let read_file_rev name =
    let ic = open_in name in
    let try_read () =
        try Some (input_line ic)
        with End_of_file -> None in
    let rec loop acc =
        match try_read () with
        | Some s -> loop (s::acc)
        | None -> close_in ic; acc in
    List.rev (loop [])


let stepped_on_a_tree line pos =
    let width = String.length line in
    String.get line (pos mod width) = '#'


let rec skiii count pos_x pos_y right down = function
    | [] -> count
    | h :: t -> if pos_y mod down <> 0 then skiii count pos_x (pos_y + 1) right down t
                else skiii (count + (if (stepped_on_a_tree h pos_x) then 1 else 0))
                     (pos_x + right)
                     (pos_y + 1)
                     right down t

let part1 l =
    skiii 0 0 0 3 1 l

let part2 l =
    (skiii 0 0 0 1 1 l) *
    (skiii 0 0 0 3 1 l) *
    (skiii 0 0 0 5 1 l) *
    (skiii 0 0 0 7 1 l) *
    (skiii 0 0 0 1 2 l)


let do_count filename = function
    | 1 -> read_file_rev filename |> part1
    | 2 -> read_file_rev filename |> part2


let () =
    assert ((do_count "day3.test" 1) = 7)
let () =
    print_endline (string_of_int (do_count "day3.input" 1))

let () =
    assert ((do_count "day3.test" 2) = 336)
let () =
    print_endline (string_of_int (do_count "day3.input" 2))