defmodule Services.NamePool do
  @moduledoc """
  A service that manages a pool of AI agent names, batch-allocating them from
  the nomenclater for efficiency. Names can be checked out and optionally
  checked back in for reuse within the same session.

  Each checked-out name is now associated with the caller's `pid`, and you can
  retrieve it via `get_name_by_pid/1`.

  The pool allocates names in chunks to maximize API efficiency without
  overwhelming the connection pool.
  """

  use GenServer

  @chunk_size 12

  @name_chunk_timeout_ms Application.compile_env(:fnord, :name_chunk_timeout_ms, 30_000)

  @name __MODULE__

  @type t :: %__MODULE__{
          available: [String.t()],
          checked_out: MapSet.t(String.t()),
          all_used: MapSet.t(String.t()),
          chunk_size: pos_integer(),
          pid_to_name: %{optional(pid()) => String.t()},
          name_to_pid: %{optional(String.t()) => pid()}
        }

  defstruct available: [],
            checked_out: MapSet.new(),
            all_used: MapSet.new(),
            chunk_size: 12,
            pid_to_name: %{},
            name_to_pid: %{}

  # -----------------------------------------------------------------------------
  # Public API
  # -----------------------------------------------------------------------------
  @default_name "Fnord Prefect"

  def default_name, do: @default_name

  @doc "Starts the name pool service"
  def start_link(opts \\ []) do
    name = Keyword.get(opts, :name, @name)
    init_opts = Keyword.drop(opts, [:name])
    GenServer.start_link(__MODULE__, init_opts, name: name)
  end

  @doc """
  Checks out a name from the pool. If the pool is empty or running low,
  automatically allocates a new chunk of names.

  Returns `{:ok, name}` or `{:error, reason}`.
  """
  @spec checkout_name(atom() | pid()) :: {:ok, String.t()} | {:error, term()}
  @spec checkout_name(atom() | pid(), pos_integer()) :: {:ok, String.t()} | {:error, term()}
  def checkout_name(server \\ @name) do
    # Backward-compatible: default timeout of 30_000 ms
    checkout_name(server, 30_000)
  end

  def checkout_name(server, timeout_ms) do
    try do
      GenServer.call(server, :checkout_name, timeout_ms)
    catch
      :exit, {:timeout, _} -> {:error, :timeout}
      :exit, :timeout -> {:error, :timeout}
    end
  end

  @doc """
  Checks a name back into the pool for potential reuse. This is optional -
  names that are never checked back in will simply be lost when the session ends.
  """
  @spec checkin_name(String.t(), atom() | pid()) :: :ok
  def checkin_name(name, server \\ @name)

  def checkin_name(@default_name, _), do: :ok

  def checkin_name(name, server) do
    GenServer.cast(server, {:checkin_name, name})
  end

  @doc """
  Restores the association between the current process and a `name`.

  This path is used when a worker process needs the same speaking identity as
  its parent so UI reporting and tool logs remain attributed to the right
  character. The update is synchronous because callers may read the association
  immediately after restoring it.
  """
  @spec associate_name(String.t() | nil, atom() | pid(), timeout()) :: :ok | {:error, :timeout}
  def associate_name(name, server \\ @name, timeout_ms \\ 30_000)

  def associate_name(nil, _, _), do: :ok

  def associate_name(name, server, timeout_ms) do
    try do
      GenServer.call(server, {:associate_name, name}, timeout_ms)
    catch
      :exit, {:timeout, _} -> {:error, :timeout}
      :exit, :timeout -> {:error, :timeout}
    end
  end

  @doc "Returns pool statistics for debugging/monitoring"
  def pool_stats(server \\ @name) do
    GenServer.call(server, :pool_stats)
  end

  @doc "Resets the pool state (mainly for testing)"
  def reset(server \\ @name) do
    GenServer.call(server, :reset)
  end

  @doc """
  Returns `{:ok, name}` if the given `pid` has a checked‐out name,
  or `{:error, :not_found}` otherwise.
  """
  @spec get_name_by_pid(pid(), atom() | pid(), timeout()) ::
          {:ok, String.t()} | {:error, :not_found} | {:error, :timeout}
  def get_name_by_pid(pid, server \\ @name, timeout_ms \\ 5_000) when is_pid(pid) do
    try do
      GenServer.call(server, {:get_name_by_pid, pid}, timeout_ms)
    catch
      :exit, {:timeout, _} -> {:error, :timeout}
      :exit, :timeout -> {:error, :timeout}
    end
  end

  # -----------------------------------------------------------------------------
  # GenServer Callbacks
  # -----------------------------------------------------------------------------

  @impl GenServer
  def init(_opts) do
    state = %__MODULE__{
      available: [],
      checked_out: MapSet.new(),
      all_used: MapSet.new(),
      chunk_size: @chunk_size,
      pid_to_name: %{},
      name_to_pid: %{}
    }

    {:ok, state}
  end

  @impl GenServer

  def handle_call(:checkout_name, {caller_pid, _ref}, state) do
    case ensure_names_available(state) do
      {:ok, updated_state} ->
        case updated_state.available do
          [name | remaining] ->
            new_state = %{
              updated_state
              | available: remaining,
                checked_out: MapSet.put(updated_state.checked_out, name),
                pid_to_name: Map.put(updated_state.pid_to_name, caller_pid, name),
                name_to_pid: Map.put(updated_state.name_to_pid, name, caller_pid)
            }

            {:reply, {:ok, name}, new_state}

          [] ->
            UI.error("Unable to allocate names from pool")
            {:reply, {:error, "No names available"}, updated_state}
        end

      {:error, reason} ->
        UI.error("Failed to ensure names available", reason)
        {:reply, {:error, reason}, state}
    end
  end

  @impl GenServer
  def handle_call({:associate_name, name}, {caller_pid, _ref}, state) do
    prev_pid = Map.get(state.name_to_pid, name)
    caller_name = Map.get(state.pid_to_name, caller_pid)

    new_state =
      state
      |> maybe_delete_pid_name(prev_pid, name, caller_pid)
      |> maybe_delete_pid_name(caller_pid, caller_name, name)
      |> then(fn state ->
        %{
          state
          | available: List.delete(state.available, name),
            checked_out: MapSet.put(state.checked_out, name),
            pid_to_name: Map.put(state.pid_to_name, caller_pid, name),
            name_to_pid: Map.put(state.name_to_pid, name, caller_pid)
        }
      end)

    {:reply, :ok, new_state}
  end

  @impl GenServer
  def handle_call(:pool_stats, _from, state) do
    stats = %{
      available_count: length(state.available),
      checked_out_count: MapSet.size(state.checked_out),
      all_used_count: MapSet.size(state.all_used),
      chunk_size: state.chunk_size
    }

    {:reply, stats, state}
  end

  @impl GenServer
  def handle_call(:reset, _from, state) do
    new_state = %{
      state
      | available: [],
        checked_out: MapSet.new(),
        all_used: MapSet.new(),
        pid_to_name: %{},
        name_to_pid: %{}
    }

    {:reply, :ok, new_state}
  end

  @impl GenServer
  def handle_call({:get_name_by_pid, pid}, _from, state) do
    case Map.fetch(state.pid_to_name, pid) do
      {:ok, name} ->
        {:reply, {:ok, name}, state}

      :error ->
        {:reply, {:error, :not_found}, state}
    end
  end

  @impl GenServer
  def handle_cast({:checkin_name, name}, state) do
    if MapSet.member?(state.checked_out, name) do
      pid = Map.get(state.name_to_pid, name)

      new_state = %{
        state
        | available: [name | state.available],
          checked_out: MapSet.delete(state.checked_out, name),
          pid_to_name: Map.delete(state.pid_to_name, pid),
          name_to_pid: Map.delete(state.name_to_pid, name)
      }

      {:noreply, new_state}
    else
      UI.warn("Attempted to check in name that wasn't checked out", name)
      {:noreply, state}
    end
  end

  defp maybe_delete_pid_name(state, pid, mapped_name, keep_name)
       when is_pid(pid) and is_binary(mapped_name) and mapped_name != keep_name do
    %{
      state
      | pid_to_name: Map.delete(state.pid_to_name, pid),
        name_to_pid: Map.delete(state.name_to_pid, mapped_name),
        checked_out: MapSet.delete(state.checked_out, mapped_name)
    }
  end

  defp maybe_delete_pid_name(state, _pid, _mapped_name, _keep_name), do: state

  # Ensures there are names available; allocates a new chunk when empty.
  # This is a private helper; do not use @doc on private functions to avoid warnings.
  # Returns {:ok, state} or {:error, reason}.
  # Ensures there are names available, allocating a new chunk if the pool is empty
  defp ensure_names_available(%__MODULE__{available: []} = state) do
    allocate_name_chunk(state)
  end

  defp ensure_names_available(state) do
    {:ok, state}
  end

  # Allocates a chunk of names from the nomenclater
  defp allocate_name_chunk(state) do
    Services.Globals.get_env(:fnord, :nomenclater, :real)
    |> case do
      :fake ->
        start_num = MapSet.size(state.all_used) + 1
        end_num = start_num + state.chunk_size - 1

        names =
          start_num..end_num
          |> Enum.map(&"NPC ##{&1}")

        new_state = %{
          state
          | available: names ++ state.available,
            all_used: MapSet.union(state.all_used, MapSet.new(names))
        }

        {:ok, new_state}

      :real ->
        used_names = MapSet.to_list(state.all_used)

        task =
          Services.Globals.Spawn.async(fn ->
            AI.Agent.Nomenclater
            # `named?: false` prevents circular dependency with ourselves
            |> AI.Agent.new(named?: false)
            |> AI.Agent.get_response(%{
              want: state.chunk_size,
              used: used_names
            })
          end)

        # Use Task.yield to allow configurable timeout
        case Task.yield(task, @name_chunk_timeout_ms) do
          {:ok, {:ok, names}} when is_list(names) ->
            new_state = %{
              state
              | available: names ++ state.available,
                all_used: MapSet.union(state.all_used, MapSet.new(names))
            }

            {:ok, new_state}

          {:ok, {:error, reason}} ->
            UI.error("Failed to make up names for your agents", reason)
            {:error, reason}

          nil ->
            Task.shutdown(task, :brutal_kill)
            UI.error("Name allocation task timed out")
            {:error, :timeout}
        end
    end
  end
end