"""VAM v3 ASN.1 UPER decoding (ETSI TS 103 300-3 V2.3.1 / CDD V2.4.1).

Pinned schemas and notices are bundled in asn1/. See ../docs/vam-support.md.
"""
from functools import lru_cache
from pathlib import Path



@lru_cache(maxsize=1)
def codec():
    try:
        import asn1tools
    except ImportError as exc:
        raise ValueError("VAM decoder unavailable: install bridge/requirements.txt") from exc
    directory = Path(__file__).resolve().parent / "asn1"
    return asn1tools.compile_files([
        str(directory / "TS102894-2v241-CDD.asn"),
        str(directory / "VAM-PDU-Descriptions.asn"),
    ], codec="uper")


def json_value(value):
    """Preserve ASN.1 CHOICE/bit-string values in JSON-safe form."""
    if isinstance(value, bytes):
        return value.hex()
    if isinstance(value, dict):
        return {k: json_value(v) for k, v in value.items()}
    if isinstance(value, (tuple, list)):
        return [json_value(v) for v in value]
    return value


def decode_vam(data):
    if not data or len(data) > 4096:
        raise ValueError("empty/oversized VAM")
    decoder = codec()
    import asn1tools
    try:
        message = decoder.decode("VAM", data, check_constraints=True)
        # Detect trailing bytes, nonzero padding and unknown extensions that the
        # library may skip. Do not silently call partially understood data valid.
        if decoder.encode("VAM", message, check_constraints=True) != data:
            raise ValueError("noncanonical VAM or unsupported extension/trailing data")
    except (asn1tools.DecodeError, asn1tools.EncodeError, asn1tools.ConstraintsError) as exc:
        raise ValueError("invalid VAM UPER: " + str(exc)) from exc
    header = message["header"]
    if header["protocolVersion"] != 3 or header["messageId"] != 16:
        raise ValueError("unsupported VAM protocol version/message ID")
    body = message["vam"]
    containers = body["vamParameters"]
    basic = containers["basicContainer"]
    pos = basic["referencePosition"]
    hf = containers["vruHighFrequencyContainer"]
    result = dict(station_id=header["stationId"],
                  generation_delta_time=body["generationDeltaTime"],
                  station_type=basic["stationType"],
                  containers=json_value(containers))
    lat, lon = pos["latitude"], pos["longitude"]
    if lat != 900000001 and lon != 1800000001:
        result.update(lat=lat / 1e7, lon=lon / 1e7)
    altitude = pos["altitude"]["altitudeValue"]
    if altitude != 800001:
        result["altitude_m"] = altitude / 100
    heading = hf["heading"]["value"]
    if heading != 3601:
        result["heading_deg"] = heading / 10
    speed = hf["speed"]["speedValue"]
    if speed != 16383:
        result["speed_mps"] = speed / 100
    acceleration = hf["longitudinalAcceleration"]["longitudinalAccelerationValue"]
    if acceleration != 161:
        result["longitudinal_acceleration_mps2"] = acceleration / 10
    return result