defmodule Vela.Backend.DETS do
  @moduledoc """
  DETS-backed cache backend. Data persists to disk and survives restarts.

  Slower than ETS (~10-100x) but durable. Good for caches where a cold
  start is expensive and the dataset fits within DETS limits (~2GB).

  Uses two DETS tables: one for data, one for the TTL index (same
  pattern as the ETS backend).

  ## Backend Options

      backend_opts: [
        data_dir: "/tmp/vela"   # optional, defaults to "vela_data"
      ]
  """

  @behaviour Vela.Backend

  alias Vela.Cache.Entry

  defstruct [:data_table, :ttl_table]

  @impl true
  def init(config) do
    opts = config.backend_opts
    data_dir = Keyword.get(opts, :data_dir, "vela_data")
    File.mkdir_p!(data_dir)

    data_path = Path.join(data_dir, "#{config.name}_data.dets") |> String.to_charlist()
    ttl_path = Path.join(data_dir, "#{config.name}_ttl.dets") |> String.to_charlist()

    {:ok, data_table} =
      :dets.open_file(:"vela_dets_data_#{config.name}",
        file: data_path,
        type: :set
      )

    {:ok, ttl_table} =
      :dets.open_file(:"vela_dets_ttl_#{config.name}",
        file: ttl_path,
        type: :set
      )

    state = %__MODULE__{
      data_table: data_table,
      ttl_table: ttl_table
    }

    {:ok, state}
  end

  @impl true
  def get(%__MODULE__{data_table: table}, key) do
    case :dets.lookup(table, key) do
      [{^key, entry}] -> {:ok, entry}
      [] -> {:error, :not_found}
    end
  end

  @impl true
  def put(%__MODULE__{data_table: data_table, ttl_table: ttl_table} = state, %Entry{} = entry) do
    # Clean up old TTL entry if overwriting
    case :dets.lookup(data_table, entry.key) do
      [{_key, old_entry}] ->
        unless old_entry.expires_at == :infinity do
          :dets.delete(ttl_table, {old_entry.expires_at, entry.key})
        end

      [] ->
        :ok
    end

    :dets.insert(data_table, {entry.key, entry})

    unless entry.expires_at == :infinity do
      :dets.insert(ttl_table, {{entry.expires_at, entry.key}, entry.key})
    end

    {:ok, state}
  end

  @impl true
  def delete(%__MODULE__{data_table: data_table, ttl_table: ttl_table} = state, key) do
    case :dets.lookup(data_table, key) do
      [{^key, entry}] ->
        :dets.delete(data_table, key)

        unless entry.expires_at == :infinity do
          :dets.delete(ttl_table, {entry.expires_at, key})
        end

      [] ->
        :ok
    end

    {:ok, state}
  end

  @impl true
  def get_many(%__MODULE__{} = state, keys) do
    result =
      Enum.reduce(keys, %{}, fn key, acc ->
        case get(state, key) do
          {:ok, entry} -> Map.put(acc, key, entry)
          _ -> acc
        end
      end)

    {:ok, result}
  end

  @impl true
  def put_many(state, entries) do
    Enum.reduce(entries, {:ok, state}, fn entry, {:ok, acc} ->
      put(acc, entry)
    end)
  end

  @impl true
  def flush(%__MODULE__{data_table: data_table, ttl_table: ttl_table} = state) do
    :dets.delete_all_objects(data_table)
    :dets.delete_all_objects(ttl_table)
    {:ok, state}
  end

  @impl true
  def flush_expired(%__MODULE__{data_table: data_table, ttl_table: ttl_table} = state, now) do
    # Scan TTL table for expired entries
    expired =
      :dets.foldl(
        fn {{expires_at, key}, _value}, acc ->
          if expires_at <= now, do: [key | acc], else: acc
        end,
        [],
        ttl_table
      )

    # Delete from both tables
    Enum.each(expired, fn key ->
      case :dets.lookup(data_table, key) do
        [{^key, entry}] ->
          :dets.delete(ttl_table, {entry.expires_at, key})

        [] ->
          :ok
      end

      :dets.delete(data_table, key)
    end)

    {:ok, length(expired), state}
  end

  @impl true
  def size(%__MODULE__{data_table: table}) do
    :dets.info(table, :size)
  end

  @impl true
  def delete_by_tag(%__MODULE__{data_table: data_table, ttl_table: ttl_table} = state, tag) do
    matching =
      :dets.foldl(
        fn {key, entry}, acc ->
          if tag in entry.tags, do: [{key, entry} | acc], else: acc
        end,
        [],
        data_table
      )

    Enum.each(matching, fn {key, entry} ->
      :dets.delete(data_table, key)

      unless entry.expires_at == :infinity do
        :dets.delete(ttl_table, {entry.expires_at, key})
      end
    end)

    {:ok, length(matching), state}
  end
end