feat(CCString): add CCString.foldi

src/core/CCString.ml

@@ -760,6 +760,12 @@ let fold f acc s =
     else fold_rec f (f acc s.[i]) s (i+1)
   in fold_rec f acc s 0
 
+let foldi f acc s =
+  let rec fold_rec f acc s i =
+    if i = String.length s then acc
+    else fold_rec f (f acc i s.[i]) s (i+1)
+  in fold_rec f acc s 0
+
 let pad ?(side=`Left) ?(c=' ') n s =
   let len_s = String.length s in
   if len_s >= n then s

src/core/CCString.mli

@@ -34,6 +34,11 @@ val fold : ('a -> char -> 'a) -> 'a -> t -> 'a
 (** [fold f init s] folds on chars by increasing index. Computes [f(… (f (f init s.[0]) s.[1]) …) s.[n-1]].
     @since 0.7 *)
 
+val foldi : ('a -> int -> char -> 'a) -> 'a -> t -> 'a
+(** [foldi f init s] is just like {!fold}, but it also passes in the index of each chars
+    as second argument to the folded function [f].
+    @since NEXT_RELEASE *)
+
 (** {2 Conversions} *)
 
 val to_gen : t -> char gen

src/core/CCStringLabels.mli

@@ -34,6 +34,11 @@ val fold : f:('a -> char -> 'a) -> init:'a -> t -> 'a
 (** [fold ~f ~init s] folds on chars by increasing index. Computes [f(… (f (f init s.[0]) s.[1]) …) s.[n-1]].
     @since 0.7 *)
 
+val foldi : f:('a -> int -> char -> 'a) -> 'a -> t -> 'a
+(** [foldi ~f init s] is just like {!fold}, but it also passes in the index of each chars
+    as second argument to the folded function [f].
+    @since NEXT_RELEASE *)
+
 (** {2 Conversions} *)
 
 val to_gen : t -> char gen