Add Zipper.subtree/1 (#133)

* Add Zipper.subtree/1

* Update test/zipper_test.exs

Co-authored-by: Zach Allaun <zach.allaun@gmail.com>

---------

Co-authored-by: Dorgan <dorgandash@gmail.com>
Co-authored-by: Zach Allaun <zach.allaun@gmail.com>

lib/sourceror/zipper.ex

@@ -501,4 +501,14 @@ defmodule Sourceror.Zipper do
 
   defp do_append_child(list, child) when is_list(list), do: list ++ [child]
   defp do_append_child({left, right}, child), do: {:{}, [], [left, right, child]}
+
+  @doc """
+  Returns a new zipper that is a subtree of the currently focused node.
+  """
+  @spec subtree(t) :: t
+  def subtree(%Z{} = zipper) do
+    zipper
+    |> node()
+    |> zip()
+  end
 end

test/zipper_test.exs

@@ -545,6 +545,18 @@ defmodule SourcerorTest.ZipperTest do
     end
   end
 
+  describe "subtree/1" do
+    test "returns a new zipper isolated on the focused of the parent zipper" do
+      zipper =
+        [1, [2, 3], 4, 5]
+        |> Z.zip()
+        |> Z.next()
+        |> Z.next()
+
+      assert Z.subtree(zipper) |> Z.root() |> Z.node() == [2, 3]
+    end
+  end
+
   describe "Zipper.Inspect" do
     test "inspect/2 defaults to using zippers: :as_ast" do
       zipper = Z.zip([1, [2], 3])