defmodule Vela.Backend.ETS do
  @moduledoc """
  ETS-backed cache backend.

  Reads and writes go directly to ETS tables (bypassing any GenServer
  mailbox) for maximum throughput. The tables are owned and protected
  by Vela.Backend.ETS.TableOwner.
  """

  @behaviour Vela.Backend

  alias Vela.Backend.ETS.TableOwner
  alias Vela.Cache.Entry

  # State for this backend — just the table names
  defstruct [:data_table, :ttl_table]

  @impl true
  def init(config) do
    state = %__MODULE__{
      data_table: TableOwner.data_table(config.name),
      ttl_table: TableOwner.ttl_table(config.name)
    }

    {:ok, state}
  end

  @impl true
  def get(%__MODULE__{data_table: table}, key) do
    case :ets.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 an existing key
    case :ets.lookup(data_table, entry.key) do
      [{_key, old_entry}] ->
        unless old_entry.expires_at == :infinity do
          :ets.delete(ttl_table, {old_entry.expires_at, entry.key})
        end

      [] ->
        :ok
    end

    # Store the entry in the main data table
    :ets.insert(data_table, {entry.key, entry})

    # Register the expiry in the TTL index table
    # Key format: {expires_at, key} — ordered_set sorts by this,
    # so a range scan finds all expired entries efficiently
    unless entry.expires_at == :infinity do
      :ets.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
    # Look up the entry first so we can remove it from the TTL index
    case :ets.lookup(data_table, key) do
      [{^key, entry}] ->
        :ets.delete(data_table, key)

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

      [] ->
        :ok
    end

    {:ok, state}
  end

  @impl true
  def get_many(%__MODULE__{data_table: table}, keys) do
    result =
      Enum.reduce(keys, %{}, fn key, acc ->
        case :ets.lookup(table, key) do
          [{^key, entry}] -> Map.put(acc, key, entry)
          [] -> acc
        end
      end)

    {:ok, result}
  end

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

    {:ok, new_state}
  end

  @impl true
  def flush(%__MODULE__{data_table: data_table, ttl_table: ttl_table} = state) do
    :ets.delete_all_objects(data_table)
    :ets.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
    # TTL table rows are {{expires_at, key}, key}
    # Match the nested tuple to extract expires_at for the guard
    match_spec = [
      {
        # pattern: {{expires_at, key}, key}
        {{:"$1", :"$2"}, :"$3"},
        # guard: expires_at <= now
        [{:"=<", :"$1", now}],
        # action: delete it
        [true]
      }
    ]

    # Collect expired keys so we can delete them from the data table too
    expired_keys =
      :ets.select(ttl_table, [
        {
          {{:"$1", :"$2"}, :"$3"},
          [{:"=<", :"$1", now}],
          # return the data key
          [:"$3"]
        }
      ])

    count = length(expired_keys)

    # Delete from both tables
    :ets.select_delete(ttl_table, match_spec)
    Enum.each(expired_keys, &:ets.delete(data_table, &1))

    {:ok, count, state}
  end

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

  @impl true
  def delete_by_tag(%__MODULE__{data_table: data_table, ttl_table: ttl_table} = state, tag) do
    # Scan data table for entries whose tags list contains the given tag.
    # ETS match spec: the value is an Entry struct stored as {key, entry}.
    # Entry has a :tags field — we select entries where tag is in the list.
    matching =
      :ets.foldl(
        fn {key, entry}, acc ->
          if tag in entry.tags, do: [{key, entry} | acc], else: acc
        end,
        [],
        data_table
      )

    # Delete each matching entry from both tables
    Enum.each(matching, fn {key, entry} ->
      :ets.delete(data_table, key)

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

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