`module Hashtbl: sig .. end`

Hash tables and hash functions.

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

```
type t('a, 'b);
```

The type of hash tables from type

`'a`

to type `'b`

.```
let create: (~random: bool=?, int) => t('a, 'b);
```

`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.

```
let clear: t('a, 'b) => unit;
```

Empty a hash table. Use

`reset`

instead of `clear`

to shrink the
size of the bucket table to its initial size.```
let reset: t('a, 'b) => unit;
```

Empty a hash table and shrink the size of the bucket table
to its initial size.

**Since** 4.00.0

```
let copy: t('a, 'b) => t('a, 'b);
```

Return a copy of the given hashtable.

```
let add: (t('a, 'b), '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.)```
let find: (t('a, 'b), 'a) => 'b;
```

`Hashtbl.find tbl x`

returns the current binding of `x`

in `tbl`

,
or raises `Not_found`

if no such binding exists.```
let find_all: (t('a, 'b), 'a) => list('b);
```

`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.```
let mem: (t('a, 'b), 'a) => bool;
```

`Hashtbl.mem tbl x`

checks if `x`

is bound in `tbl`

.```
let remove: (t('a, 'b), '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`

.```
let replace: (t('a, 'b), '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`

.```
let iter: (('a, 'b) => unit, t('a, 'b)) => 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.

```
let fold: (('a, 'b, 'c) => 'c, t('a, 'b), '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.

```
let length: t('a, 'b) => 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.```
let randomize: unit => unit;
```

After a call to

`Hashtbl.randomize()`

, hash tables are created in
randomized mode by default: `Hashtbl.create`

returns randomized
hash tables, unless the `~random:false`

optional parameter is given.
The same effect can be achieved by setting the `R`

parameter in
the `OCAMLRUNPARAM`

environment variable.
It is recommended that applications or Web frameworks that need to
protect themselves against the denial-of-service attack described
in `Hashtbl.create`

call `Hashtbl.randomize()`

at initialization
time.

Note that once `Hashtbl.randomize()`

was called, there is no way
to revert to the non-randomized default behavior of `Hashtbl.create`

.
This is intentional. Non-randomized hash tables can still be
created using `Hashtbl.create ~random:false`

.

**Since** 4.00.0

`type statistics = {`

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

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

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

` ` |
`bucket_histogram : int array;` |
`(*` |
Histogram of bucket sizes. This array
`histo` has
length `max_bucket_length + 1` . The value of
`histo.(i)` is the number of buckets whose size is `i` . | `*)` |

```
let stats: t('a, 'b) => 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
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: (H: HashedType) => S with type key = H.t;
```

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: (H: SeededHashedType) => SeededS with type key = H.t;
```

Functor building an implementation of the hashtable structure.

```
let 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.```
let seeded_hash: (int, 'a) => int;
```

```
let 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`

.```
let 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`

.