ex_clock_bound_client

defmodule ExClockBoundClient do
  @moduledoc """
    The elixir version of aws clock bound client.
  """

  alias ExClockBoundClient.Server

  @type nanosecond :: non_neg_integer
  @type unix_time_with_unit ::
          {nanosecond, :second | :millisecond | :microsecond | :nanosecond}

  @doc """
  Returns the bounds of the current system time +/- the error calculated from chrony.

  ## Examples

      iex> ExClockBoundClient.now()
      {:ok, {~U[2024-02-01 11:14:57.656524Z], ~U[2024-02-01 11:14:57.706487Z]}}

  """
  @spec now() ::
          {:ok, {earliest_datetime :: DateTime.t(), latest_datetime :: DateTime.t()}}
          | {:error, :clock_bound_server_error}
  def now do
    GenServer.call(Server, :now)
  end

  @doc """
  Execute `function` and return bounds on execution time.

  ## Options

  - `exec_timeout` - The maximum time in milliseconds to wait for the function to complete. Default is 5000.

  ## Examples

      iex> ExClockBoundClient.timing(fn -> :timer.sleep(1000) end)
      {:ok, {{~U[2024-02-01 11:14:57.656524Z], ~U[2024-02-01 11:14:57.706487Z]}, {1011, 1033}}}
  """
  @spec timing(function()) ::
          {:ok,
           {{earliest_start :: DateTime.t(), latest_finish :: DateTime.t()},
            {min_execution_time :: non_neg_integer, max_execution_time :: non_neg_integer}}}
          | {:error, :clock_bound_server_error | :timeout}
          | {:error, :raise | :throw, inspection :: String.t()}
  @spec timing(function(), opts :: keyword) ::
          {:ok,
           {{earliest_start :: DateTime.t(), latest_finish :: DateTime.t()},
            {min_execution_time :: non_neg_integer, max_execution_time :: non_neg_integer}}}
          | {:error, :clock_bound_server_error | :timeout}
          | {:error, :raise | :throw, inspection :: String.t()}
  def timing(func, opts \\ [exec_timeout: 5000]) do
    with {:ok, {earliest_start_datetime, latest_start_datetime}} <- now(),
         {:ok, _} <- run_function_in_task(func, opts[:exec_timeout]),
         {:ok, {earliest_finish_datetime, latest_finish_datetime}} <- now() do
      earliest_start = DateTime.to_unix(earliest_start_datetime, :nanosecond)
      latest_start = DateTime.to_unix(latest_start_datetime, :nanosecond)
      earliest_finish = DateTime.to_unix(earliest_finish_datetime, :nanosecond)
      latest_finish = DateTime.to_unix(latest_finish_datetime, :nanosecond)

      start_midpoint = (earliest_start + latest_start) / 2
      end_midpoint = (earliest_finish + latest_finish) / 2

      execution_time = end_midpoint - start_midpoint

      error_rate = execution_time * config()[:frequency_error] / 1_000_000

      min_execution_time = (execution_time - error_rate) |> ceil()
      max_execution_time = (execution_time + error_rate) |> ceil()
      earliest_start = DateTime.from_unix!(earliest_start, :nanosecond)
      latest_finish = DateTime.from_unix!(latest_finish, :nanosecond)
      {:ok, {{earliest_start, latest_finish}, {min_execution_time, max_execution_time}}}
    end
  end

  defp run_function_in_task(func, timeout) do
    try do
      task =
        Task.async(fn ->
          try do
            func.()
          rescue
            exception -> {:error, :raise, inspect(exception)}
          catch
            anything -> {:error, :throw, inspect(anything)}
          end
        end)

      {:ok, Task.await(task, timeout)}
    catch
      :exit, {:timeout, _} -> {:error, :timeout}
    end
  end

  @doc """
  Returns the deviation in nanosecond of the current system time from the chrony server.

  ## Examples

      iex> ExClockBoundClient.deviation()
      {:ok, 1314}
  """
  @spec deviation() :: {:ok, nanosecond()} | {:error, :clock_bound_server_error}
  def deviation do
    GenServer.call(Server, :deviation)
  end

  @doc """
  Returns true if the provided timestamp is before the earliest error bound. Otherwise, returns false.

  ## DateTime Format

  See `DateTime Format` in `after?/1` .

  ## Examples

      iex> ExClockBoundClient.before?(~U[2024-02-01 11:14:57.656524Z])
      {:ok, true}
  """
  @spec before?(DateTime.t() | unix_time_with_unit()) ::
          {:ok, boolean} | {:error, :clock_bound_server_error | :invalid_datetime}
  def before?(datetime) do
    with {:ok, datetime} <- normalize_datetime(datetime) do
      GenServer.call(Server, {:before?, datetime})
    end
  end

  @doc """
  Returns true if the provided timestamp is after the latest error bound. Otherwise, returns false.

  ## DateTime Format

  The `datetime` argument could be one of :
  - a DateTime struct.
  - a tuple of the form `{unix_time, unit}`. `unix_time` is an integer in unit, and `unit` is one of `:second`, `:millisecond`, `:microsecond`, or `:nanosecond`.

  ## Examples

      iex> ExClockBoundClient.before?(~U[2024-02-01 11:14:57.656524Z])
      {:ok, true}
  """
  @spec after?(DateTime.t() | unix_time_with_unit()) ::
          {:ok, boolean} | {:error, :clock_bound_server_error | :invalid_datetime}
  def after?(datetime) do
    with {:ok, datetime} <- normalize_datetime(datetime) do
      GenServer.call(Server, {:after?, datetime})
    end
  end

  defp normalize_datetime(datetime) do
    with {:ok, unix_nanosecond} <- do_normalize_datetime(datetime),
         {:ok, _datetime} <- DateTime.from_unix(unix_nanosecond, :nanosecond) do
      {:ok, unix_nanosecond}
    else
      _ -> {:error, :invalid_datetime}
    end
  end

  defp do_normalize_datetime(%DateTime{} = datetime),
    do: {:ok, DateTime.to_unix(datetime, :nanosecond)}

  defp do_normalize_datetime(unix_time) when is_integer(unix_time),
    do: do_normalize_datetime({unix_time, :second})

  defp do_normalize_datetime({unix_time, :second}) when is_integer(unix_time) do
    {:ok, unix_time * 1_000_000_000}
  end

  defp do_normalize_datetime({unix_time, :millisecond}) when is_integer(unix_time) do
    {:ok, unix_time * 1_000_000}
  end

  defp do_normalize_datetime({unix_time, :microsecond}) when is_integer(unix_time) do
    {:ok, unix_time * 1_000}
  end

  defp do_normalize_datetime({unix_time, :nanosecond}) when is_integer(unix_time),
    do: {:ok, unix_time}

  defp do_normalize_datetime(_), do: {:error, :invalid_datetime}

  @doc false
  def config do
    config = Application.get_all_env(:ex_clock_bound_client)
    Keyword.merge(default_config(), config)
  end

  defp default_config do
    [
      clock_bound_socket_path: "/run/clockboundd/clockboundd.sock",
      client_socket_path: Path.expand("./client.sock"),
      recv_timeout: 100,
      # 1 ppm for chrony
      frequency_error: 1
    ]
  end
end