From d80d36106b07500b5d610cae488d96d4d7605029 Mon Sep 17 00:00:00 2001
From: Emmanuel Arrighi <emmanuel.arrighi@ahrefs.com>
Date: Fri, 6 Feb 2026 16:22:12 +0100
Subject: [PATCH] CCSeq(chore): sync with stdlib

---
 src/core/CCSeq.ml  | 178 +++++++++++++++++++++++++++++++--------------
 src/core/CCSeq.mli | 120 ++++++++++++++++++++++++------
 2 files changed, 221 insertions(+), 77 deletions(-)

diff --git a/src/core/CCSeq.ml b/src/core/CCSeq.ml
index c3b8dd34..4230e319 100644
--- a/src/core/CCSeq.ml
+++ b/src/core/CCSeq.ml
@@ -9,10 +9,19 @@ type 'a printer = Format.formatter -> 'a -> unit
 include Seq
 
 let nil () = Nil
+
+[@@@iflt 4.11]
+
 let cons a b () = Cons (a, b)
-let empty = nil
+
+[@@@endif]
+[@@@iflt 5.4]
+
 let singleton x () = Cons (x, nil)
 
+[@@@endif]
+[@@@iflt 4.11]
+
 let init n f =
   let rec aux i () =
     if i >= n then
@@ -22,6 +31,8 @@ let init n f =
   in
   aux 0
 
+[@@@endif]
+
 let rec _forever x () = Cons (x, _forever x)
 
 let rec _repeat n x () =
@@ -37,11 +48,15 @@ let repeat ?n x =
 
 let rec forever f () = Cons (f (), forever f)
 
+[@@@iflt 4.14]
+
 let is_empty l =
   match l () with
   | Nil -> true
   | Cons _ -> false
 
+[@@@endif]
+
 let head_exn l =
   match l () with
   | Nil -> raise Not_found
@@ -62,11 +77,15 @@ let tail l =
   | Nil -> None
   | Cons (_, l) -> Some l
 
+[@@@iflt 4.14]
+
 let uncons l =
   match l () with
   | Nil -> None
   | Cons (h, t) -> Some (h, t)
 
+[@@@endif]
+
 let rec equal eq l1 l2 =
   match l1 (), l2 () with
   | Nil, Nil -> true
@@ -100,14 +119,9 @@ let foldi f acc res =
   in
   aux acc 0 res
 
-let fold_lefti = foldi
+[@@@iflt 4.14]
 
-let rec iter f l =
-  match l () with
-  | Nil -> ()
-  | Cons (x, l') ->
-    f x;
-    iter f l'
+let fold_lefti = foldi
 
 let iteri f l =
   let rec aux f l i =
@@ -151,11 +165,6 @@ let rec drop_while p l () =
   | Cons (x, l') when p x -> drop_while p l' ()
   | Cons _ as res -> res
 
-let rec map f l () =
-  match l () with
-  | Nil -> Nil
-  | Cons (x, l') -> Cons (f x, map f l')
-
 let mapi f l =
   let rec aux f l i () =
     match l () with
@@ -164,36 +173,55 @@ let mapi f l =
   in
   aux f l 0
 
-let rec fmap f (l : 'a t) () =
-  match l () with
-  | Nil -> Nil
-  | Cons (x, l') ->
-    (match f x with
-    | None -> fmap f l' ()
-    | Some y -> Cons (y, fmap f l'))
+[@@@endif]
+[@@@iflt 5.4]
 
-let rec filter p l () =
-  match l () with
-  | Nil -> Nil
-  | Cons (x, l') ->
-    if p x then
-      Cons (x, filter p l')
-    else
-      filter p l' ()
+let filteri f l =
+  let rec aux f l i () =
+    match l () with
+    | Nil -> Nil
+    | Cons (x, tl) ->
+      if f i x then
+        Cons (x, aux f tl (i + 1))
+      else
+        aux f tl (i + 1) ()
+  in
+  aux f l 0
+
+[@@@endif]
+
+let fmap = filter_map
+
+[@@@iflt 4.11]
 
 let rec append l1 l2 () =
   match l1 () with
   | Nil -> l2 ()
   | Cons (x, l1') -> Cons (x, append l1' l2)
 
-let rec cycle l () = append l (cycle l) ()
+[@@@endif]
+[@@@iflt 4.14]
+
+let rec cycle l =
+  if is_empty l then
+    l
+  else
+    fun () ->
+  append l (cycle l) ()
+
 let rec iterate f a () = Cons (a, iterate f (f a))
 
+[@@@endif]
+[@@@iflt 4.11]
+
 let rec unfold f acc () =
   match f acc with
   | None -> Nil
   | Some (x, acc') -> Cons (x, unfold f acc')
 
+[@@@endif]
+[@@@iflt 4.14]
+
 let rec for_all p l =
   match l () with
   | Nil -> true
@@ -221,6 +249,35 @@ let rec find_map f l =
     | None -> find_map f tl
     | e -> e)
 
+[@@@endif]
+[@@@iflt 5.1]
+
+let find_index p l =
+  let rec aux i l =
+    match l () with
+    | Nil -> None
+    | Cons (x, tl) ->
+      if p x then
+        Some i
+      else
+        aux (i + 1) tl
+  in
+  aux 0 l
+
+let find_mapi f l =
+  let rec aux i l =
+    match l () with
+    | Nil -> None
+    | Cons (x, tl) ->
+      (match f i x with
+      | Some _ as res -> res
+      | None -> aux (i + 1) tl)
+  in
+  aux 0 l
+
+[@@@endif]
+[@@@iflt 5.1]
+
 let rec scan f acc res () =
   Cons
     ( acc,
@@ -229,18 +286,13 @@ let rec scan f acc res () =
         | Nil -> Nil
         | Cons (s, cont) -> scan f (f acc s) cont () )
 
-let rec flat_map f l () =
-  match l () with
-  | Nil -> Nil
-  | Cons (x, l') -> _flat_map_app f (f x) l' ()
-
-and _flat_map_app f l l' () =
-  match l () with
-  | Nil -> flat_map f l' ()
-  | Cons (x, tl) -> Cons (x, _flat_map_app f tl l')
+[@@@endif]
+[@@@iflt 4.13]
 
 let concat_map = flat_map
 
+[@@@endif]
+
 let product_with f l1 l2 =
   let rec _next_left h1 tl1 h2 tl2 () =
     match tl1 () with
@@ -264,6 +316,8 @@ let product_with f l1 l2 =
   in
   _next_left [] l1 [] l2
 
+[@@@iflt 4.14]
+
 let map_product = product_with
 let product l1 l2 = product_with (fun x y -> x, y) l1 l2
 
@@ -273,6 +327,8 @@ let rec group eq l () =
   | Cons (x, l') ->
     Cons (cons x (take_while (eq x) l'), group eq (drop_while (eq x) l'))
 
+[@@@endif]
+
 let rec _uniq eq prev l () =
   match prev, l () with
   | _, Nil -> Nil
@@ -285,16 +341,13 @@ let rec _uniq eq prev l () =
 
 let uniq eq l = _uniq eq None l
 
-let rec filter_map f l () =
-  match l () with
-  | Nil -> Nil
-  | Cons (x, l') ->
-    (match f x with
-    | None -> filter_map f l' ()
-    | Some y -> Cons (y, filter_map f l'))
+[@@@iflt 4.13]
 
-let flatten l = flat_map (fun x -> x) l
-let concat = flatten
+let concat l = flat_map (fun x -> x) l
+
+[@@@endif]
+
+let flatten = concat
 
 let range i j =
   let rec aux i j () =
@@ -317,12 +370,18 @@ let ( --^ ) i j =
   else
     range i (j + 1)
 
-let rec fold2 f acc l1 l2 =
+[@@@iflt 4.14]
+
+let rec fold_left2 f acc l1 l2 =
   match l1 (), l2 () with
   | Nil, _ | _, Nil -> acc
-  | Cons (x1, l1'), Cons (x2, l2') -> fold2 f (f acc x1 x2) l1' l2'
+  | Cons (x1, l1'), Cons (x2, l2') -> fold_left2 f (f acc x1 x2) l1' l2'
 
-let fold_left2 = fold2
+[@@@endif]
+
+let fold2 = fold_left2
+
+[@@@iflt 4.14]
 
 let rec map2 f l1 l2 () =
   match l1 (), l2 () with
@@ -346,17 +405,21 @@ let rec exists2 f l1 l2 =
   | Nil, _ | _, Nil -> false
   | Cons (x1, l1'), Cons (x2, l2') -> f x1 x2 || exists2 f l1' l2'
 
-let rec merge cmp l1 l2 () =
+let rec sorted_merge cmp l1 l2 () =
   match l1 (), l2 () with
   | Nil, tl2 -> tl2
   | tl1, Nil -> tl1
   | Cons (x1, l1'), Cons (x2, l2') ->
     if cmp x1 x2 < 0 then
-      Cons (x1, merge cmp l1' l2)
+      Cons (x1, sorted_merge cmp l1' l2)
     else
-      Cons (x2, merge cmp l1 l2')
+      Cons (x2, sorted_merge cmp l1 l2')
 
-let sorted_merge = merge
+[@@@endif]
+
+let merge = sorted_merge
+
+[@@@iflt 4.14]
 
 let rec zip a b () =
   match a (), b () with
@@ -377,6 +440,8 @@ let unzip l =
 
 let split = unzip
 
+[@@@endif]
+
 let zip_i seq =
   let rec loop i seq () =
     match seq () with
@@ -387,7 +452,6 @@ let zip_i seq =
 
 (** {2 Implementations} *)
 
-let return x () = Cons (x, nil)
 let pure = return
 let ( >>= ) xs f = flat_map f xs
 let ( >|= ) xs f = map f xs
@@ -530,11 +594,15 @@ let rec memoize f =
 
 (** {2 Fair Combinations} *)
 
+[@@@iflt 4.14]
+
 let rec interleave a b () =
   match a () with
   | Nil -> b ()
   | Cons (x, tail) -> Cons (x, interleave b tail)
 
+[@@@endif]
+
 let rec fair_flat_map f a () =
   match a () with
   | Nil -> Nil
diff --git a/src/core/CCSeq.mli b/src/core/CCSeq.mli
index a26fa1a1..cea1951f 100644
--- a/src/core/CCSeq.mli
+++ b/src/core/CCSeq.mli
@@ -17,38 +17,60 @@ include module type of Seq
 (** @inline *)
 
 val nil : 'a t
-val empty : 'a t
+
+[@@@iflt 4.11]
+
 val cons : 'a -> 'a t -> 'a t
+
+[@@@endif]
+[@@@iflt 5.4]
+
 val singleton : 'a -> 'a t
 
+[@@@endif]
+[@@@iflt 4.14]
+
 val init : int -> (int -> 'a) -> 'a t
 (** [init n f] corresponds to the sequence [f 0; f 1; ...; f (n-1)].
     @raise Invalid_argument if n is negative.
     @since 3.10 *)
 
+[@@@endif]
+
 val repeat : ?n:int -> 'a -> 'a t
 (** [repeat ~n x] repeats [x] [n] times then stops. If [n] is omitted,
     then [x] is repeated forever. *)
 
+[@@@iflt 4.14]
+
 val forever : (unit -> 'a) -> 'a t
 (** [forever f] corresponds to the infinite sequence containing all the [f ()].
     @since 3.10 *)
 
 val cycle : 'a t -> 'a t
-(** Cycle through the iterator infinitely. The iterator shouldn't be empty. *)
+(** Cycle through the sequence infinitely. The sequence should be persistent.
+  @since NEXT_RELEASE the sequence can be empty, in this case cycle return an empty sequence. *)
 
 val iterate : ('a -> 'a) -> 'a -> 'a t
 (** [iterate f a] corresponds to the infinite sequence containing [a], [f a], [f (f a)],
     ...
     @since 3.10 *)
 
+[@@@endif]
+[@@@iflt 4.11]
+
 val unfold : ('b -> ('a * 'b) option) -> 'b -> 'a t
 (** [unfold f acc] calls [f acc] and:
     - if [f acc = Some (x, acc')], yield [x], continue with [unfold f acc'].
     - if [f acc = None], stops. *)
 
+[@@@endif]
+[@@@iflt 4.14]
+
 val is_empty : 'a t -> bool
-(** [is_empty xs] checks in the sequence [xs] is empty *)
+(** [is_empty xs] checks in the sequence [xs] is empty. [is_empty] acces the first element of the sequence, this can causes issue if the sequence is ephemeral. *)
+
+[@@@endif]
 
 val head : 'a t -> 'a option
 (** Head of the list. *)
@@ -64,10 +86,14 @@ val tail_exn : 'a t -> 'a t
 (** Unsafe version of {!tail}.
     @raise Not_found if the list is empty. *)
 
+[@@@iflt 4.14]
+
 val uncons : 'a t -> ('a * 'a t) option
 (** [uncons xs] return [None] if [xs] is empty other
     @since 3.10 *)
 
+[@@@endif]
+
 val equal : 'a equal -> 'a t equal
 (** Equality step by step. Eager. *)
 
@@ -86,12 +112,12 @@ val foldi : ('a -> int -> 'b -> 'a) -> 'a -> 'b t -> 'a
     0) and [x] is the element of the sequence.
     @since 3.10 *)
 
+[@@@iflt 4.14]
+
 val fold_lefti : ('a -> int -> 'b -> 'a) -> 'a -> 'b t -> 'a
 (** Alias of {!foldi}.
     @since 3.10 *)
 
-val iter : ('a -> unit) -> 'a t -> unit
-
 val iteri : (int -> 'a -> unit) -> 'a t -> unit
 (** Iterate with index (starts at 0). *)
 
@@ -104,19 +130,33 @@ val take : int -> 'a t -> 'a t
 val take_while : ('a -> bool) -> 'a t -> 'a t
 val drop : int -> 'a t -> 'a t
 val drop_while : ('a -> bool) -> 'a t -> 'a t
-val map : ('a -> 'b) -> 'a t -> 'b t
 
 val mapi : (int -> 'a -> 'b) -> 'a t -> 'b t
 (** Map with index (starts at 0). *)
 
+[@@@endif]
+[@@@iflt 5.4]
+
+val filteri : (int -> 'a -> bool) -> 'a t -> 'a t
+(** Similar to {!filter} but the predicate takes aditionally the index of the elements. *)
+
+[@@@endif]
+
 val fmap : ('a -> 'b option) -> 'a t -> 'b t
-val filter : ('a -> bool) -> 'a t -> 'a t
+(** Alias of {!filter_map}. *)
+
+[@@@iflt 4.11]
+
 val append : 'a t -> 'a t -> 'a t
 
+[@@@endif]
+
 val product_with : ('a -> 'b -> 'c) -> 'a t -> 'b t -> 'c t
 (** Fair product of two (possibly infinite) lists into a new list. Lazy.
     The first parameter is used to combine each pair of elements. *)
 
+[@@@iflt 4.14]
+
 val map_product : ('a -> 'b -> 'c) -> 'a t -> 'b t -> 'c t
 (** Alias of {!product_with}.
     @since 3.10 *)
@@ -129,11 +169,15 @@ val group : 'a equal -> 'a t -> 'a t t
     For instance [group (=) [1;1;1;2;2;3;3;1]] yields
       [[1;1;1]; [2;2]; [3;3]; [1]]. *)
 
+[@@@endif]
+
 val uniq : 'a equal -> 'a t -> 'a t
 (** [uniq eq l] returns [l] but removes consecutive duplicates. Lazy.
     In other words, if several values that are equal follow one another,
     only the first of them is kept. *)
 
+[@@@iflt 4.14]
+
 val for_all : ('a -> bool) -> 'a t -> bool
 (** [for_all p [a1; ...; an]] checks if all elements of the sequence satisfy the
     predicate [p].  That is, it returns [(p a1) && ... && (p an)] for a
@@ -158,23 +202,37 @@ val find_map : ('a -> 'b option) -> 'a t -> 'b option
     [f ai = Some _] and return [None] otherwise.
     @since 3.10 *)
 
+[@@@endif]
+[@@@iflt 5.1]
+
+val find_index : ('a -> bool) -> 'a t -> int option
+(** [find_index p xs] returns [Some i], where [i] is the index of the first value of [xs] satisfying [p]. It returns [None] if no value of [xs] satifies [p]. *)
+
+val find_mapi : (int -> 'a -> 'b option) -> 'a t -> 'b option
+(** Similar to {!find_map} but the predicate take aditionnaly the index of the element. *)
+
+[@@@endif]
+[@@@iflt 4.14]
+
 val scan : ('a -> 'b -> 'a) -> 'a -> 'b t -> 'a t
 (** [scan f init xs] is the sequence containing the intermediate result of
     [fold f init xs].
     @since 3.10 *)
 
-val flat_map : ('a -> 'b t) -> 'a t -> 'b t
+[@@@endif]
+[@@@iflt 4.13]
 
 val concat_map : ('a -> 'b t) -> 'a t -> 'b t
 (** Alias of {!flat_map}
     @since 3.10 *)
 
-val filter_map : ('a -> 'b option) -> 'a t -> 'b t
-val flatten : 'a t t -> 'a t
-
 val concat : 'a t t -> 'a t
-(** Alias of {!flatten}.
-    @since 3.10 *)
+(** @since 3.10 *)
+
+[@@@endif]
+
+val flatten : 'a t t -> 'a t
+(** Alias of {!concat} *)
 
 val range : int -> int -> int t
 
@@ -187,12 +245,18 @@ val ( --^ ) : int -> int -> int t
 
 (** {2 Operations on two Collections} *)
 
-val fold2 : ('acc -> 'a -> 'b -> 'acc) -> 'acc -> 'a t -> 'b t -> 'acc
-(** Fold on two collections at once. Stop as soon as one of them ends. *)
+[@@@iflt 4.14]
 
 val fold_left2 : ('acc -> 'a -> 'b -> 'acc) -> 'acc -> 'a t -> 'b t -> 'acc
-(** Alias for {!fold2}.
-    @since 3.10 *)
+(** Fold on two collections at once. Stop as soon as one of them ends.
+@since 3.10 *)
+
+[@@@endif]
+
+val fold2 : ('acc -> 'a -> 'b -> 'acc) -> 'acc -> 'a t -> 'b t -> 'acc
+(** Alias for {!fold_left2}. *)
+
+[@@@iflt 4.14]
 
 val map2 : ('a -> 'b -> 'c) -> 'a t -> 'b t -> 'c t
 (** Map on two collections at once. Stop as soon as one of the
@@ -204,12 +268,19 @@ val iter2 : ('a -> 'b -> unit) -> 'a t -> 'b t -> unit
 val for_all2 : ('a -> 'b -> bool) -> 'a t -> 'b t -> bool
 val exists2 : ('a -> 'b -> bool) -> 'a t -> 'b t -> bool
 
-val merge : 'a ord -> 'a t -> 'a t -> 'a t
-(** Merge two sorted iterators into a sorted iterator. *)
+[@@@endif]
+[@@@iflt 4.14]
 
 val sorted_merge : 'a ord -> 'a t -> 'a t -> 'a t
-(** Alias of {!merge}.
-    @since 3.10 *)
+(** Merge two sorted iterators into a sorted iterator.
+  @since 3.10 *)
+
+[@@@endif]
+
+val merge : 'a ord -> 'a t -> 'a t -> 'a t
+(** Alias of {!sorted_merge}. *)
+
+[@@@iflt 4.14]
 
 val zip : 'a t -> 'b t -> ('a * 'b) t
 (** Combine elements pairwise. Stop as soon as one of the lists stops. *)
@@ -221,6 +292,8 @@ val split : ('a * 'b) t -> 'a t * 'b t
 (** Alias of {!unzip}.
     @since 3.10 *)
 
+[@@@endif]
+
 val zip_i : 'a t -> (int * 'a) t
 (** [zip_i seq] zips the index of each element with the element itself.
     @since 3.8
@@ -241,9 +314,13 @@ val memoize : 'a t -> 'a t
 
 (** {2 Fair Combinations} *)
 
+[@@@iflt 4.14]
+
 val interleave : 'a t -> 'a t -> 'a t
 (** Fair interleaving of both streams. *)
 
+[@@@endif]
+
 val fair_flat_map : ('a -> 'b t) -> 'a t -> 'b t
 (** Fair version of {!flat_map}. *)
 
@@ -252,7 +329,6 @@ val fair_app : ('a -> 'b) t -> 'a t -> 'b t
 
 (** {2 Implementations} *)
 
-val return : 'a -> 'a t
 val pure : 'a -> 'a t
 val ( >>= ) : 'a t -> ('a -> 'b t) -> 'b t
 val ( >|= ) : 'a t -> ('a -> 'b) -> 'b t