#!/usr/bin/env elixir
defmodule Day9 do
  def part1(input) do
    {size, _, spaces, _, disk, files} =
      Enum.reduce(input, {0, 0, 0, true, %{}, []}, fn len,
                                                      {idx, id, spaces, file?, disk, files} ->
        if file? do
          disk = Enum.into(idx..(idx + len - 1), disk, fn i -> {i, id} end)
          files = if file?, do: for(_ <- 1..len, do: id) ++ files, else: files
          {idx + len, id + 1, spaces, not file?, disk, files}
        else
          {idx + len, id, spaces + len, not file?, disk, files}
        end
      end)

    for i <- 0..(size - spaces - 1), reduce: {0, files} do
      {checksum, files} ->
        case Map.fetch(disk, i) do
          {:ok, block} ->
            {checksum + block * i, files}

          :error ->
            [block | files] = files
            {checksum + block * i, files}
        end
    end
    |> elem(0)
  end

  # Wrote a crazy long implementation, but was fatally flawed because it didn't consider that
  # moving a block needs to leave contiguous spaces behind if there were spaces on either side.
  # Maybe can store the spaces as ranges, then merge the ranges?
  def part2(_input) do
    :ok
  end

  def input do
    with [input_filename] <- System.argv(),
         {:ok, input} <- File.read(input_filename) do
      for <<c::binary-1 <- String.trim_trailing(input, "\n")>>, do: String.to_integer(c)
    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 day9.exs input_filename")
  end
end

Day9.run()