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Hash table.

The interface provides a subset of the OCaml `Stdlib.Hashtbl`

module with some changes:

- The functorial interface of the
`Stdlib.Hashtbl`

is not provided. Instead the constructor functions,`create`

,`of_seq`

, and`rebuild`

, take an optional`HashedType`

module as an argument. By default`create`

returns a randomized hash table. - The
`add_seq`

and`replace_seq`

operations are not provided at all. - Non-instance specific operations related to randomization (e.g.
`randomize`

,`is_randomized`

) are not provided. - Non-instance specific operations related to hashing (e.g.
`hash`

,`seeded_hash`

,`hash_param`

,`seeded_hash_param`

) are not provided.

Compositional versions of `find`

, `to_seq`

, `to_seq_keys`

, `to_seq_values`

, `rebuild`

, `copy`

, `iter`

, `filter_map_inplace`

, `fold`

, and `stats`

are not provided.

Please note that the design is intentionally based on `Stdlib.Hashtbl`

and copies its semantics as accurately as possible. Some of the operations come with warnings.

The hash table implementation is designed to avoid starvation. Read-only accesses can generally proceed in parallel without interference. Write accesses that do not change the number of bindings can proceed in parallel as long as they hit different internal buckets. Write accesses that change the number of bindings use a scalable `Accumulator`

and only make infrequent random checks to determine whether the hash table should be resized.

`type 'k hashed_type = (module Hashtbl.HashedType with type t = 'k)`

First-class `HashedType`

module type abbreviation.

```
val create :
?hashed_type:'k hashed_type ->
?min_buckets:int ->
?max_buckets:int ->
unit ->
('k, 'v) t
```

`create ()`

returns a new empty hash table.

- The default
`hash`

is computed as`Stdlib.Hashtbl.hash (Random.bits ())`

. - The default
`equal`

is`(=)`

. - The default
`min_buckets`

is unspecified and a given`min_buckets`

may be adjusted by the implementation. - The default
`max_buckets`

is the minimum of`1 lsl 30`

and suitably adjusted`Sys.max_array_length`

and a given`max_buckets`

may be adjusted by the implementation.

Hash tables are automatically internally resized.

`val hashed_type_of : ('k, 'v) t -> 'k hashed_type`

`hashed_type_of t`

returns a copy of the hashed type used when the hash table `t`

was `create`

d.

`val min_buckets_of : ('k, 'v) t -> int`

`min_buckets_of t`

returns the minimum number of buckets of the hash table `t`

.

**NOTE**: The returned value may not be the same as given to `create`

.

`val max_buckets_of : ('k, 'v) t -> int`

`max_buckets_of t`

returns the maximum number of buckets of the hash table `t`

.

**NOTE**: The returned value may not be the same as given to `create`

.

```
val of_seq :
?hashed_type:'k hashed_type ->
?min_buckets:int ->
?max_buckets:int ->
('k * 'v) Seq.t ->
('k, 'v) t
```

`of_seq assoc`

creates a new hash table from the given association sequence `assoc`

. The associations are added in the same order as they appear in the sequence, using `replace`

, which means that if two pairs have the same key, only the latest one will appear in the table. See `create`

for the optional arguments.

⚠️ `of_seq (to_seq t)`

does not necessarily copy the bindings of a hash table correctly.

`module Xt : sig ... end`

Explicit transaction log passing on hash tables.

`val length : ('k, 'v) t -> int`

`length t`

returns the number of *bindings* in the hash table `t`

.

⚠️ The returned value may be greater than the number of *distinct keys* in the hash table.

`val reset : ('k, 'v) t -> unit`

`reset t`

remove all bindings from the hash table `t`

and shrinks the capacity of the table back to the minimum.

`swap t1 t2`

exchanges the contents of the hash tables `t1`

and `t2`

.

`val remove : ('k, 'v) t -> 'k -> unit`

`remove t k`

removes the most recent existing binding of key `k`

, if any, from the hash table `t`

thereby revealing the earlier binding of `k`

, if any.

`val add : ('k, 'v) t -> 'k -> 'v -> unit`

`add t k v`

adds a binding of key `k`

to value `v`

to the hash table shadowing the previous binding of the key `k`

, if any.

⚠️ Consider using `replace`

instead of `add`

.

`val replace : ('k, 'v) t -> 'k -> 'v -> unit`

`replace t k v`

adds a binding of key `k`

to value `v`

or replaces the most recent existing binding of key `k`

in the hash table `t`

.

`val mem : ('k, 'v) t -> 'k -> bool`

`mem t k`

is equivalent to `Option.is_some (find_opt t k)`

.

`val find_opt : ('k, 'v) t -> 'k -> 'v option`

`find_opt t k`

returns the current binding of key `k`

in the hash table `t`

, or `None`

if no such binding exists.

`val find_all : ('k, 'v) t -> 'k -> 'v list`

`find_all t k`

returns a list of all the bindings of the key `k`

in the hash table in reverse order of their introduction.

`val find : ('k, 'v) t -> 'k -> 'v`

`find t k`

returns the current binding of `k`

in hash table `t`

, or raises `Not_found`

if no such binding exists.

`to_seq t`

takes a snapshot of the keys and values in the hash table and returns them as an association sequence. Bindings of each individual key appear in the sequence in reverse order of their introduction.

⚠️ `of_seq (to_seq t)`

does not necessarily copy the bindings of a hash table correctly.

`to_seq_keys t`

is equivalent to `to_seq t |> Seq.map fst`

.

⚠️ The sequence may include duplicates.

`to_seq_values t`

is equivalent to `to_seq t |> Seq.map snd`

.

⚠️ The sequence may include values of bindings that are hidden.

```
val rebuild :
?hashed_type:'k hashed_type ->
?min_buckets:int ->
?max_buckets:int ->
('k, 'v) t ->
('k, 'v) t
```

`copy t`

is equivalent to `rebuild t`

. In other words, the returned hash table uses the same `hashed_type`

(and other parameters) as the given hash table `t`

.

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

`iter f t`

is equivalent to `Seq.iter (fun (k, v) -> f k v) (to_seq t)`

.

`val filter_map_inplace : ('k -> 'v -> 'v option) -> ('k, 'v) t -> unit`

`filter_map_inplace f t`

applies `f`

to all bindings in the hash table `t`

and updates each binding depending on the result of `f`

. If `f`

returns `None`

, the binding is discarded. If `f`

returns `Some new_value`

, the binding is updated to associate the key to the `new_value`

.

⚠️ The given `f`

may be called multiple times for the same bindings from multiple domains in parallel.

`val fold : ('a -> 'b -> 'c -> 'c) -> ('a, 'b) t -> 'c -> 'c`

`fold f t a`

is equivalent to `Seq.fold_left (fun a (k, v) -> f k v a) a (to_seq t)`

.

`val stats : ('a, 'b) t -> Hashtbl.statistics`

`stats t`

returns statistics about the hash table `t`

.

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