Do not compose List.length with List.filter.

2 files changed, 9 insertions, 0 deletions

diff --git a/lib/cList.ml b/lib/cList.ml
index 0ac372d8d..72f892a09 100644
--- a/lib/cList.ml
+++ b/lib/cList.ml
@@ -48,6 +48,7 @@ sig
   val filteri :
     (int -> 'a -> bool) -> 'a list -> 'a list
   val smartfilter : ('a -> bool) -> 'a list -> 'a list
+  val count : ('a -> bool) -> 'a list -> int
   val index : 'a eq -> 'a -> 'a list -> int
   val index0 : 'a eq -> 'a -> 'a list -> int
   val iteri :  (int -> 'a -> unit) -> 'a list -> unit
@@ -375,6 +376,12 @@ let rec smartfilter f l = match l with
           else h :: tl'
         else tl'
 
+let count f l =
+  let rec aux acc = function
+    | [] -> acc
+    | h :: t -> if f h then aux (acc + 1) t else aux acc t in
+  aux 0 l
+
 let rec index_f f x l n = match l with
 | [] -> raise Not_found
 | y :: l -> if f x y then n else index_f f x l (succ n)
diff --git a/lib/cList.mli b/lib/cList.mli
index 19eeb2509..1487f67a3 100644
--- a/lib/cList.mli
+++ b/lib/cList.mli
@@ -94,6 +94,8 @@ sig
   (** [smartfilter f [a1...an] = List.filter f [a1...an]] but if for all i
     [f ai = true], then [smartfilter f l == l] *)
 
+  val count : ('a -> bool) -> 'a list -> int
+
   val index : 'a eq -> 'a -> 'a list -> int
   (** [index] returns the 1st index of an element in a list (counting from 1). *)