(* Common util. Should probably put this in a module or something.. *)
let get_one_line file =
  try Some (input_line file) with
    End_of_file -> None

let get_lines file =
  let rec input lines =
    match get_one_line file with
      Some line -> input (line :: lines)
    | None -> List.rev lines
  in input []

let read_file file =
  let channel = open_in(file) in
  get_lines channel

let explode s = (* convert a string to a list of chars *)
  let rec exp i l =
   if i < 0 then l else exp (i - 1) (s.[i] :: l) in
  exp (String.length s - 1) [];;

(* --- *)

let wrapped_y y map = (* wrap the y (horizontal) coordinate when off the map *)
  y mod (List.length (List.nth map 1))

let read_cell map x y = (* return Some char at these coordinates, or None if below the map *)
  if (x < List.length map)
  then Some (List.nth (List.nth map x) (wrapped_y y map))
  else None

let rec go map position slope = (* move across the map, counting trees *)
  let (x, y) = position in
  let (dy, dx) = slope in (* suddenly regretting having x be the vertical axis.. *)
  match read_cell map x y with
  | None -> 0 (* off the map! stop counting *)
  | Some '#' -> 1 + go map (x+dx, y+dy) slope
  | Some a -> go map (x+dx, y+dy) slope

let map = read_file "day3.txt" |> List.map explode
let slopes = [(1, 1); (3, 1); (5, 1); (7, 1); (1, 2)]
let tree_counts = slopes |> List.map (fun slope -> go map (0, 0) slope);;

List.iter (fun count -> Printf.printf "Number of trees: %i\n" count) tree_counts;;

let product = List.fold_left ( * ) 1 tree_counts;;
Printf.printf "Product: %i\n" product