"""SU3 file format parser — signed update file format.

Parses the header of I2P's SU3 update files which contain
signed router updates, plugins, and other content.

Ported from net.i2p.crypto.SU3File.
"""

from __future__ import annotations

import struct
from dataclasses import dataclass

_SU3_MAGIC = b"I2Psu3"
_MIN_HEADER_SIZE = 40


@dataclass
class SU3Header:
    """Parsed SU3 file header."""
    magic: str
    format_version: int
    sig_type: int
    sig_length: int
    content_type: int
    file_type: int
    version_length: int
    signer_length: int
    content_length: int


def parse_su3_header(data: bytes) -> SU3Header | None:
    """Parse an SU3 file header from raw bytes.

    Returns None if the data is too short or has invalid magic.
    """
    if len(data) < _MIN_HEADER_SIZE:
        return None

    # Check magic (first 6 bytes)
    if data[:6] != _SU3_MAGIC:
        return None

    # Parse header fields
    # Byte 6: unused
    # Byte 7: format version (currently 0)
    format_version = data[7]

    # Bytes 8-9: signature type (big-endian uint16)
    sig_type = struct.unpack(">H", data[8:10])[0]

    # Bytes 10-11: signature length
    sig_length = struct.unpack(">H", data[10:12])[0]

    # Byte 12: unused
    # Byte 13: content type
    content_type = data[13] if len(data) > 13 else 0

    # Byte 14: unused
    # Byte 15: file type
    file_type = data[15] if len(data) > 15 else 0

    # Byte 16: version string length
    version_length = data[16] if len(data) > 16 else 0

    # Byte 17: unused
    # Byte 18: signer ID length
    signer_length = data[18] if len(data) > 18 else 0

    # Bytes 20-27: content length (big-endian uint64)
    content_length = 0
    if len(data) >= 28:
        content_length = struct.unpack(">Q", data[20:28])[0]

    return SU3Header(
        magic=_SU3_MAGIC.decode("ascii"),
        format_version=format_version,
        sig_type=sig_type,
        sig_length=sig_length,
        content_type=content_type,
        file_type=file_type,
        version_length=version_length,
        signer_length=signer_length,
        content_length=content_length,
    )