defmodule Snapcast.Clock do
  @moduledoc """
  The server's monotonic audio clock, shared by all sessions and streams.

  Snapcast timestamps are `{sec, usec}` on an arbitrary steady clock — only
  *differences* matter (the client derives its offset from Time sync), so we use a
  single process-wide epoch and express everything relative to it. Keeping values
  small also keeps `sec` within int32.
  """

  @key {__MODULE__, :epoch_us}

  @doc "Fix the epoch (call once at startup). Safe to call repeatedly."
  def init, do: epoch()

  @doc "Microseconds since the server epoch (monotonic, non-negative)."
  def now_us, do: System.monotonic_time(:microsecond) - epoch()

  @doc "Convert an absolute `System.monotonic_time(:microsecond)` value to this clock."
  def from_monotonic_us(monotonic_us) when is_integer(monotonic_us), do: monotonic_us - epoch()

  @doc "Current time as a snapcast `{sec, usec}` tv."
  def now_tv, do: us_to_tv(now_us())

  @doc "`{sec, usec}` → microseconds."
  def tv_to_us({sec, usec}), do: sec * 1_000_000 + usec

  @doc """
  Microseconds → `{sec, usec}`. Uses truncating div/rem so `sec*1e6 + usec == us`
  exactly, including for negatives (Time-sync deltas can be negative).
  """
  def us_to_tv(us), do: {div(us, 1_000_000), rem(us, 1_000_000)}

  defp epoch do
    case :persistent_term.get(@key, nil) do
      nil ->
        e = System.monotonic_time(:microsecond)
        :persistent_term.put(@key, e)
        e

      e ->
        e
    end
  end
end