let add_t (a, b) (c, d) = (a + c, b + d)
let two_to_pov x = int_of_float (2. ** (float_of_int x))


let seat_of_ticket ticket =
    let len = (String.length ticket) - 1 in
    let rec aux idx res s =
        let current = abs (len - idx - 3) in
        if idx < 0 then res
        else match s.[idx] with
             | 'B' -> aux (idx - 1) (add_t res (two_to_pov current, 0)) s
             | 'R' -> aux (idx - 1) (add_t res (0, two_to_pov (3 - current))) s
             | _ -> aux (idx - 1) res s in
    let (row, col) = aux len (0, 0) ticket in
    (row * 8) + col


let try_read f =
    try Some (input_line f)
    with End_of_file -> None

let part1 filename =
    let f = open_in filename in
    let rec loop max =
        match try_read f with
        | Some t -> let seat_id = seat_of_ticket t in
                    if seat_id > max then loop seat_id else loop max
        | None -> close_in f; max in
    loop 0

let part2 filename =
    let f = open_in filename in
    let rec read_file_to_list l =
        match try_read f with
        | Some s -> read_file_to_list ((seat_of_ticket s) :: l)
        | None -> close_in f; l in
    let find (prev_id, bingo) seat_id =
        (seat_id,
         Option.fold ~none:(if seat_id = prev_id + 2 then Some (seat_id - 1) else None)
                     ~some:(fun x -> Some x) bingo) in
    let (_, ticket_id) = read_file_to_list []
                         |> List.sort compare
                         |> List.fold_left (find) (-1, None) in
    Option.get ticket_id

let () = print_endline (string_of_int (part1 "day5.input"))
let () = print_endline (string_of_int (part2 "day5.input"))