ocaml-containers/pHashtbl.ml

(*
Copyright (c) 2013, Simon Cruanes
All rights reserved.

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*)

(** {1 Open addressing hashtable, with linear probing} *)

type ('a, 'b) t = {
  mutable buckets : ('a * 'b * state) array;
  mutable size : int;
  eq : 'a -> 'a -> bool;
  hash : 'a -> int;
  max_load : float;
} (** A hashtable is an array of (key, value) buckets that have a state,
      plus the size of the table and equality/hash functions *)
and state = Used | Empty | Deleted
  (** state of a bucket *)

let my_null () = (Obj.magic None, Obj.magic None, Empty)

let my_deleted key = (key, Obj.magic None, Deleted)

(** Create a table. Size will be >= 2 *)
let create ?(max_load=0.8) ?(eq=fun x y -> x = y)
           ?(hash=fun x -> Hashtbl.hash x) size =
  let size = max 2 size in
  let null = my_null () in
  { buckets = Array.make size null;
    size = 0;
    max_load;
    eq;
    hash; }

(** Copy of the hashtable *)
let copy t = {
  eq = t.eq;
  hash = t.hash;
  max_load = t.max_load;
  size = t.size;
  buckets = Array.copy t.buckets;
}

(** clear the table, by resetting all states to Empty *)
let clear t =
  let null = my_null () in
  Array.fill t.buckets 0 (Array.length t.buckets) null;
  t.size <- 0

(** Index of slot, for i-th probing starting from hash [h] in
    a table of length [n] *)
let addr h n i = (h + i) mod n
  
(** Insert (key -> value) in buckets, starting with the hash. *)
let insert ~eq buckets h key value =
  let n = Array.length buckets in
  (* lookup an empty slot to insert the key->value in. *)
  let rec lookup h n i =
    let j = addr h n i in
    match buckets.(j) with
    | (_, _, Empty) -> buckets.(j) <- (key, value, Used)
    | (key', _, _) when eq key key' -> ()
    | _ -> lookup h n (i+1)
  in
  lookup h n 0

(** Resize the array, by inserting its content into twice as large an array *)
let resize ~eq ~hash buckets =
  let buckets' = Array.make (Array.length buckets * 2) (my_null ()) in
  for i = 0 to Array.length buckets - 1 do
    match buckets.(i) with
    | (key, value, Used) ->
      insert ~eq buckets' (hash key) key value  (* insert key -> value into new array *)
    | _ -> ()
  done;
  buckets'

(** Lookup [key] in the table *)
let find t key =
  let n = Array.length t.buckets in
  let h = t.hash key in
  let buckets = t.buckets in
  let rec probe h n i num =
    if num = n then raise Not_found
    else
    let j = addr h n i in
    match buckets.(j) with
    | (key', value, Used) when t.eq key key' ->
      value  (* found value for this key *)
    | (_, _, Deleted) | (_, _, Used) ->
      probe h n (i+1) (num + 1) (* try next bucket *)
    | (_, _, Empty) -> raise Not_found
  in
  probe h n 0 0

(** put [key] -> [value] in the hashtable *)
let replace t key value =
  let load = float_of_int t.size /. float_of_int (Array.length t.buckets) in
  (if load > t.max_load then t.buckets <- resize ~eq:t.eq ~hash:t.hash t.buckets);
  let n = Array.length t.buckets in
  let h = t.hash key in
  let buckets = t.buckets in
  let rec probe h n i =
    let j = addr h n i in
    match buckets.(j) with
    | (key', _, Used) when t.eq key key' ->
      buckets.(j) <- (key, value, Used)  (* replace value *)
    | (_, _, Deleted) | (_, _, Empty) ->
      buckets.(j) <- (key, value, Used);
      t.size <- t.size + 1 (* insert and increment size *)
    | (_, _, Used) ->
      probe h n (i+1) (* go further *)
  in
  probe h n 0

(** alias for replace *)
let add t key value = replace t key value

(** Remove the key from the table *)
let remove t key =
  let n = Array.length t.buckets in
  let h = t.hash key in
  let buckets = t.buckets in
  let rec probe h n i =
    let j = addr h n i in
    match buckets.(j) with
    | (key', _, Used) when t.eq key key' ->
      buckets.(j) <- (my_deleted key'); t.size <- t.size - 1  (* remove slot *)
    | (_, _, Deleted) | (_, _, Used) ->
      probe h n (i+1) (* search further *)
    | (_, _, Empty) -> ()  (* not present *)
  in
  probe h n 0

(** size of the table *)
let length t = t.size

(** Is the key member of the table? *)
let mem t key =
  try ignore (find t key); true
  with Not_found -> false

(** Iterate on key -> value pairs *)
let iter k t =
  let buckets = t.buckets in
  for i = 0 to Array.length buckets - 1 do
    match buckets.(i) with
    | (key, value, Used) -> k key value
    | _ -> ()
  done

(** Fold on key -> value pairs *)
let fold f acc t =
  let acc = ref acc in
  let buckets = t.buckets in
  for i = 0 to Array.length buckets - 1 do
    match buckets.(i) with
    | (key, value, Used) -> acc := f !acc key value
    | _ -> ()
  done;
  !acc

(** Add the given pairs to the hashtable *)
let of_seq t seq =
  Sequence.iter (fun (k,v) -> add t k v) seq

(** Sequence of pairs *)
let to_seq t =
  Sequence.from_iter
    (fun kont -> iter (fun k v -> kont (k,v)) t)

(** Statistics on the table *)
let stats t = (Array.length t.buckets, t.size, t.size, 0, 0, 1)