From 05e1cc51b2fb0824580925b55319377305105c44 Mon Sep 17 00:00:00 2001
From: Federico Igne <undyamon@disroot.org>
Date: Sun, 28 Jan 2024 23:40:30 +0100
Subject: feat(zipper): add option to not move past last element when moving
 right

---
 lib/zipper.ml  | 24 +++++++++++++++---------
 lib/zipper.mli | 26 ++++++++++++++++----------
 2 files changed, 31 insertions(+), 19 deletions(-)

diff --git a/lib/zipper.ml b/lib/zipper.ml
index 5f3b37a..6077f21 100644
--- a/lib/zipper.ml
+++ b/lib/zipper.ml
@@ -1,4 +1,4 @@
-(* Module [Zipper]: functional zippers *)
+(* Module [Zipper]: functional linear zippers *)
 
 open Base
 
@@ -11,7 +11,10 @@ let after z = z.after
 let focus z = after z |> Sequence.next |> Option.map ~f:fst
 let focus_or ~default z = Option.value ~default (focus z)
 let is_far_left z = before z |> Sequence.is_empty
-let is_far_right z = after z |> Sequence.is_empty
+
+let is_far_right ?(by_one = false) z =
+  after z |> Sequence.length_is_bounded_by ~max:(if by_one then 1 else 0)
+
 let is_empty z = is_far_left z && is_far_right z
 let left_length z = z.pos
 let right_length z = after z |> Sequence.length
@@ -30,23 +33,26 @@ let rec left_while f z =
 
 let rec far_left z = if is_far_left z then z else z |> left |> far_left
 
-let right z =
+let right ?(by_one = false) z =
   match Sequence.next z.after with
   | None -> z
+  | Some (_, t) when by_one && Sequence.is_empty t -> z
   | Some (h, t) ->
       { pos = z.pos + 1; before = Sequence.shift_right z.before h; after = t }
 
-let rec right_while f z =
+let rec right_while ?(by_one = false) f z =
   if
-    (not (is_far_right z)) && Option.(focus z |> map ~f |> value ~default:false)
-  then right z |> right_while f
+    (not (is_far_right ~by_one z))
+    && Option.(focus z |> map ~f |> value ~default:false)
+  then right z |> right_while ~by_one f
   else z
 
-let rec far_right z = if is_far_right z then z else z |> right |> far_right
+let rec far_right ?(by_one = false) z =
+  if is_far_right ~by_one z then z else z |> right |> far_right ~by_one
 
-let goto n z =
+let goto ?(by_one = false) n z =
   let n = n - z.pos in
-  let step = if n < 0 then left else right in
+  let step = if n < 0 then left else right ~by_one in
   Fn.apply_n_times ~n:(abs n) step z
 
 let pop ?(n = 1) z =
diff --git a/lib/zipper.mli b/lib/zipper.mli
index a300b12..0481cfe 100644
--- a/lib/zipper.mli
+++ b/lib/zipper.mli
@@ -46,8 +46,10 @@ val focus_or : default:'a -> 'a zipper -> 'a
 val is_far_left : 'a zipper -> bool
 (** Return whether the cursor is at the beginning of the zipper. *)
 
-val is_far_right : 'a zipper -> bool
-(** Return whether the cursor is at the end of the zipper. *)
+val is_far_right : ?by_one:bool -> 'a zipper -> bool
+(** Return whether the cursor is at the end of the zipper.
+    If [by_one] is [true], the cursor is considered at the far right
+    even when pointing {b to} the last element of the sequence. *)
 
 val is_empty : 'a zipper -> bool
 (** Return whether the zipper is empty. *)
@@ -79,26 +81,30 @@ val left_while : ('a -> bool) -> 'a zipper -> 'a zipper
 val far_left : 'a zipper -> 'a zipper
 (** Move the cursor to the left, as much as possible. *)
 
-val right : 'a zipper -> 'a zipper
-(** Move the cursor one step to the right, if possible.
+val right : ?by_one:bool -> 'a zipper -> 'a zipper
+(** Move the cursor one step to the right, if possible. If [by_one] is
+    [true], the cursor won't move past the last element.
     Calling [right z],
 
         - if [z] is [([3; 2; 1], [4; 5])], the result is [([4; 3; 2; 1], [5])], 
         - if [z] is [([1; 2; 3], [])], the result is [([1; 2; 3], [])].
     *)
 
-val right_while : ('a -> bool) -> 'a zipper -> 'a zipper
+val right_while : ?by_one:bool -> ('a -> bool) -> 'a zipper -> 'a zipper
 (** [right_while f z] moves the cursor in [z] to the right as long as
     the predicate [f] is [true] when applied to the focus, or the right
-    end of the zipper is reached. *)
+    end of the zipper is reached. If [by_one] is [true], the cursor won't
+    move past the last element. *)
 
-val far_right : 'a zipper -> 'a zipper
-(** Move the cursor to the right, as much as possible. *)
+val far_right : ?by_one:bool -> 'a zipper -> 'a zipper
+(** Move the cursor to the right, as much as possible. If [by_one] is
+    [true], the cursor won't move past the last element.*)
 
-val goto : int -> 'a zipper -> 'a zipper
+val goto : ?by_one:bool -> int -> 'a zipper -> 'a zipper
 (** Move the cursor to a specific (absolute) position in the zipper.
     Depending on the current position, it either moves the cursor
-    forward or backwards, without crossing the zipper boundaries. *)
+    forward or backwards, without crossing the zipper boundaries. If
+    [by_one] is [true], the cursor won't move past the last element. *)
 
 (** {1 Changes at the cursor} *)
 
-- 
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