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Tree reconstruction from a list of leave depths

Problem 4 of the second verified software competition.

The ZIP file below contains both the source code, the Why3 proof session file and the Coq scripts of the proofs made in Coq. The Why3 source code is then displayed, followed by a summary of the proofs.

Authors: Jean-Christophe Filliâtre / Andrei Paskevich

Topics: Trees

Tools: Why3

References: The 2nd Verified Software Competition

see also the index (by topic, by tool, by reference, by year)

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(* The 2nd Verified Software Competition (VSTTE 2012)

   Problem 4: Tree Reconstruction
   Build a binary tree from a list of leaf depths, if any

   This is a purely applicative implementation, using immutable
   lists from Why3's standard library.

module Tree

  use export int.Int
  use export list.List
  use export list.Append

  type tree = Leaf | Node tree tree

  (* the list of leaf depths for tree t, if root is at depth d *)
  let rec function depths (d: int) (t: tree) : list int =
    match t with
    | Leaf -> Cons d Nil
    | Node l r -> depths (d+1) l ++ depths (d+1) r

  (* lemmas on depths *)

  lemma depths_head:
    forall t: tree, d: int.
    match depths d t with Cons x _ -> x >= d | Nil -> false end

  let rec lemma depths_unique (t1 t2: tree) (d: int) (s1 s2: list int)
    requires { depths d t1 ++ s1 = depths d t2 ++ s2 }
    variant { t1 }
    ensures { t1 = t2 && s1 = s2 }
  = let d' = d+1 in
    match t1,t2 with
    | Leaf,Leaf -> ()
    | Node t11 t12, Node t21 t22 ->
      depths_unique t11 t21 d' (depths d' t12 ++ s1) (depths d' t22 ++ s2);
      depths_unique t12 t22 d' s1 s2
    | Leaf, (Node t _) | (Node t _), Leaf ->
      match depths d' t with
      | Nil -> absurd
      | Cons x _ -> assert { x >= d' }

  lemma depths_prefix:
    forall t: tree, d1 d2: int, s1 s2: list int.
    depths d1 t ++ s1 = depths d2 t ++ s2 -> d1 = d2

  lemma depths_prefix_simple:
    forall t: tree, d1 d2: int.
    depths d1 t = depths d2 t -> d1 = d2

  let rec lemma depths_subtree (t1 t2: tree) (d1 d2:int) (s1:list int)
    requires { depths d1 t1 ++ s1 = depths d2 t2 }
    variant { t1 }
    ensures { d1 >= d2 }
  = assert { depths d2 t2 = depths d2 t2 ++ Nil };
    match t1 with
    | Leaf -> ()
    | Node t3 t4 ->
        depths_subtree t3 t2 (d1+1) d2 (depths (d1+1) t4 ++ s1)

  lemma depths_unique2:
    forall t1 t2: tree, d1 d2: int.
    depths d1 t1 = depths d2 t2 -> d1 = d2 && t1 = t2


module TreeReconstruction

  use export Tree
  use list.Length
  use list.HdTlNoOpt

  exception Failure
    (* used to signal the algorithm's failure i.e. there is no tree *)

  let rec build_rec (d: int) (s: list int) : (t: tree, s': list int)
    variant { length s, hd s - d }
    ensures { s = depths d t ++ s' }
    raises  { Failure -> forall t: tree, s' : list int. depths d t ++ s' <> s }
  = match s with
    | Nil ->
        raise Failure
    | Cons h t ->
        if h < d then raise Failure;
        if h = d then
          Leaf, t
          let l, s = build_rec (d+1) s in
          let r, s = build_rec (d+1) s in
          Node l r, s

  let build (s: list int) : tree
    ensures { depths 0 result = s }
    raises  { Failure -> forall t: tree. depths 0 t <> s }
  = let t, s = build_rec 0 s in
    match s with
    | Nil -> t
    | _ -> raise Failure


module Harness

  use TreeReconstruction

  let harness ()
    ensures { result = Node Leaf (Node (Node Leaf Leaf) Leaf) }
    raises  { Failure -> false }
  = build (Cons 1 (Cons 3 (Cons 3 (Cons 2 Nil))))

  let harness2 () : (_:tree)
    ensures { false } raises { Failure -> true }
  = build (Cons 1 (Cons 3 (Cons 2 (Cons 2 Nil))))


  A variant implementation proposed by Jayadev Misra

  Given the input list [x1; x2; ...; xn], we first turn it into the
  list of pairs [(x1, Leaf); (x2, Leaf); ...; (xn, Leaf)].  Then,
  repeatedly, we scan this list from left to right, looking for two
  consecutive pairs (v1, t1) and (v2, t2) with v1 = v2.  Then we
  replace them with the pair (v1-1, Node t1 t2) and we start again.
  We stop when there is only one pair left (v,t). Then we must have v=0.

  The implementation below achieves linear complexity using a zipper
  data structure to traverse the list of pairs. The left list contains
  the elements already traversed (thus on the left), in reverse order,
  and the right list contains the elements yet to be traversed.


(* Proving termination is quite easy and we do it first (though we could,
   obviously, do it together with proving correctness) *)

module ZipperBasedTermination

  use Tree
  use list.Length
  use list.Reverse

  exception Failure

  let rec tc (left: list (int, tree)) (right: list (int, tree)) : tree
    variant { length left + length right, length right }
    raises  { Failure }
  = match left, right with
    | _, Nil ->
        raise Failure
    | Nil, Cons (v, t) Nil ->
        if v = 0 then t else raise Failure
    | Nil, Cons (v, t) right' ->
        tc (Cons (v, t) Nil) right'
    | Cons (v1, t1) left', Cons (v2, t2) right' ->
        if v1 = v2 then tc left' (Cons (v1 - 1, Node t1 t2) right')
        else tc (Cons (v2, t2) left) right'


(* Now soundness and completeness *)

module ZipperBased

  use Tree
  use list.Length
  use list.Reverse

  (* the following function generalizes function [depths] to a forest, that
     is a list of pairs (depth, tree) *)

  function forest_depths (f: list (int, tree)) : list int = match f with
  | Nil -> Nil
  | Cons (d, t) r -> depths d t ++ forest_depths r

  (* an obvious lemma on [forest_depths] *)

  lemma forest_depths_append:
    forall f1 f2: list (int, tree).
    forest_depths (f1 ++ f2) = forest_depths f1 ++ forest_depths f2

  (* to prove completeness, one needs an invariant over the list [left].
     The main ingredient is predicate [greedy] below, which states that
     [d] is distinct from all depths along the left branch of [d1, t1]. *)

  predicate greedy (d: int) (d1: int) (t1: tree) =
    d <> d1 /\ match t1 with Leaf -> true | Node l1 _ -> greedy d (d1+1) l1 end

  (* then we extend it to a list of pairs [(dn,tn); ...; (d2,t2); (d1,t1)]
     as follows: [greedy d2 d1 t1], [greedy d3 d2 t2], etc.
     this is inductive predicate [g] *)

  inductive g (l: list (int, tree)) =
    | Gnil: g Nil
    | Gone: forall d: int, t: tree. g (Cons (d, t) Nil)
    | Gtwo: forall d1 d2: int, t1 t2: tree, l: list (int, tree).
        greedy d1 d2 t2 -> g (Cons (d1, t1) l) ->
        g (Cons (d2, t2) (Cons (d1, t1) l))

  (* an easy lemma on [g] *)

  lemma g_append:
    forall l1 [@induction] l2: list (int, tree). g (l1 ++ l2) -> g l1

  (* key lemma for completeness: whenever we fail because [right] is empty,
     we have to prove that there is no solution

     Note: the proof first generalizes the statement as follows:
       forest_depths ((d1,t1) :: l) <> depths d t + s
     whenever d < d1 (see the corresponding Coq file) *)

  lemma depths_length: forall t d. length (depths d t) >= 1
  lemma forest_depths_length: forall l. length (forest_depths l) >= 0
  lemma g_tail: forall l1 l2: list (int, tree). g (l1 ++ l2) -> g l2

  lemma key_lemma : forall t l d d1 t1 s. d < d1 ->
    1 <= length l -> g (reverse (Cons (d1, t1) l)) ->
    not (forest_depths (Cons (d1, t1) l) = (depths d t) ++ s)

  lemma right_nil:
    forall l: list (int, tree). length l >= 2 -> g l ->
    forall t: tree, d: int.
    forest_depths (reverse l) <> depths d t

  (* key lemma for soundness: preservation of the invariant when we move
     a tree from [right] to [left] *)

  lemma main_lemma:
    forall l: list (int, tree), d1 d2: int, t1 t2: tree. d1 <> d2 ->
    g (Cons (d1, t1) l) ->
    match t2 with Node l2 _ -> greedy d1 (d2+1) l2 | Leaf -> true end ->
    g (Cons (d2, t2) (Cons (d1, t1) l))

  (* finally, we need a predicate to state that a forest [l] contains only
     leaves *)

  predicate only_leaf (l: list (int, tree)) = match l with
    | Nil -> true
    | Cons (_, t) r -> t = Leaf /\ only_leaf r

  exception Failure

  let rec tc (left: list (int, tree)) (right: list (int, tree)) : tree
    requires { (* list [left] satisfies the invariant *)
      g left /\
      (* when [left] has one element, it can't be a solution *)
      match left with Cons (d1, _) Nil -> d1 <> 0 \/ right <> Nil
                    | _                -> true                    end /\
      (* apart (possibly) from its head, all elements in [right] are leaves;
         moreover the left branch of [right]'s head already satisfies
         invariant [g] when consed to [left] *)
      match right with
        | Cons (d2, t2) right' -> only_leaf right' /\
            match t2 with Node l2 _ -> g (Cons (d2+1, l2) left)
                        | Leaf      -> true end
        | Nil -> true end }
    variant { length left + 2 * length right }
    ensures { depths 0 result = forest_depths (reverse left ++ right) }
    raises { Failure ->
      forall t: tree. depths 0 t <> forest_depths (reverse left ++ right) }
  = match left, right with
    | _, Nil ->
        raise Failure
    | Nil, Cons (v, t) Nil ->
        if v = 0 then t else raise Failure
    | Nil, Cons (v, t) right' ->
        tc (Cons (v, t) Nil) right'
    | Cons (v1, t1) left', Cons (v2, t2) right' ->
        if v1 = v2 then tc left' (Cons (v1 - 1, Node t1 t2) right')
        else tc (Cons (v2, t2) left) right'

  (* Getting function [build] from [tc] is easy: from the list
     [x1; x2; ...; xn] we simply build the list of pairs
     [(x1, Leaf); (x2, Leaf); ...; (xn, Leaf)].
     Function [map_leaf] below does this. *)

  let rec function map_leaf (l: list int) : list (int, tree) =
    match l with
    | Nil -> Nil
    | Cons d r -> Cons (d, Leaf) (map_leaf r)

  (* two lemmas on [map_leaf] *)

  lemma map_leaf_depths:
    forall l: list int. forest_depths (map_leaf l) = l

  lemma map_leaf_only_leaf:
    forall l: list int. only_leaf (map_leaf l)

  let build (s: list int)
    ensures { depths 0 result = s }
    raises  { Failure -> forall t: tree. depths 0 t <> s }
  = tc Nil (map_leaf s)


Why3 Proof Results for Project "vstte12_tree_reconstruction"

Theory "vstte12_tree_reconstruction.Tree": fully verified

ObligationsAlt-Ergo 2.4.2Eprover 2.0Z3 4.11.2
VC for depths_unique0.17------
VC for depths_subtree0.10------

Theory "vstte12_tree_reconstruction.TreeReconstruction": fully verified

ObligationsCVC4 1.5CVC5 1.0.5Coq 8.11.2Eprover 2.0Z3 4.11.2
VC for build_rec---------------
exceptional postcondition------------0.01
exceptional postcondition0.06------------
variant decrease------------0.01
variant decrease------------0.02
exceptional postcondition------0.44------
exceptional postcondition------0.32------
VC for build---------0.02---

Theory "vstte12_tree_reconstruction.Harness": fully verified

ObligationsCoq 8.11.2
VC for harness---
exceptional postcondition0.28
VC for harness20.45

Theory "vstte12_tree_reconstruction.ZipperBasedTermination": fully verified

ObligationsCVC4 1.5
VC for tc0.72

Theory "vstte12_tree_reconstruction.ZipperBased": fully verified

ObligationsAlt-Ergo 2.3.3CVC4 1.5CVC5 1.0.5Coq 8.11.2
VC for tc------------
exceptional postcondition---0.16------
exceptional postcondition---0.05------
variant decrease---0.04------
exceptional postcondition---0.48------
variant decrease---0.04------
exceptional postcondition---0.06------
variant decrease---0.05------
exceptional postcondition---0.06------
variant decrease---0.06------
exceptional postcondition---0.51------
variant decrease---0.06------
exceptional postcondition---0.08------
VC for build---0.06------