aoc2020/day3.ml

type map = string array

let (.%[]) str i = str.[i mod String.length str]
let (+=) r x = r := !r + x

let iter_map ?(step_x=1) ?(step_y=1) f arr =
  let i = ref 0 in let j = ref 0 in
  while !i < Array.length arr do
    f arr.(!i).%[!j];
    i += step_x; j += step_y
  done


let read_file = Bracket.infile_lines ~line:Fun.id ~of_seq:Array.of_seq


let count_trees ?step_x ?step_y map =
  let count = ref 0 in
  iter_map ?step_x ?step_y (fun c -> if c = '#' then incr count) map;
  !count

let main1 input =
  Format.printf "%d\n" (count_trees ~step_y:3 (read_file input))

let main2 input =
  let map = read_file input in
  Misc.print_prod
    [count_trees map;
     count_trees map ~step_y:3;
     count_trees map ~step_y:5;
     count_trees map ~step_y:7;
     count_trees map ~step_x:2]

let main = Misc.main 3 [|main1; main2|]

let%expect_test _ =
  main1 "../../data/day3";
  [%expect{| 244 |}]

let%expect_test _ =
  main2 "../../data/day3";
  [%expect{| 90 * 244 * 97 * 92 * 48 = 9406609920 |}]