debouncer

defmodule Debouncer do
  use Application
  use GenServer

  @moduledoc """
  Debouncer executes a function call debounced. Debouncing is done one a per key basis:

  ```
  Debouncer.apply(Key, fn() -> IO.puts("Hello World, debounced") end)
  ```

  The third optional parameter is the timeout period in milliseconds

  ```
  Debouncer.apply(Key, fn() -> IO.puts("Hello World, once per minute max") end, 60_000)
  ```

  The variants supported are:

  * `apply/3`      => Events are executed after the timeout
  * `immediate/3`  => Events are executed immediately, and further events are delayed for the timeout
  * `immediate2/3` => Events are executed immediately, and further events are IGNORED for the timeout
  * `delay/3`      => Each event delays the execution of the next event

  ```
  EVENT        X1---X2------X3-------X4----------
  TIMEOUT      ----------|----------|----------|-
  ===============================================
  apply()      ----------X2---------X3---------X4
  immediate()  X1--------X2---------X3---------X4
  immediate2() X1-----------X3-------------------
  delay()      --------------------------------X4
  ```
  """

  defstruct events: %{}, workers: %{}

  @spec immediate(term(), (() -> any()), non_neg_integer()) :: :ok
  @doc """
  Executes the function immediately but blocks any further call
  under the same key for the given timeout.
  """
  def immediate(key, fun, timeout \\ 5000) when is_integer(timeout) do
    do_cast(fn deb = %Debouncer{events: events} ->
      case Map.get(events, key) do
        nil ->
          new_event(deb, key, nil, timeout, timeout)
          |> execute(key, fun)

        {calltime, _fun, _timeout} ->
          events = Map.put(events, key, {calltime, fun, timeout})
          %Debouncer{deb | events: events}
      end
    end)
  end

  @spec immediate2(term(), (() -> any()), non_neg_integer()) :: :ok
  @doc """
  Executes the function immediately but ignores further calls
  under the same key for the given timeout.
  """
  def immediate2(key, fun, timeout \\ 5000) when is_integer(timeout) do
    do_cast(fn deb = %Debouncer{events: events} ->
      case Map.get(events, key) do
        nil ->
          new_event(deb, key, nil, timeout, timeout)
          |> execute(key, fun)

        {calltime, _fun, _timeout} ->
          events = Map.put(events, key, {calltime, nil, timeout})
          %Debouncer{deb | events: events}
      end
    end)
  end

  @spec delay(term(), (() -> any()), non_neg_integer()) :: :ok
  @doc """
  Executes the function after the specified timeout t0 + timeout,
  when delay is called multipe times the timeout is reset based on the
  most recent call (t1 + timeout, t2 + timeout) etc... the fun is also updated
  """
  def delay(key, fun, timeout \\ 5000) when is_integer(timeout) do
    do_cast(fn deb ->
      new_event(deb, key, fun, timeout, nil)
    end)
  end

  @spec apply(term(), (() -> any()), non_neg_integer()) :: :ok
  @doc """
  Executes the function after the specified timeout t0 + timeout,
  when apply is called multiple times it does not affect the point
  in time when the next call is happening (t0 + timeout) but updates the fun
  """
  def apply(key, fun, timeout \\ 5000) when is_integer(timeout) do
    do_cast(fn deb = %Debouncer{events: events} ->
      case Map.get(events, key) do
        nil ->
          new_event(deb, key, fun, timeout, timeout)

        {calltime, _fun, timeout} ->
          events = Map.put(events, key, {calltime, fun, timeout})
          %Debouncer{deb | events: events}
      end
    end)
  end

  defp new_event(deb = %Debouncer{events: events}, key, fun, timeout, stall) do
    calltime = time() + timeout
    ets_insert(calltime, key)
    events = Map.put(events, key, {calltime, fun, stall})
    %Debouncer{deb | events: events}
  end

  @spec cancel(term()) :: :ok
  @doc """
  Deletes the latest event if it hasn't triggered yet.
  """
  def cancel(key) do
    do_cast(fn deb = %Debouncer{events: events} ->
      case Map.get(events, key) do
        nil ->
          deb

        {calltime, _fun, timeout} ->
          events = Map.put(events, key, {calltime, nil, timeout})
          %Debouncer{deb | events: events}
      end
    end)
  end

  @spec worker(any()) :: pid() | nil
  @doc """
  Returns the pid of an active job worker or nil if no such job is scheduled.
  Per key the debouncer never starts more than one process at the same time.
  """
  def worker(key) do
    GenServer.call(__MODULE__, {:worker, key})
  end

  ######################## CALLBACKS       ####################
  @doc false
  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    child = %{
      id: Debouncer,
      start: {Debouncer, :start_link, []}
    }

    Supervisor.start_link([child], strategy: :one_for_one, name: Debouncer.Supervisor)
  end

  @doc false
  @spec start_link() :: :ignore | {:error, any} | {:ok, pid}
  def start_link() do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  @doc false
  def init(_arg) do
    {:ok, _} = :timer.send_interval(100, :tick)
    __MODULE__ = :ets.new(__MODULE__, [{:keypos, 1}, :ordered_set, :named_table])
    {:ok, %Debouncer{}}
  end

  ######################## INTERNAL METHOD ####################
  defp do_cast(fun) do
    GenServer.cast(__MODULE__, fun)
  end

  def handle_cast(fun, state) do
    {:noreply, fun.(state)}
  end

  def handle_call({:worker, key}, _from, state = %Debouncer{workers: workers}) do
    case Map.get(workers, key) do
      nil ->
        {:reply, nil, state}

      {pid, _fun, _repeat?} ->
        {:reply, pid, state}
    end
  end

  defp ets_insert(calltime, key) do
    case :ets.lookup(__MODULE__, calltime) do
      [] -> :ets.insert(__MODULE__, {calltime, [key]})
      [{_, keys}] -> :ets.insert(__MODULE__, {calltime, [key | keys]})
    end
  end

  def handle_info(:tick, deb) do
    {:noreply, update(deb, time())}
  end

  def handle_info({:DOWN, _ref, :process, end_pid, _reason}, deb = %Debouncer{workers: workers}) do
    {key, {_pid, fun, repeat?}} =
      Enum.find(workers, fn {_key, {pid, _fun, _repeat?}} -> pid == end_pid end)

    workers = Map.delete(workers, key)

    if map_size(workers) == 0 do
      :erlang.garbage_collect()
    end

    deb = %Debouncer{deb | workers: workers}

    if repeat? do
      {:noreply, execute(deb, key, fun)}
    else
      {:noreply, deb}
    end
  end

  defp update(deb, now) do
    case :ets.first(__MODULE__) do
      :"$end_of_table" ->
        deb

      ts when ts > now ->
        deb

      ts ->
        hd(:ets.take(__MODULE__, ts))
        |> elem(1)
        |> Enum.reduce(deb, fn key, deb = %Debouncer{events: events} ->
          case Map.get(events, key) do
            # Handling apply(), immediate(), immediate2()
            {^ts, nil, _timeout} ->
              events = Map.delete(events, key)
              %Debouncer{deb | events: events}

            # Executing and putting marker for next event
            {^ts, fun, timeout} when is_integer(timeout) ->
              calltime = ts + timeout
              ets_insert(calltime, key)
              events = Map.put(events, key, {calltime, nil, timeout})

              %Debouncer{deb | events: events}
              |> execute(key, fun)

            # delay() goes here
            {^ts, fun, nil} ->
              events = Map.delete(events, key)

              %Debouncer{deb | events: events}
              |> execute(key, fun)

            _ ->
              deb
          end
        end)
        |> update(now)
    end
  end

  defp execute(deb, _key, nil) do
    deb
  end

  defp execute(deb = %Debouncer{workers: workers}, key, fun) do
    worker =
      case Map.get(workers, key) do
        nil ->
          pid = spawn(fun)
          Process.monitor(pid)
          {pid, fun, false}

        {pid, _fun, _repeat?} ->
          # Execute this after the current job finishes
          {pid, fun, true}
      end

    %Debouncer{deb | workers: Map.put(workers, key, worker)}
  end

  defp time() do
    System.monotonic_time(:millisecond)
  end
end