"""C7 inference-runtime composition-root factory (AZ-297).

:func:`build_inference_runtime` selects exactly one strategy by
``config.components['c7_inference'].runtime`` and respects compile-time
``BUILD_*`` gating: requesting a strategy whose flag is OFF raises
:class:`RuntimeNotAvailableError` at composition time (NOT at first
inference call).

The concrete strategy modules (``tensorrt_runtime``, ``onnx_trt_ep_runtime``,
``pytorch_fp16_runtime``) are imported lazily — a Tier-0 workstation
build with ``BUILD_TENSORRT_RUNTIME=OFF`` MUST NOT load
``c7_inference.tensorrt_runtime`` (Invariant I-5; verifiable via
``sys.modules``).
"""

from __future__ import annotations

import os
from typing import TYPE_CHECKING

from gps_denied_onboard.runtime_root.errors import RuntimeNotAvailableError

if TYPE_CHECKING:
    from gps_denied_onboard.components.c7_inference import (
        C7InferenceConfig,
        InferenceRuntime,
    )
    from gps_denied_onboard.config.schema import Config

__all__ = ["build_inference_runtime"]


_RUNTIME_TO_BUILD_FLAG: dict[str, str] = {
    "tensorrt": "BUILD_TENSORRT_RUNTIME",
    "onnx_trt_ep": "BUILD_ONNX_TRT_EP_RUNTIME",
    "pytorch_fp16": "BUILD_PYTORCH_FP16_RUNTIME",
}

_RUNTIME_TO_MODULE: dict[str, tuple[str, str]] = {
    "tensorrt": (
        "gps_denied_onboard.components.c7_inference.tensorrt_runtime",
        "TensorrtRuntime",
    ),
    "onnx_trt_ep": (
        "gps_denied_onboard.components.c7_inference.onnx_trt_ep_runtime",
        "OnnxTrtEpRuntime",
    ),
    "pytorch_fp16": (
        "gps_denied_onboard.components.c7_inference.pytorch_fp16_runtime",
        "PytorchFp16Runtime",
    ),
}


def _is_build_flag_on(flag_name: str) -> bool:
    """Read a compile-time ``BUILD_*`` flag from the environment.

    ``ON`` / ``1`` / ``true`` / ``yes`` (case-insensitive) → ``True``;
    anything else (including unset) → ``False``. Defaults to OFF so
    test environments must opt-in explicitly per strategy.
    """
    raw = os.environ.get(flag_name, "")
    return raw.strip().lower() in {"on", "1", "true", "yes"}


def _c7_config(config: "Config") -> "C7InferenceConfig":
    """Pull the registered C7 config block.

    ``c7_inference.__init__`` registers it on import; a missing
    registration is a developer error and surfaces as ``KeyError``
    rather than a silent fallback.
    """
    return config.components["c7_inference"]


def build_inference_runtime(config: "Config") -> "InferenceRuntime":
    """Construct the :class:`InferenceRuntime` impl selected by config.

    The factory:

    1. Reads ``config.components['c7_inference'].runtime``.
    2. Checks the matching ``BUILD_*`` flag — if OFF, raises
       :class:`RuntimeNotAvailableError` BEFORE any import.
    3. Lazily imports the concrete strategy module.
    4. Constructs and returns the strategy instance, passing ``config``.

    Raises :class:`RuntimeNotAvailableError` when:

    - The compile-time flag is OFF (the canonical Tier-0 path).
    - The concrete strategy module has not been built yet (AZ-298 /
      AZ-299 / AZ-300 are still pending) — the import fails and the
      factory wraps :class:`ModuleNotFoundError`.
    """
    block = _c7_config(config)
    runtime = block.runtime
    flag_name = _RUNTIME_TO_BUILD_FLAG.get(runtime)
    module_info = _RUNTIME_TO_MODULE.get(runtime)
    if flag_name is None or module_info is None:
        raise RuntimeNotAvailableError(
            f"InferenceRuntime runtime {runtime!r} is not buildable in "
            "this binary."
        )
    if not _is_build_flag_on(flag_name):
        raise RuntimeNotAvailableError(
            f"InferenceRuntime runtime {runtime!r} requires "
            f"{flag_name}=ON in this binary; the flag is OFF."
        )
    module_name, class_name = module_info
    try:
        module = __import__(module_name, fromlist=[class_name])
    except ModuleNotFoundError as exc:
        raise RuntimeNotAvailableError(
            f"InferenceRuntime runtime {runtime!r} is configured but its "
            f"concrete impl module {module_name!r} has not been built into "
            "this binary yet (AZ-298 / AZ-299 / AZ-300 pending)."
        ) from exc
    strategy_cls = getattr(module, class_name)
    return strategy_cls(config)
