--- /dev/null
+1. Complete the definitions of `move_botleft` and `move_right_or_up` from the same-fringe solution in the [[week11]] notes. Test your attempts against some example trees to see if the resulting `make_fringe_enumerator` and `same_fringe` functions work as expected.
+
+ type 'a tree = Leaf of 'a | Node of ('a tree * 'a tree)
+
+ type 'a starred_tree = Root | Starring_Left of 'a starred_pair | Starring_Right of 'a starred_pair
+ and 'a starred_pair = { parent : 'a starred_tree; sibling: 'a tree }
+ and 'a zipper = { tree : 'a starred_tree; filler: 'a tree };;
+
+ let rec move_botleft (z : 'a zipper) : 'a zipper =
+ (* returns z if the targetted node in z has no children *)
+ (* else returns move_botleft (zipper which results from moving down and left in z) *)
+ YOU SUPPLY THE DEFINITION
+
+
+ let rec move_right_or_up (z : 'a zipper) : 'a zipper option =
+ (* if it's possible to move right in z, returns Some (the result of doing so) *)
+ (* else if it's not possible to move any further up in z, returns None *)
+ (* else returns move_right_or_up (result of moving up in z) *)
+ YOU SUPPLY THE DEFINITION
+
+
+ let new_zipper (t : 'a tree) : 'a zipper =
+ {tree = Root; filler = t}
+ ;;
+
+ let make_fringe_enumerator (t: 'a tree) =
+ (* create a zipper targetting the root of t *)
+ let zstart = new_zipper t
+ in let zbotleft = move_botleft zstart
+ (* create a refcell initially pointing to zbotleft *)
+ in let zcell = ref (Some zbotleft)
+ (* construct the next_leaf function *)
+ in let next_leaf () : 'a option =
+ match !zcell with
+ | None -> (* we've finished enumerating the fringe *)
+ None
+ | Some z -> (
+ (* extract label of currently-targetted leaf *)
+ let Leaf current = z.filler
+ (* update zcell to point to next leaf, if there is one *)
+ in let () = zcell := match move_right_or_up z with
+ | None -> None
+ | Some z' -> Some (move_botleft z')
+ (* return saved label *)
+ in Some current
+ )
+ (* return the next_leaf function *)
+ in next_leaf
+ ;;
+
+ let same_fringe (t1 : 'a tree) (t2 : 'a tree) : bool =
+ let next1 = make_fringe_enumerator t1
+ in let next2 = make_fringe_enumerator t2
+ in let rec loop () : bool =
+ match next1 (), next2 () with
+ | Some a, Some b when a = b -> loop ()
+ | None, None -> true
+ | _ -> false
+ in loop ()
+ ;;
+