defmodule Belay.Clock do
  @moduledoc """
  Injectable time source. Everything time-dependent in the engine (backoff,
  lease expiry, rate windows, await deadlines, cron slots) reads through this,
  so tests advance a `Belay.Clock.Sim` instead of sleeping.
  """

  @callback now(term()) :: DateTime.t()

  @doc "Read the current time from a clock module or `{module, argument}` pair."
  @spec now(module() | {module(), term()}) :: DateTime.t()
  def now({mod, arg}), do: mod.now(arg)
  def now(mod) when is_atom(mod), do: mod.now(nil)
end

defmodule Belay.Clock.System do
  @moduledoc "Real time."
  @behaviour Belay.Clock

  @impl true
  def now(_), do: DateTime.utc_now()
end

defmodule Belay.Clock.Sim do
  @moduledoc "A settable, advanceable clock for deterministic tests."
  @behaviour Belay.Clock

  @doc "Start a simulated clock at `start`."
  @spec start_link(DateTime.t()) :: Agent.on_start()
  def start_link(start \\ ~U[2026-01-01 00:00:00.000000Z]) do
    Agent.start_link(fn -> start end)
  end

  @impl true
  def now(pid), do: Agent.get(pid, & &1)

  @doc "Advance the simulated clock by a number of seconds."
  @spec advance(Agent.agent(), number()) :: :ok
  def advance(pid, seconds) do
    Agent.update(pid, &DateTime.add(&1, round(seconds * 1_000_000), :microsecond))
  end

  @doc "Set the simulated clock to an exact value."
  @spec set(Agent.agent(), DateTime.t()) :: :ok
  def set(pid, %DateTime{} = at), do: Agent.update(pid, fn _ -> at end)
end