defmodule FastLocalDatetime do
  @moduledoc """
  Fast conversions of UTC epoch timestamps (Unix timestamps) into a local DateTime.
  """
  alias FastLocalDatetime.{Table, TableRegistry, TableSupervisor}

  @doc """
  Convert a UTC epoch timestamp (Unix timestamp) to a DateTime in the given timezone.

      iex> {:ok, dt} = unix_to_datetime(1522888537, "America/New_York")
      iex> dt
      #DateTime<2018-04-04 20:35:37-04:00 EDT America/New_York>

      iex> unix_to_datetime(1522888537, "not found")
      {:error, :not_found}

  """
  @spec unix_to_datetime(timestamp, timezone) :: {:ok, DateTime.t()} | {:error, :not_found}
        when timestamp: integer, timezone: String.t()
  def unix_to_datetime(unix, timezone) when is_integer(unix) and is_binary(timezone) do
    with {:ok, table} <- ensure_table(timezone) do
      {:ok, Table.unix_to_datetime(table, unix)}
    end
  end

  defp ensure_table(timezone) do
    case Registry.lookup(TableRegistry, timezone) do
      [{_pid, table} | _] ->
        {:ok, table}

      [] ->
        case start_child(timezone) do
          {:ok, _} ->
            ensure_table(timezone)

          error ->
            error
        end
    end
  end

  defp start_child(timezone) do
    TableSupervisor.start_child(timezone)
  end
end