import os


def ExtractVolumeLetter(path: str) -> str:
    """从绝对路径中提取盘符"""
    drive = os.path.splitdrive(path)[0]
    if not drive:
        raise ValueError(f"无法从路径中提取盘符：{path}")
    return drive[0].upper()  # 返回 'Y'


def CopySingleFragmentFiles(source_data_dict, target_path):
    """
    根据起始字节和长度，从磁盘中读取数据并保存为目标文件

    :param source_data_dict: 包含源数据信息的字典
    :param target_path: 目标文件夹路径
    """
    start_byte = source_data_dict.get("start_byte")
    byte_length = source_data_dict.get("length")
    absolute_path = source_data_dict.get("absolute_path")
    file_name = source_data_dict.get("filename")

    if byte_length <= 0:
        print("错误：字节长度无效")
        return

    if not absolute_path or not file_name:
        print("错误：缺少必要的文件信息")
        return

    source_disk_path = ExtractVolumeLetter(absolute_path)
    target_file_path = os.path.join(target_path, file_name)

    try:
        # 创建目标目录（如果不存在）
        os.makedirs(target_path, exist_ok=True)

        with open(fr"\\.\{source_disk_path}:", 'rb') as disk:
            disk.seek(start_byte)

            with open(target_file_path, 'wb') as f:
                remaining = byte_length
                CHUNK_SIZE = 1024 * 1024  # 1MB
                while remaining > 0:
                    read_size = min(CHUNK_SIZE, remaining)
                    chunk = disk.read(read_size)
                    if not chunk:
                        print("警告：读取到空数据，可能已到达磁盘末尾。")
                        break
                    f.write(chunk)
                    remaining -= len(chunk)

        print(
            f"成功：已从字节偏移量 {start_byte} 读取 {byte_length} 字节，保存为 {target_file_path}")

    except PermissionError:
        print("错误：需要管理员权限访问磁盘设备，请以管理员身份运行此程序")
    except Exception as e:
        print(f"发生错误: {str(e)}")


def ReadDiskBytes(volume_letter: str, start_byte: int, length: int) -> bytes:
    """
    从指定磁盘的指定起始位置读取指定长度的字节。

    :param volume_letter: 盘符（如 "Y"）
    :param start_byte: 起始字节位置（整数）
    :param length: 要读取的字节数（整数）
    :return: 读取到的原始字节数据（bytes）
    """
    if not isinstance(volume_letter, str) or len(volume_letter.strip()) != 1:
        raise ValueError("drive_letter 必须是单个字母，如 'Y'")

    # 构建 Windows 设备路径格式：\\.\Y:
    disk_path = f"\\\\.\\{volume_letter.strip().upper()}:"

    try:
        with open(disk_path, "rb") as disk:
            disk.seek(start_byte)
            data = disk.read(length)
            return data
    except PermissionError:
        raise PermissionError("权限不足，请以管理员身份运行程序")
    except Exception as e:
        raise RuntimeError(f"读取磁盘失败：{e}")


def CopyMultiFragmentFiles(
        item: dict,
        fragment_lists: dict,
        target_path: str
):
    """
    处理具有多个分片的文件，读取并按顺序拼接内容，最终写入磁盘。

    :param item: 包含文件分片信息的字典
    :param fragment_lists: 存储各文件分片内容的字典
    :param target_path: 恢复文件的目标保存路径
    :return: None
    """
    file_name = item['filename']
    extent_count = item['extent_count']
    fragment_index = item['fragment_index']
    start_byte = item['start_byte']
    length_byte = item['length']

    volume_letter = ExtractVolumeLetter(item['absolute_path'])

    # 读取分片内容
    fragment_content = ReadDiskBytes(volume_letter, start_byte, length_byte)

    # 如果还没有为这个文件创建列表，则初始化
    if file_name not in fragment_lists:
        fragment_lists[file_name] = [None] * extent_count

    # 将内容插入到指定位置
    if fragment_index <= extent_count:
        fragment_lists[file_name][fragment_index - 1] = fragment_content
        print(f"已写入 {file_name} 的第 {fragment_index} 个片段。")
    else:
        print(f"警告：{file_name} 的 fragment_index 超出范围：{fragment_index} / {extent_count}")

    # 检查是否所有分片都已加载
    fragments = fragment_lists[file_name]
    if None not in fragments:
        full_content = b''.join(fragments)
        target_file_path = os.path.join(target_path, file_name)
        try:
            with open(target_file_path, 'wb') as f:
                f.write(full_content)
            print(f"已成功恢复文件：{file_name}")
        except Exception as e:
            print(f"写入文件失败：{file_name}，错误：{e}")