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class Benchmark2 {
method BinarySearch(a: array<int>, key: int) returns (result: int)
requires a != null;
requires (forall i, j :: 0 <= i && i < j && j < |a| ==> a[i] <= a[j]);
ensures -1 <= result && result < |a|;
ensures 0 <= result ==> a[result] == key;
ensures result == -1 ==> (forall i :: 0 <= i && i < |a| ==> a[i] != key);
{
var low := 0;
var high := |a|;
while (low < high)
invariant 0 <= low && low <= high && high <= |a|;
invariant (forall i :: 0 <= i && i < low ==> a[i] < key);
invariant (forall i :: high <= i && i < |a| ==> key < a[i]);
{
var mid := low + (high - low) / 2;
var midVal := a[mid];
if (midVal < key) {
low := mid + 1;
} else if (key < midVal) {
high := mid;
} else {
result := mid; // key found
return;
}
}
result := -1; // key not present
}
}
method Main() {
var a := new int[5];
a[0] := -4;
a[1] := -2;
a[2] := -2;
a[3] := 0;
a[4] := 25;
call TestSearch(a, 4);
call TestSearch(a, -8);
call TestSearch(a, -2);
call TestSearch(a, 0);
call TestSearch(a, 23);
call TestSearch(a, 25);
call TestSearch(a, 27);
}
method TestSearch(a: array<int>, key: int)
requires a != null;
requires (forall i, j :: 0 <= i && i < j && j < |a| ==> a[i] <= a[j]);
{
var b := new Benchmark2;
call r := b.BinarySearch(a, key);
print "Looking for key=", key, ", result=", r, "\n";
}
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