defmodule Plug.AccessLog.Logfiles do
  @moduledoc """
  Logfile agent.
  """

  require Logger


  @doc """
  Starts the logfile agent.
  """
  @spec start_link() :: Agent.on_start
  def start_link(), do: Agent.start_link(fn -> %{} end, name: __MODULE__)

  @doc """
  Returns the logfile device for writing.
  """
  @spec get(String.t) :: File.io_device | nil
  def get(logfile) do
    case get_device(logfile) do
      { nil, nil } -> open(logfile)

      { device, inode } when is_pid(device) ->
        get_or_reopen({ device, inode }, logfile)

      { other_device, _ } -> other_device
    end
  end

  @doc """
  Opens a logfile for future writing.
  """
  @spec open(String.t) :: File.io_device | nil
  def open(logfile) do
    case File.open(logfile, [ :append, :utf8 ]) do
      { :ok, device }   -> set(logfile, device)
      { :error, error } ->
        log_open_error(logfile, error)
        nil
    end
  end

  @doc """
  Replaces the io_device associated with a logfile.

  The already registered (if any) io_device will be closed
  using `File.close/1`.

  The new io_device associated with the logfile will be returned.
  """
  @spec replace(String.t, File.io_device) :: File.io_device
  def replace(logfile, new_device) do
    case get_device(logfile) do
      { nil, nil }      -> nil
      { old_device, _ } -> File.close(old_device)
    end

    logstate = { new_device, inode(logfile) }

    Agent.update(__MODULE__, &Map.put(&1, logfile, logstate))

    new_device
  end

  @doc """
  Sets the io_device associated with a logfile.

  If a logfile is already associated with an io_device the passed one
  will be closed using `File.close/1`.

  The new io_device associated with the logfile will be returned.
  """
  @spec set(String.t, File.io_device) :: File.io_device
  def set(logfile, new_device) do
    logstate = { new_device, inode(logfile) }

    Agent.update(__MODULE__, &Map.put_new(&1, logfile, logstate))

    case get_device(logfile) do
      { ^new_device, _ } -> new_device
      { old_device,  _ } ->
        File.close(new_device)
        old_device
    end
  end


  # Internal utility methods

  defp get_device(logfile) do
    Agent.get(__MODULE__, &Map.get(&1, logfile, { nil, nil }))
  end

  defp get_or_reopen({ device, inode }, logfile) do
    case Process.alive?(device) && inode == inode(logfile) do
      true  -> device
      false ->
        case File.open(logfile, [ :append, :utf8 ]) do
          { :ok, device }   -> replace(logfile, device)
          { :error, error } ->
            log_open_error(logfile, error)
            nil
        end
    end
  end

  defp inode(logpath) do
    case File.stat(logpath) do
      { :ok, stat } -> stat.inode
      _             -> nil
    end
  end

  defp log_open_error(logfile, error) do
    Logger.error "Failed to open logfile #{ inspect logfile } for writing: #{ inspect error }"
  end
end