#!/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]
V1_CHANNEL_LIMITS = {1: 1024, 2: 2048, 3: 65515, 10: 1179, 11: 1179, 12: 1179}
V2_CHANNEL_LIMITS = {10: 1177, 11: 1177}


def classify(raw: bytes) -> str:
    if len(raw) < 3:
        return "invalid:truncated"
    if raw[:2] != b"VD":
        return "invalid:magic"
    if raw[2] not in (1, 2):
        return "invalid:unsupported_version"
    header_bytes = 21 if raw[2] == 1 else 23
    if len(raw) < header_bytes:
        return "invalid:truncated"
    limits = V1_CHANNEL_LIMITS if raw[2] == 1 else V2_CHANNEL_LIMITS
    if raw[3] not in limits:
        return "invalid:unknown_channel"
    if raw[4] != 0:
        return "invalid:flags"
    if raw[2] == 1:
        fragment_index, fragment_count = raw[17], raw[18]
        payload_length = int.from_bytes(raw[19:21], "big")
    else:
        fragment_index = int.from_bytes(raw[17:19], "big")
        fragment_count = int.from_bytes(raw[19:21], "big")
        payload_length = int.from_bytes(raw[21:23], "big")
        if fragment_count > 891:
            return "invalid:fragment_limit"
    if fragment_count == 0 or fragment_index >= fragment_count:
        return "invalid:fragment"
    if payload_length > limits[raw[3]]:
        return "invalid:payload_limit"
    if len(raw) != header_bytes + payload_length:
        return "invalid:length_mismatch"
    if raw[2] == 1 and len(raw) > 65536 or raw[2] == 2 and len(raw) > 1200:
        return "invalid:frame_limit"
    return "valid"


def main() -> None:
    for fixture in ("datagram-v1.tsv", "datagram-v2.tsv"):
        lines = (ROOT / "fixtures/conformance" / fixture).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 in ("1", "2")
            encoded = input_value.removeprefix("hex=")
            try:
                actual = classify(binascii.unhexlify(encoded))
            except binascii.Error:
                actual = "invalid:hex"
            assert actual == expected, f"{fixture}:{identifier}: {actual} != {expected}"
    print("Datagram frame validation passed")


if __name__ == "__main__":
    main()