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

let parse_data ch =
  In_channel.input_all ch
  |> String.trim
  |> String.split_on_char '\n'
  |> List.mapi (fun i s -> (i, s))
  |> List.to_seq
  |> Seq.flat_map (fun (i, s) ->
      String.to_seq s
      |> Seq.mapi (fun j v -> (j, v = '@'))
      |> Seq.filter_map (fun (j, p) -> if p then Some ((i, j), 0) else None))
  |> Hashtbl.of_seq

let neighs =
  [ (-1, -1); (-1, 0); (-1, 1); (0, -1); (0, 1); (1, -1); (1, 0); (1, 1) ]

let neighs (i, j) = List.map (fun (a, b) -> (i + a, j + b)) neighs
let count tbl k = (k, neighs k |> List.filter (Hashtbl.mem tbl) |> List.length)
let counts tbl = Hashtbl.to_seq_keys tbl |> Seq.map (count tbl)

let part1 ch =
  parse_data ch
  |> counts
  |> Seq.filter (fun (_, v) -> v < 4)
  |> Seq.length
  |> Printf.printf "%d\n"

let part2 ch =
  let tbl = parse_data ch |> counts |> Hashtbl.of_seq in
  let rec fn acc s =
    let test k =
      match Hashtbl.find_opt tbl k with
      | None -> None
      | Some v when v = 4 ->
          Hashtbl.remove tbl k;
          Some k
      | Some v ->
          Hashtbl.replace tbl k (v - 1);
          None
    in
    match s with
    | [] -> acc
    | h :: t ->
        neighs h
        |> List.filter_map test
        |> List.fold_left (fun (acc, s) k -> (acc + 1, k :: s)) (acc, t)
        |> fun (acc, s) -> fn acc s
  in
  Hashtbl.to_seq tbl
  |> List.of_seq
  |> List.filter_map (fun (k, v) ->
      if v < 4 then (
        Hashtbl.remove tbl k;
        Some k)
      else None)
  |> (fun s -> fn (List.length s) s)
  |> Printf.printf "%d\n"