module Hashtbl:`sig`

..`end`

Hash tables and hash functions.

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

`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.

**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

`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.

The behavior is not defined 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 `Hashtbl.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 defined if the hash table is modified
by `f`

during the iteration.

`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

`val is_randomized : ``unit -> bool`

return if the tables are currently created in randomized mode by default

**Since** 4.02.0

`type `

statistics = {

` ` |
`num_bindings : ` |
`(*` |
Number of bindings present in the table.
Same value as returned by
`Hashtbl.length` . | `*)` |

` ` |
`num_buckets : ` |
`(*` |
Number of buckets in the table.
| `*)` |

` ` |
`max_bucket_length : ` |
`(*` |
Maximal number of bindings per bucket.
| `*)` |

` ` |
`bucket_histogram : ` |
`(*` |
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.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`

of tables from

IntHashtbl.t`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 building an implementation of the hashtable structure.

module type SeededHashedType =`sig`

..`end`

The input signature of the functor

`Hashtbl.MakeSeeded`

.
module type SeededS =`sig`

..`end`

The output signature of the functor

`Hashtbl.MakeSeeded`

.
module MakeSeeded:

Functor building an implementation of the hashtable structure.

`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`

`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, `Hashtbl.hash`

and `Hashtbl.seeded_hash`

take
`meaningful = 10`

and `total = 100`

.`val seeded_hash_param : ``int -> int -> int -> 'a -> int`

A variant of

**Since** 4.00.0

`Hashtbl.hash_param`

that is further parameterized by
an integer seed. Usage:
`Hashtbl.seeded_hash_param meaningful total seed x`

.