defmodule Membrane.Clock do
@moduledoc """
Clock is a Membrane utility that allows elements to measure time according to
a particular clock, which can be e.g. a soundcard hardware clock.
Internally, Clock is a GenServer process that can receive _updates_ (see `t:update_message/0`),
which are messages containing amount of time until the next update.
For example, a sink playing audio to the sound card can send an update before
each write to the sound card buffer (for practical reasons that can be done every
100 or 1000 writes). Although it might be more intuitive to send updates with
the time passed, in practice the described approach turns out to be more convenient,
as it simplifies the first update.
Basing on updates, Clock calculates the `t:ratio/0` of its time to the reference
time. The reference time can be configured with `:time_provider` option. The ratio
is broadcasted (see `t:ratio_message/0`) to _subscribers_ (see `subscribe/2`)
- processes willing to synchronize to the custom clock. Subscribers can adjust
their timers according to received ratio - timers started with
`t:Membrane.Element.Action.start_timer/0` action in elements do it automatically.
Initial ratio is equal to 1, which means that if no updates are received,
Clock is synchronized to the reference time.
## Proxy mode
Clock can work in _proxy_ mode, which means it cannot receive updates, but
it receives ratio from another clock instead, and forwards it to subscribers.
Proxy mode is enabled with `proxy_for: pid` or `proxy: true` (no initial proxy)
option, and the proxy is set/changed using `proxy_for/2`.
"""
use Bunch
use GenServer
alias Membrane.Core.Message
alias Membrane.Time
@typedoc @moduledoc
@type t :: pid
@typedoc """
Ratio of the Clock time to the reference time.
"""
@type ratio :: Ratio.t()
@typedoc """
Update message received by the Clock. It should contain the time till the next
update.
"""
@type update_message ::
{:membrane_clock_update,
milliseconds ::
non_neg_integer
| ratio
| {numerator :: non_neg_integer, denominator :: pos_integer()}}
@typedoc """
Ratio message sent by the Clock to all its subscribers. It contains the ratio
of the custom clock time to the reference time.
"""
@type ratio_message :: {:membrane_clock_ratio, clock :: pid, ratio}
@typedoc """
Options accepted by `start_link/2` and `start/2` functions.
They are the following:
- `time_provider` - function providing the reference time in milliseconds
- `proxy` - determines whether the Clock should work in proxy mode
- `proxy_for` - enables the proxy mode and sets proxied Clock to pid
Check the moduledoc for more details.
"""
@type option ::
{:time_provider, (-> Time.t())}
| {:proxy, boolean}
| {:proxy_for, pid | nil}
@spec start_link([option], GenServer.options()) :: GenServer.on_start()
def start_link(options \\ [], gen_server_options \\ []) do
GenServer.start_link(__MODULE__, options, gen_server_options)
end
@spec start([option], GenServer.options()) :: GenServer.on_start()
def start(options \\ [], gen_server_options \\ []) do
GenServer.start(__MODULE__, options, gen_server_options)
end
@doc """
Subscribes `pid` for receiving `t:ratio_message/0` messages from the clock.
This function can be called multiple times from the same process. To unsubscribe,
`unsubscribe/2` should be called the same amount of times. The subscribed pid
always receives one message, regardless of how many times it called `subscribe/2`.
"""
@spec subscribe(t, subscriber :: pid) :: :ok
def subscribe(clock, pid \\ self()) do
GenServer.cast(clock, {:clock_subscribe, pid})
end
@doc """
Unsubscribes `pid` from receiving `t:ratio_message/0` messages from the clock.
For unsubscription to take effect, `unsubscribe/2` should be called the same
amount of times as `subscribe/2`.
"""
@spec unsubscribe(t, subscriber :: pid) :: :ok
def unsubscribe(clock, pid \\ self()) do
GenServer.cast(clock, {:clock_unsubscribe, pid})
end
@doc """
Sets a new proxy clock to `clock_to_proxy_for`.
"""
@spec proxy_for(t, clock_to_proxy_for :: pid | nil) :: :ok
def proxy_for(clock, clock_to_proxy_for) do
GenServer.cast(clock, {:proxy_for, clock_to_proxy_for})
end
@impl GenServer
def init(options) do
proxy_opts = get_proxy_options(options[:proxy], options[:proxy_for])
state =
%{
ratio: Ratio.new(1),
subscribers: %{},
time_provider: options |> Keyword.get(:time_provider, fn -> Time.monotonic_time() end)
}
|> Map.merge(proxy_opts)
if pid = proxy_opts[:proxy_for], do: Message.send(pid, :clock_subscribe, self())
{:ok, state}
end
@impl GenServer
def handle_cast({:proxy_for, proxy_for}, %{proxy: true} = state) do
if state.proxy_for, do: unsubscribe(state.proxy_for)
state = %{state | proxy_for: proxy_for}
state =
if proxy_for do
subscribe(proxy_for)
state
else
broadcast_and_update_ratio(Ratio.new(1), state)
end
{:noreply, state}
end
@impl GenServer
def handle_cast({:clock_subscribe, pid}, state) do
state
|> update_in([:subscribers, pid], fn
nil ->
send_ratio(pid, state.ratio)
monitor = Process.monitor(pid)
%{monitor: monitor, subscriptions: 1}
%{subscriptions: subs} = subscriber ->
%{subscriber | subscriptions: subs + 1}
end)
~> {:noreply, &1}
end
@impl GenServer
def handle_cast({:clock_unsubscribe, pid}, state) do
if Map.has_key?(state.subscribers, pid) do
{subs, state} =
state |> Bunch.Access.get_updated_in([:subscribers, pid, :subscriptions], &(&1 - 1))
if subs == 0, do: handle_unsubscribe(pid, state), else: state
else
state
end
~> {:noreply, &1}
end
@impl GenServer
def handle_info({:membrane_clock_update, till_next}, %{proxy: false} = state) do
{:noreply, handle_clock_update(till_next, state)}
end
@impl GenServer
def handle_info({:membrane_clock_ratio, pid, ratio}, %{proxy: true, proxy_for: pid} = state) do
{:noreply, broadcast_and_update_ratio(ratio, state)}
end
@impl GenServer
# When ratio from previously proxied clock comes in after unsubscribing
def handle_info({:membrane_clock_ratio, _pid, _ratio}, %{proxy: true} = state) do
{:noreply, state}
end
@impl GenServer
def handle_info({:DOWN, _ref, :process, pid, _reason}, state) do
{:noreply, handle_unsubscribe(pid, state)}
end
defp get_proxy_options(proxy, proxy_for)
defp get_proxy_options(_proxy, pid) when is_pid(pid), do: %{proxy: true, proxy_for: pid}
defp get_proxy_options(true, _proxy_for), do: %{proxy: true, proxy_for: nil}
defp get_proxy_options(_proxy, _proxy_for),
do: %{init_time: nil, clock_time: Ratio.new(0), till_next: nil, proxy: false}
defp handle_unsubscribe(pid, state) do
Process.demonitor(state.subscribers[pid].monitor, [:flush])
{_subscriber, state} = pop_in(state, [:subscribers, pid])
state
end
defp handle_clock_update({nom, denom}, state) do
handle_clock_update(Ratio.new(nom, denom), state)
end
defp handle_clock_update(till_next, state) do
till_next = Ratio.new(till_next)
if Ratio.lt?(till_next, 0) do
raise "Clock update time cannot be negative, received: #{inspect(till_next)}"
end
till_next = Ratio.mult(till_next, Ratio.new(Time.millisecond()))
case state.init_time do
nil -> %{state | init_time: state.time_provider.(), till_next: till_next}
_init_time -> do_handle_clock_update(till_next, state)
end
end
defp do_handle_clock_update(till_next, state) do
%{till_next: from_previous, clock_time: clock_time} = state
clock_time = Ratio.add(clock_time, from_previous)
ratio = Ratio.div(clock_time, Ratio.new(state.time_provider.() - state.init_time))
state = %{state | clock_time: clock_time, till_next: till_next}
broadcast_and_update_ratio(ratio, state)
end
defp broadcast_and_update_ratio(ratio, state) do
state.subscribers |> Bunch.KVList.each_key(&send_ratio(&1, ratio))
%{state | ratio: ratio}
end
defp send_ratio(pid, ratio), do: send(pid, {:membrane_clock_ratio, self(), ratio})
end
