#!/usr/bin/env python3
"""Validate the bounded Phase 3A datagram header and fixture corpus."""

from __future__ import annotations

import binascii
import pathlib

ROOT = pathlib.Path(__file__).resolve().parents[1]
HEADER_BYTES = 21
MAX_FRAME_BYTES = 65536
CHANNEL_LIMITS = {1: 1024, 2: 2048, 3: 65515, 10: 1179, 11: 1179, 12: 1179}


def classify(raw: bytes) -> str:
    if len(raw) < HEADER_BYTES:
        return "invalid:truncated"
    if raw[:2] != b"VD":
        return "invalid:magic"
    if raw[2] != 1:
        return "invalid:unsupported_version"
    if raw[3] not in CHANNEL_LIMITS:
        return "invalid:unknown_channel"
    if raw[4] != 0:
        return "invalid:flags"
    fragment_index, fragment_count = raw[17], raw[18]
    if fragment_count == 0 or fragment_index >= fragment_count:
        return "invalid:fragment"
    payload_length = int.from_bytes(raw[19:21], "big")
    if payload_length > CHANNEL_LIMITS[raw[3]]:
        return "invalid:payload_limit"
    if len(raw) != HEADER_BYTES + payload_length:
        return "invalid:length_mismatch"
    if len(raw) > MAX_FRAME_BYTES:
        return "invalid:frame_limit"
    return "valid"


def main() -> None:
    lines = (ROOT / "fixtures/conformance/datagram-v1.tsv").read_text(encoding="utf-8").splitlines()
    assert lines[0] == "id\tversion\tkind\tinput\texpected"
    for line in lines[1:]:
        identifier, version, kind, input_value, expected = line.split("\t")
        assert kind == "datagram" and version == "1"
        encoded = input_value.removeprefix("hex=")
        try:
            actual = classify(binascii.unhexlify(encoded))
        except binascii.Error:
            actual = "invalid:hex"
        assert actual == expected, f"{identifier}: {actual} != {expected}"
    print("Datagram frame validation passed")


if __name__ == "__main__":
    main()