type t = { mutable words: int; mutable chars: int; mutable lines: int; mutable in_word: bool ; mutable finished: bool};;

  let () =
    match Core.Sys.argv with
    | [| prog_name |] -> Core.Printf.eprintf "Usage: %s file1 file2 ...\n" prog_name
    | _ -> (
      let args = Core.Array.slice Core.Sys.argv 1 @@ Core.Array.length Core.Sys.argv in
      let buf_size = 65536 in (* 64 KB -> Caml IO buffer size *)
      let buf = Core.Bytes.create buf_size in
      Core.Array.fold args ~init:() ~f:(fun _ file ->
        Core.In_channel.with_file file ~f:(fun in_ch ->
          let c = { words = 0; chars = 0; lines = 0; in_word = false; finished = false } in
          let set_words () = if c.in_word then c.words <- c.words + 1 in
          while not c.finished do
            let len = Core.In_channel.input in_ch ~buf ~pos:0 ~len:buf_size |> Core.Int.to_int in
            if len > 0 then (
              for i = 0 to (len - 1) do
                match (Core.Caml.Bytes.get buf i) with
                | ' ' | '\t' | '\r' -> (c.chars <- c.chars + 1; set_words (); c.in_word <- false)
                | '\n'              -> (c.chars <- c.chars + 1; set_words (); c.in_word <- false; c.lines <- c.lines + 1)
                | _                 -> (c.chars <- c.chars + 1; c.in_word <- true)
              done
            ) else ( c.finished <- true )
          done;
          set_words ();
          Core.Printf.printf "%s -> lines: %d, words: %d, characters: %d\n" file c.lines c.words c.chars)))
  ;;

  $ ls -lh test_file.txt 
  -rw-r--r--  1 user  group   508M Oct 16 12:01 test_file.txt

  $ time wc test_file.txt 
  4863460 54621760 532480000 test_file.txt

  real 0m3.368s
  user 0m3.137s
  sys  0m0.195s

  #Ocaml version:
  $ time ./wcl.native test_file.txt 
  test_file.txt -> lines: 4863460, words: 54621760, characters: 532480000

  real 0m2.480s
  user 0m2.345s
  sys  0m0.123s