eml_rtmp_session

defmodule RtmpSession.RtmpTime do
  @moduledoc """  

  Provides utilities to work with timestamps in an RTMP context.

  

  RTMP timestamps are 32 bits (unsigned) integers and thus roll over every ~50 days.

  All adjacent timestamps are within 2^31 - 1 milliseconds of 

  each other (e.g.  10000 comes after 4000000000, and 3000000000 comes before 4000000000).

  

  """
  
  @max_timestamp :math.pow(2, 32)
  @adjacent_threshold :math.pow(2, 31) - 1
  
  @doc """

  Converts a timestamp into a valid RTMP timestamp

  (i.e. rolls it over if it's too high or too low)

  

  ## Examples

  

    iex> RtmpSession.RtmpTime.to_rtmp_timestamp(1000)

    1000

    

    iex> RtmpSession.RtmpTime.to_rtmp_timestamp(-1000)

    4294966296

    

    iex> RtmpSession.RtmpTime.to_rtmp_timestamp(4294968296)

    1000

  """
  def to_rtmp_timestamp(timestamp) when timestamp < 0, do: to_rtmp_timestamp(@max_timestamp + timestamp)
  def to_rtmp_timestamp(timestamp) when timestamp > @max_timestamp, do: to_rtmp_timestamp(timestamp - @max_timestamp)
  def to_rtmp_timestamp(timestamp), do: trunc(timestamp)
  
  @doc """

  Applies the specified delta to a timestamp

  

  ## Examples

  

    iex> RtmpSession.RtmpTime.apply_delta(1000, 500)

    1500

    

    iex> RtmpSession.RtmpTime.apply_delta(1000, -500)

    500

    

    iex> RtmpSession.RtmpTime.apply_delta(1000, -2000)

    4294966296

    

    iex> RtmpSession.RtmpTime.apply_delta(4294966296, 2000)

    1000

  """
  def apply_delta(timestamp, delta) do
    timestamp + delta
    |> to_rtmp_timestamp
  end
  
  
  @doc """

  Gets the delta between an old RTMP timestamp and a new RTMP timestamp

  

  ## Examples

    

    iex> RtmpSession.RtmpTime.get_delta(4000000000, 4000001000)

    1000

    

    iex> RtmpSession.RtmpTime.get_delta(4000000000, 10000)

    294977296

    

    iex> RtmpSession.RtmpTime.get_delta(4000000000, 3000000000)

    -1000000000

  

  """
  def get_delta(previous_timestamp, new_timestamp) do
    difference = new_timestamp - previous_timestamp
    is_adjacent = if :erlang.abs(difference) <= @adjacent_threshold, do: true, else: false
    
    do_get_delta(previous_timestamp, new_timestamp, is_adjacent)
    |> trunc
  end
  
  defp do_get_delta(timestamp1, timestamp2, true) do
    timestamp2 - timestamp1
  end
  
  defp do_get_delta(timestamp1, timestamp2, false) when timestamp1 > timestamp2 do
    (@max_timestamp - timestamp1) + timestamp2
  end
  
  defp do_get_delta(timestamp1, timestamp2, false) do
    (@max_timestamp - timestamp2) + timestamp1
  end
end