> bool
    (** The equality predicate used to compare keys. *)

    val hash: int -> t -> int
* A seeded hashing function on keys .   The first argument is 
           the seed .   It must be the case that if [ equal x y ] is true , 
           then [ hash seed x = hash seed y ] for any value of [ seed ] . 
           A suitable choice for [ hash ] is the function 
           { !  . Hashtbl.seeded_hash } below .
          the seed.  It must be the case that if [equal x y] is true,
          then [hash seed x = hash seed y] for any value of [seed].
          A suitable choice for [hash] is the function
          {!Stdlib.Hashtbl.seeded_hash} below. *)
  end
* The input signature of the functor { ! MakeSeeded } . 
     @since 4.00.0
    @since 4.00.0 *)

module type SeededS =
  sig
    type key
    type !'a t
    val create : ?random (* thwart tools/sync_stdlib_docs *) :bool ->
                 int -> 'a t
    val clear : 'a t -> unit
    val reset : 'a t -> unit
    val copy : 'a t -> 'a t
    val add : 'a t -> key -> 'a -> unit
    val remove : 'a t -> key -> unit
    val find : 'a t -> key -> 'a
* @since 4.05.0

    val find_all : 'a t -> key -> 'a list
    val replace : 'a t -> key -> 'a -> unit
    val mem : 'a t -> key -> bool
    val iter : (key -> 'a -> unit) -> 'a t -> unit
    val filter_map_inplace: (key -> 'a -> 'a option) -> 'a t ->
      unit
* @since 4.03.0

    val fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
    val length : 'a t -> int
    val stats: 'a t -> statistics

    val to_seq : 'a t -> (key * 'a) Seq.t
* @since 4.07

    val to_seq_keys : _ t -> key Seq.t
* @since 4.07

    val to_seq_values : 'a t -> 'a Seq.t
* @since 4.07

    val add_seq : 'a t -> (key * 'a) Seq.t -> unit
* @since 4.07

    val replace_seq : 'a t -> (key * 'a) Seq.t -> unit
* @since 4.07

    val of_seq : (key * 'a) Seq.t -> 'a t
* @since 4.07
  end
* The output signature of the functor { ! MakeSeeded } . 
     @since 4.00.0
    @since 4.00.0 *)

module MakeSeeded (H : SeededHashedType) : SeededS with type key = H.t
* Functor building an implementation of the hashtable structure . 
     The functor [  . 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 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 *)


* { 1 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 [Stdlib.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 { ! hash } that is further parameterized by 
    an integer seed . 
    @since 4.00.0
   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 .   If [ total ] as specified by the user exceeds a certain 
    value , currently 256 , then it is capped to that value . 
    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 , { ! hash } and { ! seeded_hash } take 
    [ meaningful = 10 ] and [ total = 100 ] .
   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.  If [total] as specified by the user exceeds a certain
   value, currently 256, then it is capped to that value.
   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, {!hash} and {!seeded_hash} take
   [meaningful = 10] and [total = 100]. *)

val seeded_hash_param : int -> int -> int -> 'a -> int
* A variant of { ! hash_param } that is further parameterized by 
    an integer seed .   Usage : 
    [ Hashtbl.seeded_hash_param meaningful total seed x ] . 
    @since 4.00.0
   an integer seed.  Usage:
   [Hashtbl.seeded_hash_param meaningful total seed x].
   @since 4.00.0 *)

************************************************************************
                                                                        
                                 OCaml                                  
                                                                        
                                                                        
     en Automatique.                                                    
                                                                        
   All rights reserved.  This file is distributed under the terms of    
   special exception on linking described in the file LICENSE.          
                                                                        
************************************************************************
 NOTE: If this file is hashtbl.mli, do not edit it directly! Instead,
   edit templates/hashtbl.template.mli and run tools/sync_stdlib_docs 
* Hash tables and hash functions.

   Hash tables are hashed association tables, with in-place modification.

* The type of hash tables from type ['a] to type ['b]. 
 thwart tools/sync_stdlib_docs 
* Empty a hash table. Use [reset] instead of [clear] to shrink the
    size of the bucket table to its initial size. 
* Return a copy of the given hashtable. 
* [Hashtbl.find tbl x] returns the current binding of [x] in [tbl],
   or raises [Not_found] if no such binding exists. 
 thwart tools/sync_stdlib_docs 
* Number of bindings present in the table.
        Same value as returned by {!length}. 
* Number of buckets in the table. 
* Maximal number of bindings per bucket. 
* 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]. 
* The type of the hashtable keys. 
* The equality predicate used to compare keys. 
* The input signature of the functor {!Make}. 
* The output signature of the functor {!Make}. 
* The type of the hashtable keys. 
* The equality predicate used to compare keys. 
 thwart tools/sync_stdlib_docs 
* [Hashtbl.hash x] associates a nonnegative integer to any value of
   any type. It is guaranteed that
   if [x = y] or [Stdlib.compare x y = 0], then [hash x = hash y].
   Moreover, [hash] always terminates, even on cyclic structures. 
# 1 "hashtbl.mli"
                , projet Cristal , INRIA Rocquencourt           
    Copyright 1996 Institut National de Recherche en Informatique et     
    the GNU Lesser General Public License version 2.1 , with the          

open! Stdlib




* { 1 Generic interface }


type (!'a, !'b) t

             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 ] set to [ false ] uses a 
    fixed hash function ( { ! 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 ] set to [ true ] uses the seeded 
    hash function { ! 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 { ! fold } 
    or { ! 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 { ! randomize } or by 
    setting the [ R ] flag in the [ OCAMLRUNPARAM ] environment variable . 

    @before 4.00.0 the [ ~][random ] parameter was not present and all 
    hash tables were created in non - randomized mode .
   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] set to [false] uses a
   fixed hash function ({!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] set to [true] uses the seeded
   hash function {!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 {!fold}
   or {!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 {!randomize} or by
   setting the [R] flag in the [OCAMLRUNPARAM] environment variable.

   @before 4.00.0 the [~][random] parameter was not present and all
   hash tables were created in non-randomized mode. *)

val clear : ('a, 'b) t -> unit

val reset : ('a, 'b) t -> unit
* Empty a hash table and shrink the size of the bucket table 
     to its initial size . 
     @since 4.00.0
    to its initial size.
    @since 4.00.0 *)

val copy : ('a, 'b) t -> ('a, 'b) t

val add : ('a, 'b) t -> 'a -> 'b -> unit
* [ Hashtbl.add tbl key data ] adds a binding of [ key ] to [ data ] 
    in table [ tbl ] . 
    Previous bindings for [ key ] are not removed , but simply 
    hidden . That is , after performing { ! remove } [  key ] , 
    the previous binding for [ key ] , if any , is restored . 
    ( Same behavior as with association lists . )
   in table [tbl].
   Previous bindings for [key] are not removed, but simply
   hidden. That is, after performing {!remove}[ tbl key],
   the previous binding for [key], if any, is restored.
   (Same behavior as with association lists.) *)

val find : ('a, 'b) t -> 'a -> 'b

val find_opt : ('a, 'b) t -> 'a -> 'b option
* [ Hashtbl.find_opt tbl x ] returns the current binding of [ x ] in [ tbl ] , 
     or [ None ] if no such binding exists . 
     @since 4.05
    or [None] if no such binding exists.
    @since 4.05 *)

val find_all : ('a, 'b) t -> 'a -> 'b list
* [ Hashtbl.find_all  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 .
   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
* [  tbl x ] checks if [ x ] is bound in [ tbl ] .

val remove : ('a, 'b) t -> 'a -> unit
* [ Hashtbl.remove  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 ] .
   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 key data ] replaces the current binding of [ key ] 
    in [ tbl ] by a binding of [ key ] to [ data ] .   If [ key ] is unbound in [ tbl ] , 
    a binding of [ key ] to [ data ] is added to [ tbl ] . 
    This is functionally equivalent to { ! remove } [  key ] 
    followed by { ! add } [ tbl key data ] .
   in [tbl] by a binding of [key] to [data].  If [key] is unbound in [tbl],
   a binding of [key] to [data] is added to [tbl].
   This is functionally equivalent to {!remove}[ tbl key]
   followed by {!add}[ tbl key data]. *)

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 . 

    The behavior is not specified if the hash table is modified 
    by [ f ] during the iteration . 

   [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.

   The behavior is not specified if the hash table is modified
   by [f] during the iteration.
*)

val filter_map_inplace: ('a -> 'b -> 'b option) -> ('a, 'b) t ->
    unit
* [ Hashtbl.filter_map_inplace f tbl ] applies [ f ] to all bindings in 
     table [ tbl ] and update each binding depending on the result of 
     [ f ] .   If [ f ] returns [ None ] , the binding is discarded .   If it 
     returns [ Some new_val ] , the binding is update to associate the key 
     to [ new_val ] . 

     Other comments for { ! iter } apply as well . 
     @since 4.03.0
    table [tbl] and update each binding depending on the result of
    [f].  If [f] returns [None], the binding is discarded.  If it
    returns [Some new_val], the binding is update to associate the key
    to [new_val].

    Other comments for {!iter} apply as well.
    @since 4.03.0 *)

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 . 

    The behavior is not specified if the hash table is modified 
    by [ f ] during the iteration . 

   [(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.

   The behavior is not specified if the hash table is modified
   by [f] during the iteration.
*)

val length : ('a, 'b) t -> int
* [ Hashtbl.length  ] 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 .
   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 : { ! 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 { ! 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 { ! create } . 
     This is intentional .   Non - randomized hash tables can still be 
     created using [ Hashtbl.create ~random : false ] . 

     @since 4.00.0
    randomized mode by default: {!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 {!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 {!create}.
    This is intentional.  Non-randomized hash tables can still be
    created using [Hashtbl.create ~random:false].

    @since 4.00.0 *)

val is_randomized : unit -> bool
* Return [ true ] if the tables are currently created in randomized mode 
     by default , [ false ] otherwise . 
     @since 4.03.0
    by default, [false] otherwise.
    @since 4.03.0 *)

    ('a, 'b) t -> ('a, 'b) t
* Return a copy of the given hashtable .   Unlike { ! copy } , 
     { ! rebuild } [ h ] re - hashes all the ( key , value ) entries of 
     the original table [ h ] .   The returned hash table is randomized if 
     [ h ] was randomized , or the optional [ random ] parameter is true , or 
     if the default is to create randomized hash tables ; see 
     { ! create } for more information . 

     { ! rebuild } can safely be used to import a hash table built 
     by an old version of the { !  } module , then marshaled to 
     persistent storage .   After unmarshaling , apply { ! rebuild } 
     to produce a hash table for the current version of the { !  } 
     module . 

     @since 4.12.0
    {!rebuild}[ h] re-hashes all the (key, value) entries of
    the original table [h].  The returned hash table is randomized if
    [h] was randomized, or the optional [random] parameter is true, or
    if the default is to create randomized hash tables; see
    {!create} for more information.

    {!rebuild} can safely be used to import a hash table built
    by an old version of the {!Hashtbl} module, then marshaled to
    persistent storage.  After unmarshaling, apply {!rebuild}
    to produce a hash table for the current version of the {!Hashtbl}
    module.

    @since 4.12.0 *)

* @since 4.00.0
type statistics = {
  num_bindings: int;
  num_buckets: int;
  max_bucket_length: int;
  bucket_histogram: int array
}

val stats : ('a, 'b) t -> statistics
* [ Hashtbl.stats  ] returns statistics about the table [ tbl ] : 
    number of buckets , size of the biggest bucket , distribution of 
    buckets by size . 
    @since 4.00.0
   number of buckets, size of the biggest bucket, distribution of
   buckets by size.
   @since 4.00.0 *)

* { 1 Hash tables and Sequences }

val to_seq : ('a,'b) t -> ('a * 'b) Seq.t
* Iterate on the whole table .   The order in which the bindings 
     appear in the sequence is unspecified . However , if the table contains 
     several bindings for the same key , they appear in reversed order of 
     introduction , that is , the most recent binding appears first . 

     The behavior is not specified if the hash table is modified 
     during the iteration . 

     @since 4.07
    appear in the sequence is unspecified. However, if the table contains
    several bindings for the same key, they appear in reversed order of
    introduction, that is, the most recent binding appears first.

    The behavior is not specified if the hash table is modified
    during the iteration.

    @since 4.07 *)

val to_seq_keys : ('a,_) t -> 'a Seq.t
* Same as [ Seq.map fst ( to_seq m ) ] 
     @since 4.07
    @since 4.07 *)

val to_seq_values : (_,'b) t -> 'b Seq.t
* Same as [ Seq.map snd ( to_seq m ) ] 
     @since 4.07
    @since 4.07 *)

val add_seq : ('a,'b) t -> ('a * 'b) Seq.t -> unit
* Add the given bindings to the table , using { ! add } 
     @since 4.07
    @since 4.07 *)

val replace_seq : ('a,'b) t -> ('a * 'b) Seq.t -> unit
* Add the given bindings to the table , using { ! replace } 
     @since 4.07
    @since 4.07 *)

val of_seq : ('a * 'b) Seq.t -> ('a, 'b) t
* Build a table from the given bindings . The bindings are added 
     in the same order they appear in the sequence , using { ! replace_seq } , 
     which means that if two pairs have the same key , only the latest one 
     will appear in the table . 
     @since 4.07
    in the same order they appear in the sequence, using {!replace_seq},
    which means that if two pairs have the same key, only the latest one
    will appear in the table.
    @since 4.07 *)

* { 1 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  = 
         struct 
           type t = int 
           let equal i j = i = j 
           let hash i = i land max_int 
         end 

       module IntHashtbl =  .  ) 

       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 ] . 

    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
    type t

    val equal : t -> t -> bool

    val hash : t -> int
* A hashing function on keys . It must be such that if two keys are 
           equal according to [ equal ] , then they have identical hash values 
           as computed by [ hash ] . 
           Examples : suitable ( [ equal ] , [ hash ] ) pairs for arbitrary key 
           types include 
 -          ( [ ( =) ] , { ! hash } ) for comparing objects by structure 
               ( provided objects do not contain floats ) 
 -          ( [ ( fun x y - > compare x y = 0 ) ] , { ! hash } ) 
               for comparing objects by structure 
               and handling { !  } correctly 
 -          ( [ (= =) ] , { ! hash } ) for comparing objects by physical 
               equality ( e.g. for mutable or cyclic objects ) .
          equal according to [equal], then they have identical hash values
          as computed by [hash].
          Examples: suitable ([equal], [hash]) pairs for arbitrary key
          types include
-         ([(=)], {!hash}) for comparing objects by structure
              (provided objects do not contain floats)
-         ([(fun x y -> compare x y = 0)], {!hash})
              for comparing objects by structure
              and handling {!Stdlib.nan} correctly
-         ([(==)], {!hash}) for comparing objects by physical
              equality (e.g. for mutable or cyclic objects). *)
   end

module type S =
  sig
    type key
    type !'a t
    val create : int -> 'a t
    val clear : 'a t -> unit
* @since 4.00.0

    val copy : 'a t -> 'a t
    val add : 'a t -> key -> 'a -> unit
    val remove : 'a t -> key -> unit
    val find : 'a t -> key -> 'a
    val find_opt : 'a t -> key -> 'a option
* @since 4.05.0

    val find_all : 'a t -> key -> 'a list
    val replace : 'a t -> key -> 'a -> unit
    val mem : 'a t -> key -> bool
    val iter : (key -> 'a -> unit) -> 'a t -> unit
    val filter_map_inplace: (key -> 'a -> 'a option) -> 'a t ->
      unit
* @since 4.03.0

    val fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
    val length : 'a t -> int
* @since 4.00.0

    val to_seq : 'a t -> (key * 'a) Seq.t
* @since 4.07

    val to_seq_keys : _ t -> key Seq.t
* @since 4.07

    val to_seq_values : 'a t -> 'a Seq.t
* @since 4.07

    val add_seq : 'a t -> (key * 'a) Seq.t -> unit
* @since 4.07

    val replace_seq : 'a t -> (key * 'a) Seq.t -> unit
* @since 4.07

    val of_seq : (key * 'a) Seq.t -> 'a t
* @since 4.07
  end

module Make (H : HashedType) : S with type key = H.t
* Functor building an implementation of the hashtable structure . 
     The functor [  . 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 .
    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
    type t

    val equal: t -> t -> bool

    val hash: int -> t -> int
* A seeded hashing function on keys .   The first argument is 
           the seed .   It must be the case that if [ equal x y ] is true , 
           then [ hash seed x = hash seed y ] for any value of [ seed ] . 
           A suitable choice for [ hash ] is the function 
           { !  . Hashtbl.seeded_hash } below .
          the seed.  It must be the case that if [equal x y] is true,
          then [hash seed x = hash seed y] for any value of [seed].
          A suitable choice for [hash] is the function
          {!Stdlib.Hashtbl.seeded_hash} below. *)
  end
* The input signature of the functor { ! MakeSeeded } . 
     @since 4.00.0
    @since 4.00.0 *)

module type SeededS =
  sig
    type key
    type !'a t
                 int -> 'a t
    val clear : 'a t -> unit
    val reset : 'a t -> unit
    val copy : 'a t -> 'a t
    val add : 'a t -> key -> 'a -> unit
    val remove : 'a t -> key -> unit
    val find : 'a t -> key -> 'a
* @since 4.05.0

    val find_all : 'a t -> key -> 'a list
    val replace : 'a t -> key -> 'a -> unit
    val mem : 'a t -> key -> bool
    val iter : (key -> 'a -> unit) -> 'a t -> unit
    val filter_map_inplace: (key -> 'a -> 'a option) -> 'a t ->
      unit
* @since 4.03.0

    val fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b
    val length : 'a t -> int
    val stats: 'a t -> statistics

    val to_seq : 'a t -> (key * 'a) Seq.t
* @since 4.07

    val to_seq_keys : _ t -> key Seq.t
* @since 4.07

    val to_seq_values : 'a t -> 'a Seq.t
* @since 4.07

    val add_seq : 'a t -> (key * 'a) Seq.t -> unit
* @since 4.07

    val replace_seq : 'a t -> (key * 'a) Seq.t -> unit
* @since 4.07

    val of_seq : (key * 'a) Seq.t -> 'a t
* @since 4.07
  end
* The output signature of the functor { ! MakeSeeded } . 
     @since 4.00.0
    @since 4.00.0 *)

module MakeSeeded (H : SeededHashedType) : SeededS with type key = H.t
* Functor building an implementation of the hashtable structure . 
     The functor [  . 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 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 *)


* { 1 The polymorphic hash functions }


val hash : 'a -> int

val seeded_hash : int -> 'a -> int
* A variant of { ! hash } that is further parameterized by 
    an integer seed . 
    @since 4.00.0
   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 .   If [ total ] as specified by the user exceeds a certain 
    value , currently 256 , then it is capped to that value . 
    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 , { ! hash } and { ! seeded_hash } take 
    [ meaningful = 10 ] and [ total = 100 ] .
   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.  If [total] as specified by the user exceeds a certain
   value, currently 256, then it is capped to that value.
   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, {!hash} and {!seeded_hash} take
   [meaningful = 10] and [total = 100]. *)

val seeded_hash_param : int -> int -> int -> 'a -> int
* A variant of { ! hash_param } that is further parameterized by 
    an integer seed .   Usage : 
    [ Hashtbl.seeded_hash_param meaningful total seed x ] . 
    @since 4.00.0
   an integer seed.  Usage:
   [Hashtbl.seeded_hash_param meaningful total seed x].
   @since 4.00.0 *)
