xapi-stdext-std: add Listext.List.find_first_sorted

Useful to get the lowest or highest element in a list in linear time, as
it avoid sorting the whole list.

Signed-off-by: Pau Ruiz Safont <[email protected]>

CHANGES.md

@@ -1,3 +1,6 @@
+## Unreleased
+- std: add Listext.List.find_first_sorted
+
 ## v4.23.0 (30-Oct-2023)
 - unix: fix blkgetsize return type mismatch (CA-382014)
 - unix: add function to recursively remove files

lib/xapi-stdext-std/listext.ml

@@ -196,4 +196,14 @@ module List = struct
         aux ((upper - 1) :: accu) (upper - 1)
     in
     aux []
+
+  let find_first_sorted compare =
+    let rec loop ref = function
+      | [] ->
+          ref
+      | x :: xs ->
+          let ref = match compare ref x with c when c <= 0 -> ref | _ -> x in
+          loop ref xs
+    in
+    function [] -> None | ref :: xs -> Some (loop ref xs)
 end

lib/xapi-stdext-std/listext.mli

@@ -17,6 +17,10 @@ module List : sig
   val is_sorted : ('a -> 'a -> int) -> 'a list -> bool
   (** [is_sorted cmp l] returns whether [l] is sorted according to [cmp]. *)
 
+  val find_first_sorted : ('a -> 'a -> int) -> 'a list -> 'a option
+  (** [find_first_sorted cmp l] returns the first element in [l] according to
+      the sort order of [cmp], or [None] if the list is empty. *)
+
   (** {1 Iterators} *)
 
   val take : int -> 'a list -> 'a list

lib/xapi-stdext-std/listext_test.ml

@@ -180,6 +180,36 @@ let test_sub =
   let tests = List.map test specs in
   ("sub", tests)
 
+let test_find_first_sorted =
+  let ascending = (Int.compare, "ascending") in
+  let descending = ((fun a b -> Int.compare b a), "descending") in
+  let specs =
+    [
+      (ascending, [], None)
+    ; (ascending, [1; 2; 3; 4; 5], Some 1)
+    ; (ascending, [2; 3; 1; 5; 4], Some 1)
+    ; (descending, [], None)
+    ; (descending, [1; 2; 3; 4; 5], Some 5)
+    ; (descending, [2; 3; 1; 5; 4], Some 5)
+    ]
+  in
+  let test ((cmp, cmp_name), input, expected) =
+    let name =
+      Printf.sprintf "find_first_sorted %s of [%s]" cmp_name
+        (String.concat "; " (List.map string_of_int input))
+    in
+    test_option (Listext.find_first_sorted cmp) (name, input, expected)
+  in
+  let tests = List.map test specs in
+  ("find_first_sorted", tests)
+
 let () =
   Alcotest.run "Listext"
-    [test_iteri_right; test_take; test_drop; test_chop; test_sub]
+    [
+      test_iteri_right
+    ; test_take
+    ; test_drop
+    ; test_chop
+    ; test_sub
+    ; test_find_first_sorted
+    ]