make Stack.length constant-time

stdlib/stack.ml

@@ -11,30 +11,30 @@
 (*                                                                     *)
 (***********************************************************************)
 
-type 'a t = { mutable c : 'a list }
+type 'a t = { mutable c : 'a list; mutable len : int; }
 
 exception Empty
 
-let create () = { c = [] }
+let create () = { c = []; len = 0; }
 
-let clear s = s.c <- []
+let clear s = s.c <- []; s.len <- 0
 
-let copy s = { c = s.c }
+let copy s = { c = s.c; len = s.len; }
 
-let push x s = s.c <- x :: s.c
+let push x s = s.c <- x :: s.c; s.len <- s.len + 1
 
 let pop s =
   match s.c with
-    hd::tl -> s.c <- tl; hd
+  | hd::tl -> s.c <- tl; s.len <- s.len - 1; hd
   | []     -> raise Empty
 
 let top s =
   match s.c with
-    hd::_ -> hd
+  | hd::_ -> hd
   | []     -> raise Empty
 
 let is_empty s = (s.c = [])
 
-let length s = List.length s.c
+let length s = s.len
 
 let iter f s = List.iter f s.c

stdlib/stack.mli

@@ -47,7 +47,7 @@ val is_empty : 'a t -> bool
 (** Return [true] if the given stack is empty, [false] otherwise. *)
 
 val length : 'a t -> int
-(** Return the number of elements in a stack. *)
+(** Return the number of elements in a stack. Time complexity O(1) *)
 
 val iter : ('a -> unit) -> 'a t -> unit
 (** [iter f s] applies [f] in turn to all elements of [s],