defmodule Attesto.CodeStore.ETS do
  @moduledoc """
  Single-node ETS implementation of `Attesto.CodeStore`.

  Codes live in a public ETS table owned by a `GenServer` that sweeps
  expired rows on a fixed interval. `take/1` uses `:ets.take/2`, which
  fetches and deletes a row in one atomic step, so the single-use
  guarantee holds against concurrent redemptions on a node. This is a
  per-node store: a multi-node deployment MUST back `Attesto.CodeStore`
  with a shared store (e.g. Postgres `DELETE ... RETURNING`) so a code
  issued on one node can be redeemed (once) on another.

  ## Start options

    * `:sweep_interval_ms` (default `30_000`) - how often expired rows are
      bulk-deleted. Correctness does not depend on sweeping (`take/1`
      returns the row and `Attesto.AuthorizationCode` re-checks expiry);
      the sweeper only bounds table size.

  ## Wiring

      children = [Attesto.CodeStore.ETS]

  then pass the module as the store:

      Attesto.AuthorizationCode.issue(Attesto.CodeStore.ETS, attrs)
  """

  @behaviour Attesto.CodeStore

  use GenServer

  @table __MODULE__
  @default_sweep_interval_ms 30_000

  @spec start_link(keyword()) :: GenServer.on_start()
  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

  @doc false
  def child_spec(opts) do
    %{id: __MODULE__, start: {__MODULE__, :start_link, [opts]}, type: :worker}
  end

  @impl Attesto.CodeStore
  def put(%{code_hash: code_hash, expires_at: expires_at} = record)
      when is_binary(code_hash) and is_integer(expires_at) do
    # expires_at is hoisted into its own tuple element so the sweep
    # match spec is a plain guard, never a map pattern.
    true = :ets.insert(@table, {code_hash, expires_at, record})
    :ok
  end

  @impl Attesto.CodeStore
  def take(code_hash) when is_binary(code_hash) do
    case :ets.take(@table, code_hash) do
      [{^code_hash, _expires_at, record}] -> {:ok, record}
      [] -> :error
    end
  end

  @impl Attesto.CodeStore
  def get(code_hash) when is_binary(code_hash) do
    case :ets.lookup(@table, code_hash) do
      [{^code_hash, _expires_at, record}] -> {:ok, record}
      [] -> :error
    end
  end

  @doc "Clear every entry. Test-facing."
  @spec reset() :: :ok
  def reset do
    if :ets.whereis(@table) != :undefined, do: :ets.delete_all_objects(@table)
    :ok
  end

  @impl GenServer
  def init(opts) do
    Attesto.ClusterGuard.assert_single_node!(
      __MODULE__,
      Keyword.get(opts, :multi_node_acknowledged?, false)
    )

    sweep_interval_ms = Keyword.get(opts, :sweep_interval_ms, @default_sweep_interval_ms)

    :ets.new(@table, [:set, :public, :named_table, read_concurrency: true, write_concurrency: true])

    schedule_sweep(sweep_interval_ms)
    {:ok, %{sweep_interval_ms: sweep_interval_ms}}
  end

  @impl GenServer
  def handle_info(:sweep, state) do
    now = System.system_time(:second)
    # Delete every row whose expiry is strictly in the past.
    :ets.select_delete(@table, [{{:"$1", :"$2", :"$3"}, [{:<, :"$2", now}], [true]}])

    schedule_sweep(state.sweep_interval_ms)
    {:noreply, state}
  end

  defp schedule_sweep(interval_ms), do: Process.send_after(self(), :sweep, interval_ms)
end