path: root/Test/dafny2/COST-verif-comp-2011-4-FloydCycleDetect.dfy

/*
Rustan Leino, 6 Oct 2011

COST Verification Competition, Challenge 4: Cyclic list
http://foveoos2011.cost-ic0701.org/verification-competition

Given: A Java linked data structure with the signature:

public class Node {
    Node next;

    public boolean cyclic() {
        //...
    }
}

Implement and verify the method cyclic() to return true when the data
structure is cyclic (i.e., this Node can be reached by following next
links) and false when it is not.
*/

// Remarks:

// I found the problem statement slightly ambiguous.  What I implemented was a
// method 'Cyclic' that returns true when 'this' can reach a cycle.  That is,
// 'this' does not itself have to be on the cycle in order for the method to
// return true.

// I wanted to assume as little as possible about the state of the data structure
// when 'Cyclic' is called.  The proof of the algorithm (indeed, the correctness
// of the algorithm) requires the number of nodes to be finite.  To specify
// this, I included to 'Cyclic' a parameter 'S' that contains all nodes that
// are reachable from 'this'.  The specification says that 'S' contains 'this'
// and 'null', and that is closed under the 'next' field.  This parameter and
// its associated 'IsClosed' condition are threaded through all functions and
// methods in the program.  The set 'S' is used only for specification purposes,
// so I declared it to be a ghost parameter of 'Cyclic'.  Other than including
// 'S' and 'IsClosed(S)' in the specification, the program does not assume anything
// about the state of 'this' and the other objects in 'S'.

// The algorithm I implement and verify is due to Bob Floyd and is sometimes
// known as the "tortoise and hare" algorithm.  The idea is simple:  Use 2 pointers,
// called 'tortoise' and 'hare', and advance 'hare' twice as quickly as 'tortoise'.
// Eventually, 'hare' will reach 'null' (in which case 'this' does not reach a
// cycle) or 'hare' will become equal to 'tortoise' (in which 'this' does reach a
// cycle).  Formally and mechanically proving the correctness of the algorithm
// is much harder, I found.

// Because this file is long, it is especially important to know what a human needs
// to trust in order to believe that the program is correct.  The main thing
// is the specification of 'Cyclic', and in particular its postcondition, which
// expresses what it means for 'this' to be able to reach a cycle.  Since this
// postcondition mentions the function 'Reaches', it is also necessary to trust
// the definition of 'Reaches' (but it is not necessary to trust the 'ensures'
// clause of the function).  Function 'Reaches' is in turn defined in terms
// of 'Nexxxt(k,...)' which stands for 'k' applications of the field 'next'
// (and returns 'null' if 'null' is ever reached along the way).  Other than
// these things, the rest of the program is implementation details and proof
// details.  Well, one may want to inspect the body of 'Cyclic' to see that it
// does indeed implement Floyd's algorithm--for this inspection, ignore all
// ghost things, like ghost variables, updates to ghost variables, assert
// statements, and calls to lemmas.

// The proof is long.  One interesting aspect of it is that it is all constructed
// as a program fed to a program verifier.  Since the additional properties
// are specified using ghost variables, ghost methods, and other ghost constructs,
// the run-time execution of the program is not affected by including the
// proof as part of the program, because the Dafny compiler ignores all ghost
// constructs.

// The proof (and in particular the proof of termination), makes use of two
// numbers, called 'A' and 'B' and computed by the call to the ghost method
// 'AnalyzeList'.  'A' is the number of steps from 'this' before a cycle is
// reached.  If there is no cycle, 'A' is the length of the list.  'B' is the
// length of the cycle, if any.  But, you ask, how are 'A' and 'B' obtained?
// If these are used to prove the termination of 'Cyclic', then how does one
// prove the termination of the computation of 'A' and 'B'?  The answer is
// that 'AnalyzeList' uses a simpler algorithm, namely a depth-first traversal
// from 'this' that uses a set to keep track of which nodes have been visited.
// This set would not be nice to have to represent at run time, but since
// 'AnalyzeList' is a ghost method, the variable holding the set does not survive
// compilation.  So, 'Cyclic' first calls ghost method 'Analyze' to traverse
// the list and then, equipped with 'A' and 'B' to carry out the proof of Floyd's
// algorithm, proceeds with Floyd's algorithm.

// The ghost constructs in 'Cyclic' add some clutter to the program text.  To
// alleviate matter a little, I have include the word "Lemma" in the names of
// all ghost methods (except ghost method 'AnalyzeList', which I guess didn't
// feel to me like a "lemma" per se).

// The Dafny verifier, which builds on verification engine Boogie, which in turn
// builds on the SMT solver Z3, needs help throughout this proof.  To give it
// hints about which properties to prove, the program uses assert statements and
// calls to lemmas.  These do not provide the verifier with new facts or
// assumptions--they only instruct the verifier to verify something, after which
// the verifier can make use of what it just verified.  In a number of places,
// the assert statements mention universally quantified properties whose proof
// require induction; Dafny heuristically detects these and applies its induction
// tactic (in the absence of that induction tactic, more handholding would be
// required in the program text to guide the verifier through the proof, see
// 'Lemma_NexxxtIsTransitive', for example).

// About Dafny:
// As always (when it is successful), Dafny verifies that the program does not
// cause any run-time errors (like array index bounds errors), that the program
// terminates, that expressions and functions are well defined, and that all
// specifications are satisfied.  The language prevents type errors by being type
// safe, prevents dangling pointers by not having an "address-of" or "deallocate"
// operation (which is accommodated at run time by a garbage collector), and
// prevents arithmetic overflow errors by using mathematical integers (which
// is accommodated at run time by using BigNum's).  By proving that programs
// terminate, Dafny proves that a program's time usage is finite, which implies
// that the program's space usage is finite too.  However, executing the
// program may fall short of your hopes if you don't have enough time or
// space; that is, the program may run out of space or may fail to terminate in
// your lifetime, because Dafny does not prove that the time or space needed by
// the program matches your execution environment.  The only input fed to
// the Dafny verifier/compiler is the program text below; Dafny then automatically
// verifies and compiles the program (for this program in less than 30 seconds,
// 25 seconds of which is spent verifying the ghost method AnalyzeList)
// without further human intervention.

class Node {
  var next: Node;

  function IsClosed(S: set<Node>): bool
    reads S;
  {
    this in S && null in S &&
    forall n :: n in S && n != null && n.next != null ==> n.next in S
  }

  function Nexxxt(k: int, S: set<Node>): Node
    requires IsClosed(S) && 0 <= k;
    ensures Nexxxt(k, S) in S;  // a consequence of the definition
    reads S;
    decreases k;
  {
    if k == 0 then this
    else if Nexxxt(k-1, S) == null then null
    else Nexxxt(k-1, S).next
  }

  function Reaches(sink: Node, S: set<Node>): bool
    requires IsClosed(S);
    ensures Reaches(sink, S) ==> sink in S;  // a consequence of the definition
    reads S;
  {
    exists k :: 0 <= k && Nexxxt(k, S) == sink
  }

  method Cyclic(ghost S: set<Node>) returns (reachesCycle: bool)
    requires IsClosed(S);
    ensures reachesCycle <==> exists n :: n != null && Reaches(n, S) && n.next != null && n.next.Reaches(n, S);
  {
    ghost var A, B := AnalyzeList(S);
    var tortoise, hare:= this, next;
    ghost var t, h := 0, 1;
    while (hare != tortoise)
      invariant tortoise != null && tortoise in S && hare in S;
      invariant 0 <= t < h && Nexxxt(t, S) == tortoise && Nexxxt(h, S) == hare;
      // What follows of the invariant is for proving termination:
      invariant h == 1 + 2*t && t <= A + B;
      invariant forall k :: 0 <= k < t ==> Nexxxt(k, S) != Nexxxt(1+2*k, S);
      decreases A + B - t;
    {
      if (hare == null || hare.next == null) {
        ghost var distanceToNull := if hare == null then h else h+1;
        Lemma_NullImpliesNoCycles(distanceToNull, S);
        assert !exists k,l :: 0 <= k && 0 <= l && Nexxxt(k, S) != null && Nexxxt(k, S).next != null && Nexxxt(k, S).next.Nexxxt(l, S) == Nexxxt(k, S);  // this is a copy of the postcondition of lemma NullImpliesNoCycles
        return false;
      }
      Lemma_NullIsTerminal(h+1, S);
      assert Nexxxt(t+1, S) != null;
      tortoise, t, hare, h := tortoise.next, t+1, hare.next.next, h+2;
      CrucialLemma(A, B, S);
    }
    Lemma_NullIsTerminal(h, S);
    Lemma_NexxxtIsTransitive(t+1, h - (t+1), S);
    assert tortoise.next.Reaches(tortoise, S);
    return true;
  }

  // What follows in this file are details that are relevant only to the proof.  That is,
  // to trust that the algorithm is correct, it is not necessary to go through the
  // details below--the specification of 'Cyclic' above and the fact that Dafny verifies
  // the program suffice.  (Of course, one also needs to trust the verifier.)

  ghost method AnalyzeList(S: set<Node>) returns (A: int, B: int)
    requires IsClosed(S);
    // find an A and B (0 <= A && 1 <= B) such that:
    // the first A steps are no on a cycle, and
    // either next^A == null or next^A == next^(A+B).
    ensures 0 <= A && 1 <= B;
    ensures Nexxxt(A, S) != null ==> Nexxxt(A, S) == Nexxxt(A, S).Nexxxt(B, S);
    ensures forall k,l :: 0 <= k < l < A ==> Nexxxt(k, S) != Nexxxt(l, S);
  {
    // since S is finite, we can just go ahead and compute the transitive closure of "next" from "this"
    var p, steps, Visited := this, 0, {null};
    while (p !in Visited)
      invariant 0 <= steps && p == Nexxxt(steps, S) && p in S && null in Visited && Visited <= S;
      invariant forall t :: 0 <= t < steps ==> Nexxxt(t, S) in Visited;
      invariant forall q :: q in (Visited - {null}) ==> exists t :: 0 <= t < steps && Nexxxt(t, S) == q;
      invariant forall k,l :: 0 <= k < l < steps ==> Nexxxt(k, S) != Nexxxt(l, S);
      decreases S - Visited;
    {
      p, steps, Visited := p.next, steps + 1, Visited + {p};
    }
    if (p == null) {
      A, B := steps, 1;
    } else {
      A := FindA(S, steps, p);
      B := steps - A;
      Lemma_NexxxtIsTransitive(A, B, S);
    }
  }

  ghost method FindA(S: set<Node>, steps: int, p: Node) returns (A: int)
    requires IsClosed(S);
    requires 0 <= steps && p == Nexxxt(steps, S) && p in S && p != null;
    requires exists k :: 0 <= k < steps && Nexxxt(k, S) == p;
    ensures 0 <= A < steps;
    ensures Nexxxt(A, S) == p;
    ensures forall k :: 0 <= k < A ==> Nexxxt(k, S) != p;
  {
    A := 0;
    while (Nexxxt(A, S) != p)
      invariant 0 <= A < steps;
      invariant forall k :: 0 <= k < A ==> Nexxxt(k, S) != p;
      decreases steps - A;
    {
      A := A + 1;
    }
  }

  ghost method CrucialLemma(a: int, b: int, S: set<Node>)
    requires IsClosed(S);
    requires 0 <= a && 1 <= b;
    requires forall k,l :: 0 <= k < l < a ==> Nexxxt(k, S) != Nexxxt(l, S);
    requires Nexxxt(a, S) == null || Nexxxt(a, S).Nexxxt(b, S) == Nexxxt(a, S);
    ensures exists T :: 0 <= T < a+b && Nexxxt(T, S) == Nexxxt(1+2*T, S);
  {
    if (Nexxxt(a, S) == null) {
      Lemma_NullIsTerminal(1+2*a, S);
      assert Nexxxt(a, S) == null ==> Nexxxt(1+2*a, S) == null;
    } else {
      assert Nexxxt(a, S) != null && Nexxxt(a, S).Nexxxt(b, S) == Nexxxt(a, S);
      Lemma_NexxxtIsTransitive(a, b, S);
      assert Nexxxt(a + b, S) == Nexxxt(a, S);
      // When the tortoise has done "a" steps, both it and the hare have reached the cycle.
      // Since the cycle has length "b", the hare has at most "b" steps to catch up with the
      // tortoise.  Well, you may think of the tortoise as being the one that has to catch up,
      // since the tortoise has not traveled as far.  So, let's imagine a virtual tortoise
      // that is in the same position as the tortoise, but who got there by taking at least
      // as many steps as the hare (but fewer than "b" steps more than the hare).
      var t, h := a, 1+2*a;  // steps traveled by the tortoise and the hare, respectively
      var vt := a;  // steps traveled by the virtual tortoise
      while (vt < h)
        invariant t <= vt < h+b;
        invariant Nexxxt(t, S) == Nexxxt(vt, S);
      {
        Lemma_AboutCycles(a, b, vt, S);
        vt := vt + b;  // let the virtual tortoise take another lap
      }
      // Good.  Since the virtual tortoise has now taken at least as many steps as the hare,
      // we can compute (as a non-negative number) the steps that hare is trailing behind the
      // virtual tortoise.
      var catchup := vt - h;
      assert 0 <= catchup < b;
      // Now, let the hare catch up with the virtual tortoise by simulating "catchup" steps
      // of the algorithm.
      var i := 0;
      while (i < catchup)
        invariant 0 <= i <= catchup;
        invariant t == a + i && h == 1 + 2*t && t <= vt;
        invariant Nexxxt(t, S) == Nexxxt(vt, S) == Nexxxt(h + catchup - i, S);
      {
        i, t, vt, h := i+1, t+1, vt+1, h+2;
      }
      assert a <= t < a + b && Nexxxt(t, S) == Nexxxt(1 + 2*t, S);
    }
  }

  ghost method Lemma_AboutCycles(a: int, b: int, k: int, S: set<Node>)
    requires IsClosed(S);
    requires 0 <= a <= k && 1 <= b && Nexxxt(a, S) != null && Nexxxt(a, S).Nexxxt(b, S) == Nexxxt(a, S);
    ensures Nexxxt(k + b, S) == Nexxxt(k, S);
  {
    Lemma_NexxxtIsTransitive(a, b, S);
    var n := a;
    while (n < k)
      invariant a <= n <= k;
      invariant Nexxxt(n + b, S) == Nexxxt(n, S);
    {
      n := n + 1;
    }
  }

  ghost method Lemma_NexxxtIsTransitive(x: int, y: int, S: set<Node>)
    requires IsClosed(S) && 0 <= x && 0 <= y;
    ensures Nexxxt(x, S) != null ==> Nexxxt(x, S).Nexxxt(y, S) == Nexxxt(x + y, S);
  {
    if (Nexxxt(x, S) != null)
    {
      assert forall j :: 0 <= j ==> Nexxxt(x, S).Nexxxt(j, S) == Nexxxt(x + j, S);  // Dafny's induction tactic kicks in
      /* Alternatively, here's a manual proof by induction (but only up to the needed y):
      var j := 0;
      while (j < y)
        invariant 0 <= j <= y;
        invariant Nexxxt(x, S).Nexxxt(j, S) == Nexxxt(x + j, S);
      {
        j := j + 1;
      }
      */
    }
  }

  ghost method Lemma_NullIsTerminal(d: int, S: set<Node>)
    requires IsClosed(S) && 0 <= d;
    ensures forall k :: 0 <= k < d && Nexxxt(d, S) != null ==> Nexxxt(k, S) != null;
  {
    var j := d;
    while (0 < j)
      invariant 0 <= j <= d;
      invariant forall k :: j <= k < d && Nexxxt(k, S) == null ==> Nexxxt(d, S) == null;
    {
      j := j - 1;
      if (Nexxxt(j, S) == null) {
        assert Nexxxt(j+1, S) == null;
      }
    }
  }

  ghost method Lemma_NullImpliesNoCycles(n: int, S: set<Node>)
    requires IsClosed(S) && 0 <= n && Nexxxt(n, S) == null;
    ensures !exists k,l :: 0 <= k && 0 <= l && Nexxxt(k, S) != null && Nexxxt(k, S).next != null && Nexxxt(k, S).next.Nexxxt(l, S) == Nexxxt(k, S);
  {
    // The proof of this lemma is more complicated than necessary, because Dafny does not know that
    // "if P(k,l) holds for one arbitrary (k,l), then it holds for all (k,l)".
    Lemma_NullImpliesNoCycles_part0(n, S);
    Lemma_NullImpliesNoCycles_part1(n, S);
    Lemma_NullImpliesNoCycles_part2(n, S);
  }

  ghost method Lemma_NullImpliesNoCycles_part0(n: int, S: set<Node>)
    requires IsClosed(S) && 0 <= n && Nexxxt(n, S) == null;
    ensures forall k,l :: n <= k && 0 <= l && Nexxxt(k, S) != null && Nexxxt(k, S).next != null ==> Nexxxt(k, S).next.Nexxxt(l, S) != Nexxxt(k, S);
  {
    assert forall k :: n <= k ==> Nexxxt(k, S) == null;  // Dafny proves this thanks to its induction tactic
  }

  ghost method Lemma_NullImpliesNoCycles_part1(n: int, S: set<Node>)
    requires IsClosed(S) && 0 <= n && Nexxxt(n, S) == null;
    ensures forall k,l :: 0 <= k && n <= l && Nexxxt(k, S) != null && Nexxxt(k, S).next != null ==> Nexxxt(k, S).next.Nexxxt(l, S) != Nexxxt(k, S);
  {
    // Each of the following assertions makes use of Dafny's induction tactic
    assert forall k,l :: 0 <= k && 0 <= l && Nexxxt(k, S) != null && Nexxxt(k, S).next != null ==> Nexxxt(k, S).next.Nexxxt(l, S) == Nexxxt(k+1+l, S);
    assert forall kl :: n <= kl ==> Nexxxt(kl, S) == null;
  }

  ghost method Lemma_NullImpliesNoCycles_part2(n: int, S: set<Node>)
    requires IsClosed(S) && 0 <= n && Nexxxt(n, S) == null;
    ensures forall k,l :: 0 <= k < n && 0 <= l < n && Nexxxt(k, S) != null && Nexxxt(k, S).next != null ==> Nexxxt(k, S).next.Nexxxt(l, S) != Nexxxt(k, S);
  {
    var kn := 0;
    while (kn < n)
      invariant 0 <= kn <= n;
      invariant forall k,l :: 0 <= k < kn && 0 <= l < n && Nexxxt(k, S) != null && Nexxxt(k, S).next != null ==> Nexxxt(k, S).next.Nexxxt(l, S) != Nexxxt(k, S);
    {
      var ln := 0;
      while (ln < n)
        invariant 0 <= ln <= n;
        invariant forall k,l :: 0 <= k < kn && 0 <= l < n && Nexxxt(k, S) != null && Nexxxt(k, S).next != null ==> Nexxxt(k, S).next.Nexxxt(l, S) != Nexxxt(k, S);
        invariant forall l :: 0 <= l < ln && Nexxxt(kn, S) != null && Nexxxt(kn, S).next != null ==> Nexxxt(kn, S).next.Nexxxt(l, S) != Nexxxt(kn, S);
      {
        if (Nexxxt(kn, S) != null && Nexxxt(kn, S).next != null) {
          assert Nexxxt(kn+1, S) != null;
          Lemma_NexxxtIsTransitive(kn+1, ln, S);
          assert Nexxxt(kn, S).next.Nexxxt(ln, S) == Nexxxt(kn+1+ln, S);  // follows from the transitivity lemma on the previous line
          Lemma_NexxxtIsTransitive(kn, 1+ln, S);
          assert Nexxxt(kn+1+ln, S) == Nexxxt(kn, S).Nexxxt(1+ln, S);  // follows from the transitivity lemma on the previous line
          // finally, here comes the central part of the argument, namely:
          // if Nexxxt(kn, S).Nexxxt(1+ln, S) == Nexxxt(kn, S), then for any h (0 <= h), Nexxxt(kn, S).Nexxxt(h*(1+ln), S) == Nexxxt(kn, S), but
          // that can't be for n <= h*(1+ln), since Nexxxt(kn, S) != null and Nexxxt(n.., S) == null.
          if (Nexxxt(kn, S).Nexxxt(1+ln, S) == Nexxxt(kn, S)) {
            var nn := 1+ln;
            while (nn < n)
              invariant 0 <= nn;
              invariant Nexxxt(kn, S).Nexxxt(nn, S) == Nexxxt(kn, S);
            {
              assert Nexxxt(kn, S) ==
                     Nexxxt(kn, S).Nexxxt(nn, S) ==
                     Nexxxt(kn, S).Nexxxt(1+ln, S) ==
                     Nexxxt(kn, S).Nexxxt(nn, S).Nexxxt(1+ln, S);
              Nexxxt(kn, S).Lemma_NexxxtIsTransitive(1+ln, nn, S);
              assert Nexxxt(kn, S).Nexxxt(nn+1+ln, S) == Nexxxt(kn, S);
              nn := nn + 1+ln;
            }
            Lemma_NexxxtIsTransitive(kn, nn, S);
            assert Nexxxt(kn, S).Nexxxt(nn, S) == Nexxxt(kn+nn, S);
            assert forall j :: n <= j ==> Nexxxt(j, S) == null;  // this uses Dafny's induction tactic
            assert false;  // we have reached a contradiction
          }
          assert Nexxxt(kn+1, S).Nexxxt(ln, S) != Nexxxt(kn, S);
        }
        ln := ln + 1;
      }
      kn := kn + 1;
    }
  }
}