(* This file is free software, part of containers. See file "license" for more details. *)

(** {1 Caches}

    Particularly useful for memoization. See {!with_cache} and {!with_cache_rec}
    for more details.
    @since 0.6 *)

type 'a equal = 'a -> 'a -> bool
type 'a hash = 'a -> int

(** {2 Value interface}

    Typical use case: one wants to memoize a function [f : 'a -> 'b]. Code sample:
    {[
      let f x =
        print_endline "call f";
        x + 1;;

      let f' = with_cache (lru 256) f;;
      f' 0;;  (* prints *)
      f' 1;;  (* prints *)
      f' 0;;  (* doesn't print, returns cached value *)
    ]}

    @since 0.6 *)

type ('a, 'b) t

val clear : (_,_) t -> unit
(** Clear the content of the cache. *)

type ('a, 'b) callback = in_cache:bool -> 'a -> 'b -> unit
(** Type of the callback that is called once a cached value is found
    or not.
    Should never raise.
    @param in_cache is [true] if the value was in cache, [false]
      if the value was just produced.
    @since 1.3 *)

val with_cache : ?cb:('a, 'b) callback -> ('a, 'b) t -> ('a -> 'b) -> 'a -> 'b
(** [with_cache c f] behaves like [f], but caches calls to [f] in the
    cache [c]. It always returns the same value as
    [f x], if [f x] returns, or raise the same exception.
    However, [f] may not be called if [x] is in the cache.
    @param cb called after the value is generated or retrieved. *)

val with_cache_rec : ?cb:('a, 'b) callback -> ('a,'b) t -> (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b
(** [with_cache_rec c f] is a function that first, applies [f] to
    some [f' = fix f], such that recursive calls to [f'] are cached in [c].
    It is similar to {!with_cache} but with a function that takes as
    first argument its own recursive version.
    Example (memoized Fibonacci function):
    {[
      let fib = with_cache_rec (lru 256)
          (fun fib' n -> match n with
             | 1 | 2 -> 1
             | _ -> fib' (n-1) + fib' (n-2)
          );;

      fib 70;;
    ]}
    @param cb called after the value is generated or retrieved.
*)

val size : (_,_) t -> int
(** Size of the cache (number of entries). At most linear in the number
    of entries. *)

val iter : ('a,'b) t -> ('a -> 'b -> unit) -> unit
(** Iterate on cached values. Should yield [size cache] pairs. *)

val add : ('a, 'b) t -> 'a -> 'b -> bool
(** Manually add a cached value. Return [true] if the value has successfully
    been added, and [false] if the value was already bound.
    @since 1.5 *)

val dummy : ('a,'b) t
(** Dummy cache, never stores any value. *)

val linear : eq:'a equal -> int -> ('a, 'b) t
(** Linear cache with the given size. It stores key/value pairs in
    an array and does linear search at every call, so it should only be used
    with small size.
    @param eq optional equality predicate for keys. *)

val replacing : eq:'a equal -> ?hash:'a hash ->
  int -> ('a,'b) t
(** Replacing cache of the given size. Equality and hash functions can be
    parametrized. It's a hash table that handles collisions by replacing
    the old value with the new (so a cache entry is evicted when another
    entry with the same hash (modulo size) is added).
    Never grows wider than the given size. *)

val lru : eq:'a equal -> ?hash:'a hash ->
  int -> ('a,'b) t
(** LRU cache of the given size ("Least Recently Used": keys that have not been
    used recently are deleted first). Never grows wider than the given size. *)

val unbounded : eq:'a equal -> ?hash:'a hash ->
  int -> ('a,'b) t
(** Unbounded cache, backed by a Hash table. Will grow forever
    unless {!clear} is called manually. *)