retry

defmodule Retry do
  @moduledoc """

  Provides a convenient interface to retrying behavior. All durations are
  specified in milliseconds.

  Examples

      use Retry
      import Stream

      retry with: exponential_backoff |> randomize |> cap(1_000) |> expiry(10_000) do
      # interact with external service
      end

      retry with: linear_backoff(10, 2) |> cap(1_000) |> take(10) do
      # interact with external service
      end

      retry with: cycle([500]) |> take(10) do
      # interact with external service
      end

  The first retry will exponentially increase the delay, fudging each delay up
  to 10%, until the delay reaches 1 second and then give up after 10 seconds.

  The second retry will linearly increase the retry by a factor of 2 from 10ms giving up after 10 attempts.

  The third example shows how we can produce a delay stream using standard
  `Stream` functionality. Any stream of integers may be used as the value of
  `with:`.

  """

  @default_retry_options [atoms: [:error], rescue_only: [RuntimeError]]

  @doc false
  defmacro __using__(_opts) do
    quote do
      import Retry
      import Retry.DelayStreams
    end
  end

  @doc """

  Retry a block of code delaying between each attempt the duration specified by
  the next item in the `with` delay stream.

  If the block returns any of the atoms specified in `atoms`, a retry will be attempted.
  Other atoms or atom-result tuples will not be retried. If `atoms` is not specified,
  it defaults to `[:error]`.

  Similary, if the block raises any of the exceptions specified in `rescue_only`, a retry
  will be attempted. Other exceptions will not be retried. If `rescue_only` is
  not specified, it defaults to `[RuntimeError]`.

  The `after` block evaluates only when the `do` block returns a valid value before timeout.

  On the other hand, the `else` block evaluates only when the `do` block remains erroneous after timeout.

  Example

      use Retry

      retry with: exponential_backoff() |> cap(1_000) |> expiry(1_000), rescue_only: [CustomError] do
        # interact with external service
      after
        result -> result
      else
        error -> error
      end

  """
  defmacro retry(
             [{:with, stream_builder} | opts],
             do: do_clause,
             after: after_clause,
             else: else_clause
           ) do
    opts = Keyword.merge(@default_retry_options, opts)
    atoms = Keyword.get(opts, :atoms)

    quote do
      fun = unquote(block_runner(do_clause, opts))

      unquote(delays_from(stream_builder))
      |> Enum.reduce_while(nil, fn delay, _last_result ->
        :timer.sleep(delay)
        fun.()
      end)
      |> case do
        {:exception, e} ->
          case e do
            unquote(else_clause)
          end

        e = {atom, _} when atom in unquote(atoms) ->
          case e do
            unquote(else_clause)
          end

        e when is_atom(e) and e in unquote(atoms) ->
          case e do
            unquote(else_clause)
          end

        result ->
          case result do
            unquote(after_clause)
          end
      end
    end
  end

  defmacro retry(_stream_builder, _clauses) do
    raise(ArgumentError, ~s(invalid syntax, only "retry", "after" and "else" are permitted))
  end

  @doc """

  Retry a block of code until `halt` is emitted delaying between each attempt
  the duration specified by the next item in the `with` delay stream.

  The return value for `block` is expected to be `{:cont, result}`, return
  `{:halt, result}` to end the retry early.

  Example

      retry_while with: linear_backoff(500, 1) |> take(5) do
        call_service
        |> case do
          result = %{"errors" => true} -> {:cont, result}
          result -> {:halt, result}
        end
      end

  """
  defmacro retry_while([with: stream_builder], do: block) do
    quote do
      unquote(delays_from(stream_builder))
      |> Enum.reduce_while(nil, fn delay, _last_result ->
        :timer.sleep(delay)
        unquote(block)
      end)
    end
  end

  @doc """

  Wait for a block of code to be truthy delaying between each attempt
  the duration specified by the next item in the delay stream.

  The `after` block evaluates only when the `do` block returns a truthy value.

  On the other hand, the `else` block evaluates only when the `do` block remains falsy after timeout.

  Example

      wait linear_backoff(500, 1) |> take(5) do
        we_there_yet?
      after
        _ ->
          {:ok, "We have arrived!"}
      else
        _ ->
          {:error, "We're still on our way :("}
      end

  """
  defmacro wait(stream_builder, do: do_clause, after: after_clause, else: else_clause) do
    quote do
      unquote(delays_from(stream_builder))
      |> Enum.reduce_while(nil, fn delay, _last_result ->
        :timer.sleep(delay)

        case unquote(do_clause) do
          false = result -> {:cont, result}
          nil = result -> {:cont, result}
          result -> {:halt, result}
        end
      end)
      |> case do
        x when x in [false, nil] ->
          case x do
            unquote(else_clause)
          end

        x ->
          case x do
            unquote(after_clause)
          end
      end
    end
  end

  defmacro wait(_stream_builder, _clauses) do
    raise(ArgumentError, ~s(invalid syntax, only "wait", "after" and "else" are permitted))
  end

  defp block_runner(block, opts) do
    atoms = Keyword.get(opts, :atoms)
    exceptions = Keyword.get(opts, :rescue_only)

    quote generated: true do
      fn ->
        try do
          case unquote(block) do
            {atom, _} = result ->
              if atom in unquote(atoms) do
                {:cont, result}
              else
                {:halt, result}
              end

            result ->
              if is_atom(result) and result in unquote(atoms) do
                {:cont, result}
              else
                {:halt, result}
              end
          end
        rescue
          e ->
            if e.__struct__ in unquote(exceptions) do
              {:cont, {:exception, e}}
            else
              reraise e, System.stacktrace()
            end
        end
      end
    end
  end

  defp delays_from(stream_builder) do
    quote do
      delays = unquote(stream_builder)
      [0] |> Stream.concat(delays)
    end
  end
end