Source file decompress_q.ml

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

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   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
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   WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
   DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
   FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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   CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. *)

type 'a digit =
  | Zero
  | One of 'a
  | Two of 'a * 'a
  | Three of 'a * 'a * 'a

type 'a t =
  | Shallow of 'a digit
  | Deep of int * 'a digit * ('a * 'a) t Lazy.t * 'a digit

let empty = Shallow Zero

let is_empty = function Shallow Zero -> true | _ -> false

let _single x = Shallow (One x)
let _double x y = Shallow (Two (x, y))
let _three x y z = Shallow (Three (x, y, z))
let _deep n hd middle tl =
  assert (hd <> Zero && tl <> Zero);
  Deep (n, hd, middle, tl)
let _empty = Lazy.from_val empty

let rec cons
  : 'a. 'a -> 'a t -> 'a t
  = fun x q -> match q with
    | Shallow Zero -> _single x
    | Shallow (One y) -> _double x y
    | Shallow (Two (y, z)) -> _three x y z
    | Shallow (Three (y, z, z')) ->
      _deep 4 (Two (x, y)) _empty (Two (z, z'))
    | Deep (_, Zero, _middle, _tl) -> assert false
    | Deep (n, One y, _middle, _tl) ->
      _deep (n + 1) (Two (x, y)) _middle _tl
    | Deep (n, Two (y, z), _middle, _tl) ->
      _deep (n + 1) (Three (x, y, z)) _middle _tl
    | Deep (n, Three (y, z, z'), lazy _middle, _tl) ->
      _deep (n + 1) (Two (x, y)) (lazy (cons (z, z') _middle)) _tl

exception Empty

let rec take_front_exn
  : 'a. 'a t -> ('a * 'a t)
  = fun q -> match q with
    | Shallow Zero -> raise Empty
    | Shallow (One x) -> x, empty
    | Shallow (Two (x, y)) -> x, Shallow (One y)
    | Shallow (Three (x, y, z)) -> x, Shallow (Two (y, z))
    | Deep (_, Zero, _, _) -> assert false
    | Deep (n, One x, lazy _middle, _tail) ->
      if is_empty _middle
      then x, Shallow _tail
      else
        let (y, z), _middle = take_front_exn _middle in
        x, _deep (n - 1) (Two (y, z)) (Lazy.from_val _middle) _tail
    | Deep (n, Two (x, y), _middle, _tail) ->
      x, _deep (n - 1) (One y) _middle _tail
    | Deep (n, Three (x, y, z), _middle, _tail) ->
      x, _deep (n - 1) (Two (y, z)) _middle _tail

let take_front q =
  try Some (take_front_exn q)
  with Empty -> None

let add_seq_front seq q =
  let l = ref [] in
  seq (fun x -> l := x :: !l);
  List.fold_left (fun q x -> cons x q) q !l

let of_seq seq = add_seq_front seq empty