lib/day01.ml


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45

(*
 * SPDX-FileCopyrightText: Copyright 2025 Alexandre Jesus <https://adbjesus.com>
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *)

let parse_rotation s =
  let num = lazy (int_of_string (String.sub s 1 (String.length s - 1))) in
  match s.[0] with
  | 'R' -> Lazy.force num
  | 'L' -> -Lazy.force num
  | s -> failwith (Printf.sprintf "Unknown rotation %c" s)

let parse_rotations ch =
  In_channel.input_all ch
  |> String.trim
  |> String.split_on_char '\n'
  |> List.map parse_rotation

let part1 ch =
  let m = 100 in
  let s = 50 in
  parse_rotations ch
  |> List.fold_left
       (fun (a, p) r ->
         let nxt = (p + r) mod m in
         let inc = if nxt = 0 then 1 else 0 in
         (a + inc, nxt))
       (0, s)
  |> fst
  |> Printf.printf "%d\n"

let part2 ch =
  let m = 100 in
  let s = 50 in
  parse_rotations ch
  |> List.fold_left
       (fun (a, p) r ->
         let nxt = p + r in
         let inc = (abs nxt / m) + if p > 0 && nxt <= 0 then 1 else 0 in
         let nxt = ((nxt mod m) + m) mod m in
         (a + inc, nxt))
       (0, s)
  |> fst
  |> Printf.printf "%d\n"