Source file `heap.ml`

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
(************************************************************************)
(*         *   The Coq Proof Assistant / The Coq Development Team       *)
(*  v      *         Copyright INRIA, CNRS and contributors             *)
(* <O___,, * (see version control and CREDITS file for authors & dates) *)
(*   \VV/  **************************************************************)
(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
(************************************************************************)

(*s Heaps *)

module type Ordered = sig
  type t
  val compare : t -> t -> int
end

module type S =sig

  (* Type of functional heaps *)
  type t

  (* Type of elements *)
  type elt

  (* The empty heap *)
  val empty : t

  (* [add x h] returns a new heap containing the elements of [h], plus [x];
     complexity $O(log(n))$ *)
  val add : elt -> t -> t

  (* [maximum h] returns the maximum element of [h]; raises [EmptyHeap]
     when [h] is empty; complexity $O(1)$ *)
  val maximum : t -> elt

  (* [remove h] returns a new heap containing the elements of [h], except
     the maximum of [h]; raises [EmptyHeap] when [h] is empty;
     complexity $O(log(n))$ *)
  val remove : t -> t

  (* usual iterators and combinators; elements are presented in
     arbitrary order *)
  val iter : (elt -> unit) -> t -> unit

  val fold : (elt -> 'a -> 'a) -> t -> 'a -> 'a

end

exception EmptyHeap

(*s Functional implementation *)

module Functional(X : Ordered) = struct

  (* Heaps are encoded as Braun trees, that are binary trees
     where size r <= size l <= size r + 1 for each node Node (l, x, r) *)

  type t =
    | Leaf
    | Node of t * X.t * t

  type elt = X.t

  let empty = Leaf

  let rec add x = function
    | Leaf ->
        Node (Leaf, x, Leaf)
    | Node (l, y, r) ->
        if X.compare x y >= 0 then
          Node (add y r, x, l)
        else
          Node (add x r, y, l)

  let rec extract = function
    | Leaf ->
        assert false
    | Node (Leaf, y, r) ->
        assert (r = Leaf);
        y, Leaf
    | Node (l, y, r) ->
        let x, l = extract l in
        x, Node (r, y, l)

  let is_above x = function
    | Leaf -> true
    | Node (_, y, _) -> X.compare x y >= 0

  let rec replace_min x = function
    | Node (l, _, r) when is_above x l && is_above x r ->
        Node (l, x, r)
    | Node ((Node (_, lx, _) as l), _, r) when is_above lx r ->
        (* lx <= x, rx necessarily *)
        Node (replace_min x l, lx, r)
    | Node (l, _, (Node (_, rx, _) as r)) ->
        (* rx <= x, lx necessarily *)
        Node (l, rx, replace_min x r)
    | Leaf | Node (Leaf, _, _) | Node (_, _, Leaf) ->
        assert false

  (* merges two Braun trees [l] and [r],
     with the assumption that [size r <= size l <= size r + 1] *)
  let rec merge l r = match l, r with
    | _, Leaf ->
        l
    | Node (ll, lx, lr), Node (_, ly, _) ->
        if X.compare lx ly >= 0 then
          Node (r, lx, merge ll lr)
        else
          let x, l = extract l in
          Node (replace_min x r, ly, l)
    | Leaf, _ ->
        assert false (* contradicts the assumption *)

  let maximum = function
    | Leaf -> raise EmptyHeap
    | Node (_, x, _) -> x

  let remove = function
    | Leaf ->
        raise EmptyHeap
    | Node (l, _, r) ->
        merge l r

  let rec iter f = function
    | Leaf -> ()
    | Node (l, x, r) -> iter f l; f x; iter f r

  let rec fold f h x0 = match h with
    | Leaf ->
        x0
    | Node (l, x, r) ->
        fold f l (fold f r (f x x0))

end