defmodule LoggerJSON do
  @moduledoc """
  JSON console back-end for Elixir Logger.

  It can be used as drop-in replacement for default `:console` Logger back-end in cases where you use
  use Google Cloud Logger or other JSON-based log collectors.

  ## Log Format

  Output JSON is compatible with
  [Google Cloud Logger format](https://cloud.google.com/logging/docs/reference/v1beta3/rest/v1beta3/LogLine) with
  additional properties in `serviceLocation` and `metadata` objects:

    ```json
    {
       "time":"2017-04-09T17:52:12.497Z",
       "severity":"DEBUG",
       "sourceLocation":{
          "moduleName":"Elixir.LoggerJSONTest",
          "line":62,
          "functionName":"test metadata can be configured to :all/1",
          "file":"/Users/andrew/Projects/logger_json/test/unit/logger_json_test.exs"
       },
       "metadata":{
          "user_id":11,
          "dynamic_metadata":5
       },
       "logMessage":"hello"
    }
    ```

  ## Encoders support

  You can replace default Poison encoder with other module that supports `encode!/1` function. This can be even used
  as custom formatter callback.

  Popular Poison alternatives:

   * [exjsx](https://github.com/talentdeficit/exjsx).
   * [elixir-json](https://github.com/cblage/elixir-json) - native Elixir encoder implementation.

  If your application is performance-critical, take look at [jiffy](https://github.com/davisp/jiffy).

  ## Dynamic configuration

  For dynamically configuring the endpoint, such as loading data
  from environment variables or configuration files, LoggerJSON provides
  an `:on_init` option that allows developers to set a module, function
  and list of arguments that is invoked when the endpoint starts.

    ```elixir
    config :logger_json, :backend,
      on_init: {YourApp.Logger, :load_from_system_env, []}
    ```
  """
  @behaviour :gen_event

  defstruct [metadata: nil, level: nil, device: nil, max_buffer: nil,
             buffer_size: 0, buffer: [], ref: nil, output: nil,
             json_encoder: nil, on_init: nil]

  def init(__MODULE__) do
    config = get_env()
    device = Keyword.get(config, :device, :user)

    if Process.whereis(device) do
      {:ok, init(config, %__MODULE__{})}
    else
      {:error, :ignore}
    end
  end
  def init({__MODULE__, opts}) when is_list(opts) do
    config = configure_merge(get_env(), opts)
    {:ok, init(config, %__MODULE__{})}
  end

  def handle_call({:configure, options}, state) do
    config = configure_merge(get_env(), options)
    put_env(config)
    {:ok, :ok, init(config, state)}
  end

  def handle_event({_level, gl, _event}, state) when node(gl) != node() do
    {:ok, state}
  end

  def handle_event({level, _gl, {Logger, msg, ts, md}}, state) do
    %{level: log_level,
      ref: ref,
      buffer_size: buffer_size,
      max_buffer: max_buffer} = state

    cond do
      not meet_level?(level, log_level) ->
        {:ok, state}
      is_nil(ref) ->
        {:ok, log_event(level, msg, ts, md, state)}
      buffer_size < max_buffer ->
        {:ok, buffer_event(level, msg, ts, md, state)}
      buffer_size === max_buffer ->
        state = buffer_event(level, msg, ts, md, state)
        {:ok, await_io(state)}
    end
  end

  def handle_event(:flush, state) do
    {:ok, flush(state)}
  end

  def handle_event(_, state) do
    {:ok, state}
  end

  def handle_info({:io_reply, ref, msg}, %{ref: ref} = state) do
    {:ok, handle_io_reply(msg, state)}
  end

  def handle_info({:DOWN, ref, _, pid, reason}, %{ref: ref}) do
    raise "device #{inspect pid} exited: " <> Exception.format_exit(reason)
  end

  def handle_info(_, state) do
    {:ok, state}
  end

  def code_change(_old_vsn, state, _extra) do
    {:ok, state}
  end

  def terminate(_reason, _state) do
    :ok
  end

  # Helpers

  # Somehow Logger.Watcher is started before Application loads configuration
  # so we use default value here and expect back-end to be reconfigured later.
  defp get_env,
    do: Application.get_env(:logger_json, :backend, [])

  defp put_env(env),
    do: Application.put_env(:logger_json, :backend, env)

  defp meet_level?(_lvl, nil),
    do: true
  defp meet_level?(lvl, min),
    do: Logger.compare_levels(lvl, min) != :lt

  defp init(config, state) do
    config =
      case Keyword.fetch(config, :on_init) do
        {:ok, {mod, fun, args}} ->
          {:ok, conf} = apply(mod, fun, [config | args])
          conf
        {:ok, other} ->
          raise ArgumentError, "invalid :on_init option for :logger_json application. " <>
                               "Expected a tuple with module, function and args, got: #{inspect other}"
        :error ->
          config
      end

    json_encoder = Keyword.get(config, :json_encoder)
    level = Keyword.get(config, :level)
    device = Keyword.get(config, :device, :user)
    max_buffer = Keyword.get(config, :max_buffer, 32)
    metadata =
      config
      |> Keyword.get(:metadata, [])
      |> configure_metadata()

    %{state | metadata: metadata, level: level, device: device,
              max_buffer: max_buffer, json_encoder: json_encoder}
  end

  defp configure_metadata([]),
    do: []
  defp configure_metadata(:all),
    do: :all
  defp configure_metadata(metadata) when is_list(metadata),
    do: Enum.reverse(metadata)

  defp configure_merge(env, options),
    do: Keyword.merge(env, options, fn _key, _v1, v2 -> v2 end)

  defp log_event(level, msg, ts, md, %{device: device} = state) do
    output = format_event(level, msg, ts, md, state)
    %{state | ref: async_io(device, output), output: output}
  end

  defp buffer_event(level, msg, ts, md, state) do
    %{buffer: buffer, buffer_size: buffer_size} = state
    buffer = [buffer | format_event(level, msg, ts, md, state)]
    %{state | buffer: buffer, buffer_size: buffer_size + 1}
  end

  defp async_io(name, output) when is_atom(name) do
    case Process.whereis(name) do
      device when is_pid(device) ->
        async_io(device, output)
      nil ->
        raise "no device registered with the name #{inspect name}"
    end
  end
  defp async_io(device, output) when is_pid(device) do
    ref = Process.monitor(device)
    send(device, {:io_request, self(), ref, {:put_chars, :unicode, output}})
    ref
  end

  defp await_io(%{ref: nil} = state),
    do: state
  defp await_io(%{ref: ref} = state) do
    receive do
      {:io_reply, ^ref, :ok} ->
        handle_io_reply(:ok, state)
      {:io_reply, ^ref, error} ->
        error
        |> handle_io_reply(state)
        |> await_io()
      {:DOWN, ^ref, _, pid, reason} ->
        raise "device #{inspect pid} exited: " <> Exception.format_exit(reason)
    end
  end

  defp format_event(level, msg, ts, md, state) do
    %{metadata: keys, json_encoder: json_encoder} = state

    unless json_encoder do
      raise ArgumentError, "invalid :json_encoder option for :logger_json application. " <>
                           "Expected one of supported encoders module name, got: #{inspect json_encoder}"
    end

    %{time: format_time(ts),
      severity: format_severity(level),
      logMessage: IO.iodata_to_binary(msg),
      sourceLocation: source_location(md)}
    |> Map.put(:metadata, take_metadata(md, keys))
    |> json_encoder.encode!()
    |> Kernel.<>("\n")
  end

  # RFC3339 UTC "Zulu" format
  defp format_time({date, time}) do
    [Logger.Utils.format_date(date), Logger.Utils.format_time(time)]
    |> Enum.map(&IO.iodata_to_binary/1)
    |> Enum.join("T")
    |> Kernel.<>("Z")
  end

  # Severity levels can be found in Google Cloud Logger docs:
  # https://cloud.google.com/logging/docs/reference/v1beta3/rest/v1beta3/projects.logs.entries/write#LogSeverity
  defp format_severity(:debug),
    do: "DEBUG"
  defp format_severity(:info),
    do: "INFO"
  defp format_severity(:warn),
    do: "WARNING"
  defp format_severity(:error),
    do: "ERROR"
  defp format_severity(nil),
    do: "DEFAULT"

  # Description can be found in Google Cloud Logger docs;
  # https://cloud.google.com/logging/docs/reference/v1beta3/rest/v1beta3/LogLine#SourceLocation
  defp source_location(metadata) do
    file = Keyword.get(metadata, :file)
    line = Keyword.get(metadata, :line)
    function = Keyword.get(metadata, :function)
    module = Keyword.get(metadata, :module)

    %{
      file: file,
      line: line,
      functionName: function,
      moduleName: module
    }
  end

  defp take_metadata(metadata, :all) do
    metadata
    |> Keyword.drop([:pid, :file, :line, :function, :module])
    |> Enum.into(%{})
  end
  defp take_metadata(metadata, keys) do
    Enum.reduce keys, %{}, fn key, acc ->
      case Keyword.fetch(metadata, key) do
        {:ok, val} ->
          Map.merge(acc, %{key => val})
        :error ->
          acc
      end
    end
  end

  defp log_buffer(%{buffer_size: 0, buffer: []} = state), do: state
  defp log_buffer(state) do
    %{device: device, buffer: buffer} = state
    %{state | ref: async_io(device, buffer), buffer: [], buffer_size: 0, output: buffer}
  end

  defp handle_io_reply(:ok, %{ref: ref} = state) do
    Process.demonitor(ref, [:flush])
    log_buffer(%{state | ref: nil, output: nil})
  end
  defp handle_io_reply({:error, {:put_chars, :unicode, _} = error}, state) do
    retry_log(error, state)
  end
  defp handle_io_reply({:error, :put_chars}, %{output: output} = state) do
    retry_log({:put_chars, :unicode, output}, state)
  end
  defp handle_io_reply({:error, error}, _) do
    raise "failure while logging console messages: " <> inspect(error)
  end

  defp retry_log(error, %{device: device, ref: ref, output: dirty} = state) do
    Process.demonitor(ref, [:flush])
    case :unicode.characters_to_binary(dirty) do
      {_, good, bad} ->
        clean = [good | Logger.Formatter.prune(bad)]
        %{state | ref: async_io(device, clean), output: clean}
      _ ->
        # A well behaved IO device should not error on good data
        raise "failure while logging consoles messages: " <> inspect(error)
    end
  end

  defp flush(%{ref: nil} = state),
    do: state
  defp flush(state) do
    state
    |> await_io()
    |> flush()
  end
end