defmodule Podbox.Download.Storage do
  @moduledoc """
  Responsible for storing files on the local filesystem.
  """

  defstruct [:asset, :complete_path, :complete_root, :incomplete_root, :incomplete_path, :io]
  alias Podbox.{Download.Asset, Download.InvalidStateError, Download.PosixError}

  @type t :: %__MODULE__{
          asset: nil | Asset.t(),
          complete_path: nil | Path.t(),
          complete_root: nil | Path.t(),
          incomplete_root: nil | Path.t(),
          incomplete_path: nil | Path.t(),
          io: nil | File.io_device()
        }

  @doc """
  Initialise storage for a specific asset
  """
  @spec init(Asset.t(), Keyword.t()) :: t
  def init(asset, opts \\ []) do
    config =
      storage_config()
      |> Map.merge(Map.new(opts))

    incomplete_path = generate_subpath(config.incomplete_root, asset.id)
    complete_path = generate_subpath(config.complete_root, asset.id)

    %__MODULE__{
      asset: asset,
      complete_path: complete_path,
      complete_root: config.complete_root,
      incomplete_path: incomplete_path,
      incomplete_root: config.incomplete_root
    }
  end

  @doc """
  Checks and returns the storage state.
  """
  @spec status(t) ::
          :ok | :error | {:complete, non_neg_integer()} | {:incomplete, non_neg_integer()}
  def status(storage) do
    complete_exists? = File.exists?(storage.complete_path)
    incomplete_exists? = File.exists?(storage.incomplete_path)

    cond do
      complete_exists? && incomplete_exists? -> :error
      complete_exists? -> {:complete, File.stat!(storage.complete_path).size}
      incomplete_exists? -> {:incomplete, File.stat!(storage.incomplete_path).size}
      true -> :ok
    end
  end

  @doc """
  Ensures that the directory needed to store the partial asset is present, and
  ready to be written to.
  """
  @spec prepare(t) :: {:ok, t} | {:error, any}
  def prepare(storage) do
    incomplete_dir = Path.dirname(storage.incomplete_path)

    with :ok <- handle_posix_failure(incomplete_dir, &File.mkdir_p/1, :mkdir),
         {:ok, io} <-
           handle_posix_failure(
             storage.incomplete_path,
             &File.open(&1, [:write]),
             {:open, [:write]}
           ) do
      {:ok, %{storage | io: io}}
    end
  end

  @doc """
  Write bytes to an incomplete file.
  """
  @spec append(t, binary) :: {:ok, t} | {:error, any}
  def append(storage, bytes) when not is_nil(storage.io) do
    IO.binwrite(storage.io, bytes)
    {:ok, storage}
  end

  @doc """
  All contents are written, now move the file to complete.
  """
  @spec commit(t) :: {:ok, t} | {:error, any}
  def commit(storage) do
    complete_dir = Path.dirname(storage.complete_path)

    with :ok <- handle_posix_failure(complete_dir, &File.mkdir_p/1, :mkdir),
         :ok <-
           handle_posix_failure(
             storage.incomplete_path,
             fn _ -> File.close(storage.io) end,
             :close
           ),
         :ok <-
           handle_posix_failure(
             storage.incomplete_path,
             &File.rename(&1, storage.complete_path),
             {:move, to: storage.complete_path}
           ),
         :ok <- prune(storage.incomplete_path, storage.incomplete_root) do
      {:ok, %{storage | io: nil}}
    end
  end

  @doc """
  Remove all artefacts from the filesystem.
  """
  @spec destroy(t) :: :ok | {:error, any}
  def destroy(storage) when is_nil(storage.io) do
    with :ok <- prune(storage.incomplete_path, storage.incomplete_root) do
      prune(storage.complete_path, storage.complete_root)
    end
  end

  def destroy(storage) do
    with :ok <- File.close(storage.io) do
      destroy(%{storage | io: nil})
    end
  end

  @doc """
  Read completed file contents.
  """
  @spec read_completed(t) :: {:ok, binary} | {:error, any}
  def read_completed(storage) do
    with :ok <- assert_complete(storage) do
      handle_posix_failure(storage.complete_path, &File.read/1, :read)
    end
  end

  defp assert_complete(storage) do
    case status(storage) do
      {:complete, _} ->
        :ok

      {:incomplete, _} ->
        {:error,
         InvalidStateError.exception(
           asset_id: storage.asset.id,
           current_state: :incomplete,
           expected_state: :complete
         )}

      :error ->
        {:error,
         InvalidStateError.exception(
           asset_id: storage.asset.id,
           current_state: :error,
           expected_state: :complete
         )}

      :ok ->
        {:error,
         InvalidStateError.exception(
           asset_id: storage.asset.id,
           current_state: :none,
           expected_state: :complete
         )}
    end
  end

  defp generate_subpath(
         root,
         <<a::binary-size(1), b::binary-size(1), c::binary-size(1), _::binary>> = id
       ) do
    Path.join(root, [a, b, c, id])
  end

  defp storage_config do
    config =
      :podbox
      |> Application.get_env(__MODULE__, [])

    %{
      complete_root: Keyword.fetch!(config, :complete),
      incomplete_root: Keyword.fetch!(config, :incomplete)
    }
  end

  defp handle_posix_failure(path, callback, activity) when is_function(callback, 1) do
    with {:error, posix} <- callback.(path) do
      {:error, PosixError.exception(file_path: path, posix: posix, activity: activity)}
    end
  end

  defp prune(path, down_to) when is_binary(path) do
    down_to_count =
      down_to
      |> Path.split()
      |> Enum.count()

    to_prune =
      path
      |> Path.split()
      |> Enum.drop(down_to_count)
      |> Enum.reverse()

    prune(to_prune, down_to)
  end

  defp prune([], _down_to), do: :ok

  defp prune([_ | remaining] = to_prune, down_to) do
    path =
      to_prune
      |> Enum.reverse()
      |> then(&[down_to | &1])
      |> Path.join()

    cond do
      File.dir?(path) ->
        case File.rmdir(path) do
          :ok ->
            prune(remaining, down_to)

          {:error, :eexist} ->
            :ok

          {:error, posix} ->
            {:error, PosixError.exception(file_path: path, posix: posix, activity: :delete)}
        end

      File.exists?(path) ->
        with :ok <- handle_posix_failure(path, &File.rm/1, :delete) do
          prune(remaining, down_to)
        end

      true ->
        prune(remaining, down_to)
    end
  end
end