42 lines
954 B
Text
42 lines
954 B
Text
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predicate sorted(s: seq<int>) {
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forall i,j: int :: 0 <= i < j < |s| ==> s[i] <= s[j]
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}
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method SelectionSort(a: array<int>)
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modifies a
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ensures sorted(a[..]);
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ensures multiset(a[..]) == multiset(old(a[..]));
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{
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if (a.Length == 0) {
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return;
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}
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var s := 0;
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while (s < a.Length - 1)
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invariant multiset(a[..]) == multiset(old(a[..]));
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invariant forall i,j: int :: 0 <= i < s <= j < a.Length ==> a[i] <= a[j]
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invariant s <= a.Length - 1
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invariant s >= 1 ==> sorted(a[0..s])
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decreases a.Length - s
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{
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var min: int := s;
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var i: int := s + 1;
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while i < a.Length
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invariant multiset(a[..]) == multiset(old(a[..]));
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invariant s <= min < a.Length
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invariant s + 1 <= i <= a.Length
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invariant forall j: int :: s <= j < i ==> a[min] <= a[j]
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decreases a.Length - i
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{
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if (a[i] < a[min]) {
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min := i;
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}
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i := i + 1;
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}
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a[s], a[min] := a[min], a[s];
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s := s + 1;
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}
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}
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