(* VSTTE'10 competition http://www.macs.hw.ac.uk/vstte10/Competition.html Problem 3: searching a linked list *) module SearchingALinkedList use import int.Int use export list.List use export list.Length use export list.Nth logic zero_at (l: list int) (i: int) = nth i l = Some 0 and forall j:int. 0 <= j < i -> nth j l <> Some 0 logic no_zero (l: list int) = forall j:int. 0 <= j < length l -> nth j l <> Some 0 let rec search (i: int) (l: list int) variant { length l } = {} match l with | Nil -> i | Cons x r -> if x = 0 then i else search (i+1) r end { (i <= result < i + length l and zero_at l (result - i)) or (result = i + length l and no_zero l) } let search_list (l: list int) = { } search 0 l { (0 <= result < length l and zero_at l result) or (result = length l and no_zero l) } (* imperative version, with a loop *) use import module ref.Ref use import list.HdTl let head (l : list 'a) = { l <> Nil } match l with Nil -> absurd | Cons h _ -> h end { hd l = Some result } let tail (l : list 'a) = { l <> Nil } match l with Nil -> absurd | Cons _ t -> t end { tl l = Some result } let search_loop l = {} let i = ref 0 in let s = ref l in while !s <> Nil && head !s <> 0 do invariant { 0 <= !i and !i + length !s = length l and (forall j:int. 0 <= j -> nth j !s = nth (!i + j) l) and forall j:int. 0 <= j < !i -> nth j l <> Some 0 } variant { length !s } i := !i + 1; s := tail !s done; !i { (0 <= result < length l and zero_at l result) or (result = length l and no_zero l) } end (* Local Variables: compile-command: "unset LANG; make -C ../.. examples/programs/vstte10_search_list.gui" End: *)