let usage args =
  let err = Format.sprintf "usage: %s %s" Sys.argv.(0) args in
  print_endline err;
  exit 1

let usage_default day = usage (string_of_int day ^ " <part> <infile>")

let main day mains = function
  | [part; input] -> mains.(int_of_string part - 1) input
  | _             -> usage_default day


type 'a monoid =
  {id: 'a;
   op: 'a -> 'a -> 'a;
   op_name: string;
   pp: Format.formatter -> 'a -> unit}

let mult = {id = 1; op = ( * ); op_name = "*"; pp = Format.pp_print_int}
let add  = {id = 0; op = ( + ); op_name = "+"; pp = Format.pp_print_int}

let fold_list {op; id; _} = List.fold_left op id

let%test _ = fold_list add  [] = 0
let%test _ = fold_list add  [1;2;3;4] = 10
let%test _ = fold_list mult [1;2;3;4] = 24


let print_fold mon xs =
  let res = List.fold_left mon.op mon.id xs in
  let rec go fmt = function
    | []    -> Format.fprintf fmt "%a" mon.pp mon.id
    | [x]   -> Format.fprintf fmt "%a" mon.pp x
    | x::xs -> Format.fprintf fmt "%a %s %a" mon.pp x mon.op_name go xs in
  Format.printf "%a = %a\n" go xs mon.pp res

let print_prod = print_fold mult
let print_sum  = print_fold add

let%expect_test _ =
  print_prod [];
  [%expect{| 1 = 1 |}]

let%expect_test _ =
  print_prod [1];
  [%expect{| 1 = 1 |}]

let%expect_test _ =
  print_prod [1;2;3;4];
  [%expect{| 1 * 2 * 3 * 4 = 24 |}]

let%expect_test _ =
  print_sum [];
  [%expect{| 0 = 0 |}]

let%expect_test _ =
  print_sum [1];
  [%expect{| 1 = 1 |}]

let%expect_test _ =
  print_sum [1;2;3;4];
  [%expect{| 1 + 2 + 3 + 4 = 10 |}]