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class Cell {
var n : int;
}
class A {
var x : int;
predicate valid {
acc(x) && x >= 0
}
function getX(): int requires valid
{
unfolding valid in x
}
method init()
requires acc(this.*);
ensures valid;
{
x := 0;
fold valid;
}
method inc()
requires valid;
ensures valid && getX() == old(getX()) + 1;
{
unfold valid;
x := x + 1;
fold valid;
}
method dec()
requires valid && getX() > 0;
ensures valid;
{
unfold valid;
x := x - 1;
fold valid;
}
method magic() returns (c: Cell)
requires valid;
ensures valid;
{
}
}
class C {
ghost var x : int;
var y : Cell;
var z : Cell;
function getX() : int
requires valid;
{
unfolding valid in y.n - z.n
}
predicate valid {
acc(x) && acc(y) && acc(z) && acc(y.n) && acc(z.n) &&
y != null && z != null &&
y.n >= 0 && z.n >= 0 &&
y.n - z.n == x &&
x >= 0
}
method init()
requires acc(this.*);
ensures valid;
{
x := 0;
//
y := new Cell;
z := new Cell;
y.n := 0;
z.n := 0;
fold valid;
}
method inc()
requires valid;
ensures valid && getX() == old(getX()) + 1;
{
unfold valid;
x := x + 1;
//
y.n := y.n + 1;
fold valid;
}
method dec()
requires valid && getX() > 0;
ensures valid;
{
unfold valid;
x := x - 1;
//
z.n := z.n + 1;
fold valid;
}
method magic() returns (c: Cell)
requires valid;
ensures valid;
{
unfold valid;
c := y;
fold valid;
}
}
class Client {
method main()
{
// Abstract program
var a := new A;
call a.init();
call a.inc(); // problem is here
call a.inc();
call a.dec();
call ac := a.magic();
ac.n := 0;
// Concrete program
var c := new C;
call c.init();
call c.inc();
call c.inc();
call c.dec();
call cc := c.magic();
cc.n := 0;
}
}
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