#!/usr/bin/env elixir
defmodule Day15 do
  def part1({warehouse, robot, moves}) do
    attempt_moves(moves, robot, warehouse)
    |> Enum.map(fn
      {{x, y}, :box} -> x + y * 100
      _ -> 0
    end)
    |> Enum.sum()
  end

  def attempt_moves([], _robot, warehouse), do: warehouse

  def attempt_moves([move | next_moves], robot, warehouse) do
    next = move.(robot)

    case Map.fetch(warehouse, next) do
      :error ->
        attempt_moves(next_moves, next, warehouse)

      {:ok, :wall} ->
        attempt_moves(next_moves, robot, warehouse)

      {:ok, :box} ->
        case get_pushable_boxes(next, move, warehouse, [next]) do
          :wall ->
            attempt_moves(next_moves, robot, warehouse)

          boxes ->
            warehouse =
              Enum.reduce(boxes, warehouse, fn box, warehouse ->
                warehouse |> Map.delete(box) |> Map.put(move.(box), :box)
              end)

            attempt_moves(next_moves, next, warehouse)
        end
    end
  end

  def get_pushable_boxes(box, move, warehouse, pushable_boxes) do
    next = move.(box)

    case Map.fetch(warehouse, next) do
      :error -> pushable_boxes
      {:ok, :wall} -> :wall
      {:ok, :box} -> get_pushable_boxes(next, move, warehouse, [next | pushable_boxes])
    end
  end

  def part2(_input) do
    :boring_skipped
  end

  def input do
    with [input_filename] <- System.argv(),
         {:ok, input} <- File.read(input_filename) do
      [raw_map, raw_moves] = String.split(input, "\n\n")

      {_, _y, robot, map} =
        for <<char::binary-1 <- raw_map <> "\n">>, reduce: {0, 0, nil, %{}} do
          {x, y, robot, map} ->
            case char do
              "." -> {x + 1, y, robot, map}
              "#" -> {x + 1, y, robot, Map.put(map, {x, y}, :wall)}
              "O" -> {x + 1, y, robot, Map.put(map, {x, y}, :box)}
              "@" -> {x + 1, y, {x, y}, map}
              "\n" -> {0, y + 1, robot, map}
            end
        end

      moves =
        for <<char::binary-1 <- String.replace(raw_moves, "\n", "")>> do
          case char do
            "^" -> fn {x, y} -> {x, y - 1} end
            "v" -> fn {x, y} -> {x, y + 1} end
            "<" -> fn {x, y} -> {x - 1, y} end
            ">" -> fn {x, y} -> {x + 1, y} end
          end
        end

      {map, robot, moves}
    else
      _ -> :error
    end
  end

  #######################
  # HERE BE BOILERPLATE #
  #######################

  def run do
    case input() do
      :error -> print_usage()
      input -> run_parts_with_timer(input)
    end
  end

  defp run_parts_with_timer(input) do
    run_with_timer(1, fn -> part1(input) end)
    run_with_timer(2, fn -> part2(input) end)
  end

  defp run_with_timer(part, fun) do
    {time, result} = :timer.tc(fun)
    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
    IO.puts("#{inspect(result)}\n")
  end

  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
  defp format_time(μsec), do: "#{μsec / 1_000_000}s"

  defp print_usage do
    IO.puts("Usage: elixir day15.exs input_filename")
  end
end

Day15.run()