feat: add generic tree zipper data structure

1 files changed, 45 insertions, 0 deletions

diff --git a/lib/tipper.ml b/lib/tipper.ml
new file mode 100644
index 0000000..bf09bb9
--- /dev/null
+++ b/lib/tipper.ml
@@ -0,0 +1,45 @@
+(* Module [Tipper]: functional tree zippers *)
+
+open Base
+
+type !+'a tipper = {
+  elt : 'a;
+  parent : 'a tipper option;
+  children : 'a tipper Zipper.t;
+}
+
+type !+'a t = 'a tipper
+
+let create a = { elt = a; parent = None; children = Zipper.empty }
+let focus t = t.elt
+let is_root t = Option.(t.parent |> is_none)
+let is_leaf t = Zipper.(t.children |> is_empty)
+let is_far_left t = Zipper.(t.children |> is_far_left)
+let is_far_right t = Zipper.(t.children |> is_far_right ~by_one:true)
+let left t = { t with children = Zipper.left t.children }
+let far_left t = { t with children = Zipper.far_left t.children }
+let right t = { t with children = Zipper.right ~by_one:true t.children }
+let far_right t = { t with children = Zipper.far_right ~by_one:true t.children }
+let set_focus a t = { t with elt = a }
+let push t1 t2 = { t2 with children = Zipper.push t1 t2.children }
+
+let pop t =
+  let s, cs = Zipper.pop t.children in
+  (Sequence.hd s, { t with children = cs })
+
+let up t = Option.(map ~f:(push t) t.parent |> value ~default:t)
+
+let rec up_while ~f t =
+  if (not (is_root t)) && f t then t |> up |> up_while ~f else t
+
+let root t = up_while ~f:(fun n -> not (is_root n)) t
+
+let down t =
+  match pop t with None, _ -> t | Some c, p -> { c with parent = Some p }
+
+let rec fold ~a ~f t =
+  let a = f a t in
+  if is_leaf t then
+    let t = up_while ~f:is_far_right t in
+    if is_root t && is_far_right t then a else fold ~a ~f (t |> right |> down)
+  else fold ~a ~f (t |> far_left |> down)