summaryrefslogtreecommitdiff
path: root/Test/dafny1/PriorityQueue.dfy
blob: 6e19ab8f9db4079076eb1893a0c9b98e41b748d6 (plain)
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
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
class PriorityQueue {
  var N: int;  // capacity
  var n: int;  // current size
  ghost var Repr: set<object>;  // set of objects that make up the representation of a PriorityQueue

  var a: array<int>;  // private implementation of PriorityQueue

  function Valid(): bool
    reads this, Repr;
  {
    MostlyValid() &&
    (forall j :: 2 <= j && j <= n ==> a[j/2] <= a[j])
  }

  function MostlyValid(): bool
    reads this, Repr;
  {
    this in Repr && a in Repr &&
    a != null && a.Length == N+1 &&
    0 <= n && n <= N
  }

  method Init(capacity: int)
    requires 0 <= capacity;
    modifies this;
    ensures Valid() && fresh(Repr - {this});
    ensures N == capacity;
  {
    N := capacity;
    a := new int[N+1];
    n := 0;
    Repr := {this};
    Repr := Repr + {a};
  }

  method Insert(x: int)
    requires Valid() && n < N;
    modifies this, a;
    ensures Valid() && fresh(Repr - old(Repr));
    ensures n == old(n) + 1 && N == old(N);
  {
    n := n + 1;
    a[n] := x;
    SiftUp(n);
  }

  method SiftUp(k: int)
    requires 1 <= k && k <= n;
    requires MostlyValid();
    requires (forall j :: 2 <= j && j <= n && j != k ==> a[j/2] <= a[j]);
    requires (forall j :: 1 <= j && j <= n ==> j/2 != k);  // k is a leaf
    modifies a;
    ensures Valid();
  {
    var i := k;
    assert MostlyValid();
    while (1 < i)
      invariant i <= k && MostlyValid();
      invariant (forall j :: 2 <= j && j <= n && j != i ==> a[j/2] <= a[j]);
      invariant (forall j :: 1 <= j/2/2 && j/2 == i && j <= n ==> a[j/2/2] <= a[j]);
    {
      if (a[i/2] <= a[i]) {
        return;
      }
      var tmp := a[i];  a[i] := a[i/2];  a[i/2] := tmp;
      i := i / 2;
    }
  }

  method RemoveMin() returns (x: int)
    requires Valid() && 1 <= n;
    modifies this, a;
    ensures Valid() && fresh(Repr - old(Repr));
    ensures n == old(n) - 1;
  {
    x := a[1];
    a[1] := a[n];
    n := n - 1;
    SiftDown(1);
  }

  method SiftDown(k: int)
    requires 1 <= k;
    requires MostlyValid();
    requires (forall j :: 2 <= j && j <= n && j/2 != k ==> a[j/2] <= a[j]);
    requires (forall j :: 2 <= j && j <= n && 1 <= j/2/2 && j/2/2 != k ==> a[j/2/2] <= a[j]);
    // Alternatively, the line above can be expressed as:
    //     requires (forall j :: 1 <= k/2 && j/2 == k && j <= n ==> a[j/2/2] <= a[j]);
    modifies a;
    ensures Valid();
  {
    var i := k;
    while (2*i <= n)  // while i is not a leaf
      invariant 1 <= i && MostlyValid();
      invariant (forall j :: 2 <= j && j <= n && j/2 != i ==> a[j/2] <= a[j]);
      invariant (forall j :: 2 <= j && j <= n && 1 <= j/2/2 && j/2/2 != i ==> a[j/2/2] <= a[j]);
    {
      var smallestChild;
      if (2*i + 1 <= n && a[2*i + 1] < a[2*i]) {
        smallestChild := 2*i + 1;
      } else {
        smallestChild := 2*i;
      }
      if (a[i] <= a[smallestChild]) {
        return;
      }
      var tmp := a[i];  a[i] := a[smallestChild];  a[smallestChild] := tmp;
      i := smallestChild;
      assert 1 <= i/2/2 ==> a[i/2/2] <= a[i];
    }
  }
}

// ---------- Alternative specifications ----------

class PriorityQueue_Alternative {
  var N: int;  // capacity
  var n: int;  // current size
  ghost var Repr: set<object>;  // set of objects that make up the representation of a PriorityQueue

  var a: array<int>;  // private implementation of PriorityQueue

  function Valid(): bool
    reads this, Repr;
  {
    MostlyValid() &&
    (forall j :: 2 <= j && j <= n ==> a[j/2] <= a[j])
  }

  function MostlyValid(): bool
    reads this, Repr;
  {
    this in Repr && a in Repr &&
    a != null && a.Length == N+1 &&
    0 <= n && n <= N
  }

  method Init(capacity: int)
    requires 0 <= capacity;
    modifies this;
    ensures Valid() && fresh(Repr - {this});
    ensures N == capacity;
  {
    N := capacity;
    a := new int[N+1];
    n := 0;
    Repr := {this};
    Repr := Repr + {a};
  }

  method Insert(x: int)
    requires Valid() && n < N;
    modifies this, a;
    ensures Valid() && fresh(Repr - old(Repr));
    ensures n == old(n) + 1 && N == old(N);
  {
    n := n + 1;
    a[n] := x;
    SiftUp();
  }

  method SiftUp()
    requires MostlyValid();
    requires (forall j :: 2 <= j && j <= n && j != n ==> a[j/2] <= a[j]);
    modifies a;
    ensures Valid();
  {
    var i := n;
    assert MostlyValid();
    while (1 < i)
      invariant i <= n && MostlyValid();
      invariant (forall j :: 2 <= j && j <= n && j != i ==> a[j/2] <= a[j]);
      invariant (forall j :: 1 <= j/2/2 && j/2 == i && j <= n ==> a[j/2/2] <= a[j]);
    {
      if (a[i/2] <= a[i]) {
        return;
      }
      var tmp := a[i];  a[i] := a[i/2];  a[i/2] := tmp;
      i := i / 2;
    }
  }

  method RemoveMin() returns (x: int)
    requires Valid() && 1 <= n;
    modifies this, a;
    ensures Valid() && fresh(Repr - old(Repr));
    ensures n == old(n) - 1;
  {
    x := a[1];
    a[1] := a[n];
    n := n - 1;
    SiftDown();
  }

  method SiftDown()
    requires MostlyValid();
    requires (forall j :: 4 <= j && j <= n ==> a[j/2] <= a[j]);
    modifies a;
    ensures Valid();
  {
    var i := 1;
    while (2*i <= n)  // while i is not a leaf
      invariant 1 <= i && MostlyValid();
      invariant (forall j :: 2 <= j && j <= n && j/2 != i ==> a[j/2] <= a[j]);
      invariant (forall j :: 1 <= j/2/2 && j/2 == i && j <= n ==> a[j/2/2] <= a[j]);
    {
      var smallestChild;
      if (2*i + 1 <= n && a[2*i + 1] < a[2*i]) {
        smallestChild := 2*i + 1;
      } else {
        smallestChild := 2*i;
      }
      if (a[i] <= a[smallestChild]) {
        return;
      }
      var tmp := a[i];  a[i] := a[smallestChild];  a[smallestChild] := tmp;
      i := smallestChild;
      assert 1 <= i/2/2 ==> a[i/2/2] <= a[i];
    }
  }
}