let count_chars s =
  let t = Hashtbl.create 8 in
  for i = 0 to String.length s - 1 do
    let c = s.[i] in
    let n = match Hashtbl.find_opt t c with
            | Some v -> v + 1
            | None -> 1
    in
    Hashtbl.replace t c n
  done;
  t

let max_count t =
  Hashtbl.fold (fun _ v acc -> if v > acc then v else acc) t 0

;;

max_count (count_chars "abracadabra")