#!/usr/bin/env elixir
defmodule Day11 do
  def part1(stones) do
    1..25
    |> Enum.reduce(stones, fn _, stones -> blink(stones) end)
    |> Enum.count()
  end

  def blink([]), do: []
  def blink([0 | rest]), do: [1 | blink(rest)]

  def blink([stone | rest]) do
    string = Integer.to_string(stone)
    size = byte_size(string)

    if rem(size, 2) == 0 do
      string
      |> String.split_at(div(size, 2))
      |> Tuple.to_list()
      |> Enum.map(&String.to_integer/1)
      |> Kernel.++(blink(rest))
    else
      [stone |> Kernel.*(2024) | blink(rest)]
    end
  end

  # probably have to track the changes rather than keep the whole list?
  def part2(stones) do
    1..75
    |> Enum.reduce(stones, fn _, stones -> blink(stones) end)
    |> Enum.count()
  end

  def input do
    with [input_filename] <- System.argv(),
         {:ok, input} <- File.read(input_filename) do
      input
      |> String.split([" ", "\n"], trim: true)
      |> Enum.map(&String.to_integer/1)
    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 day11.exs input_filename")
  end
end

Day11.run()