"""Robot backend factory.

Selects and constructs the concrete :class:`RobotInterface` implementation
based on the runtime config. The chosen backend is *lazy-imported* so the
unused backend's heavy dependencies are never pulled in:

  * ``cfg.mock`` is ``True``       -> :class:`~gowelcome.robot.mock_robot.MockRobot`
    (webcam / video file; no robot SDK required).
  * ``cfg.transport == "webrtc"``  -> :class:`~gowelcome.robot.webrtc_robot.Go2WebRTCRobot`
    (default; app WebRTC protocol via ``unitree_webrtc_connect``; wifi + audio).
  * ``cfg.transport == "dds"``     -> :class:`~gowelcome.robot.go2_robot.Go2Robot`
    (official ``unitree_sdk2py`` over CycloneDDS; wired / EDU).
"""

from __future__ import annotations

from typing import TYPE_CHECKING

from gowelcome.robot.interface import RobotInterface

if TYPE_CHECKING:  # pragma: no cover - type hints only
    from config import GoWelcomeConfig


def build_robot(cfg: "GoWelcomeConfig") -> RobotInterface:
    """Build the appropriate robot backend for ``cfg``.

    ``cfg.mock`` -> MockRobot; else ``cfg.transport`` selects the real backend
    (``"webrtc"`` default, ``"dds"`` for the official SDK). The unselected
    backends are never imported, so their heavy/optional dependencies need not
    be installed.

    Args:
        cfg: The fully-populated :class:`~config.GoWelcomeConfig`.

    Returns:
        A ready-to-use :class:`RobotInterface` instance.

    Raises:
        ValueError: if ``cfg.transport`` is not a known value.
    """
    if cfg.mock:
        from gowelcome.robot.mock_robot import MockRobot

        return MockRobot(cfg)

    transport = (cfg.transport or "webrtc").strip().lower()
    if transport == "webrtc":
        from gowelcome.robot.webrtc_robot import Go2WebRTCRobot

        return Go2WebRTCRobot(cfg)
    if transport == "dds":
        from gowelcome.robot.go2_robot import Go2Robot

        return Go2Robot(cfg)

    raise ValueError(
        f"Unknown transport {cfg.transport!r} (expected 'webrtc' or 'dds')."
    )