"""二进制解析辅助函数。"""

import struct
from typing import Any

from .exceptions import TdxDecodeError


def require_bytes(
    data: bytes | bytearray,
    pos: int,
    size: int,
    context: str,
) -> None:
    """确保从 pos 起至少还能读取 size 字节。"""
    if pos < 0:
        raise TdxDecodeError(f"{context}: 非法偏移 {pos}")
    end = pos + size
    if end > len(data):
        remaining = max(len(data) - pos, 0)
        raise TdxDecodeError(
            f"{context}: 数据不足，需要 {size} 字节，偏移 {pos}，实际剩余 {remaining} 字节"
        )


def unpack_from(
    fmt: str,
    data: bytes | bytearray,
    pos: int,
    context: str,
) -> tuple[Any, ...]:
    """带边界检查的 struct.unpack_from。"""
    require_bytes(data, pos, struct.calcsize(fmt), context)
    try:
        return struct.unpack_from(fmt, data, pos)
    except struct.error as e:  # pragma: no cover - require_bytes 已覆盖大部分路径
        raise TdxDecodeError(f"{context}: 解析失败: {e}") from e


def slice_bytes(
    data: bytes | bytearray,
    pos: int,
    size: int,
    context: str,
) -> bytes:
    """带边界检查的切片读取。"""
    require_bytes(data, pos, size, context)
    return bytes(data[pos : pos + size])