open Graph.Pack.Digraph;;

let read_graph filename =

  let string_of_char = String.make 1 in

  let read_int in_ch =
    let is_valid ch = ch >= '0' && ch <= '9' in
    let ioc ch = (int_of_char ch)-(int_of_char '0') in

    let res = ref 0 in
    let tmp = ref '0' in
    try
      while true do
    tmp := (input_char in_ch);
    if (is_valid !tmp) then
      begin
        res := !res * 10 + (ioc !tmp);
      end
    else
      raise Exit;
      done;
      !res;
    with
    Exit -> !res; in

  let in_ch = open_in filename in
  let nb_of_vertex = read_int in_ch in
  let arr_of_vertex = Array.make nb_of_vertex (V.create 0) in
  let graph = create () in
  begin
    for i=0 to (nb_of_vertex-1) do
      arr_of_vertex.(i)<-(V.create i);
      add_vertex graph arr_of_vertex.(i);
    done;
  end;
  try
    while true do
      let f1 = read_int in_ch in
      let f2 = read_int in_ch in
      add_edge graph (arr_of_vertex.(f1)) (arr_of_vertex.(f2));
    done;
    graph;
  with
      End_of_file -> graph;
;;

let _ = display_with_gv (read_graph "blog.txt");;