level3_refactor.py

class FileServer:
    def __init__(self):
        # TODO: initialize your data structure here
        pass

    def FILE_UPLOAD_AT(self, timestamp: int, file_name: str, size: int, ttl=None):
        """
        Upload a file at a given timestamp.
        If ttl is provided, the file expires at timestamp + ttl.
        Raise RuntimeError if file already exists.
        """
        # TODO: implement upload with ttl

    def FILE_GET_AT(self, timestamp: int, file_name: str):
        """
        Return size if file exists and is alive at given timestamp, else None.
        """
        # TODO: implement get with ttl check

    def FILE_COPY_AT(self, timestamp: int, source: str, dest: str):
        """
        Copy the source file to a new destination at the given timestamp.
        - The copied file uses the current timestamp as its upload time.
        - The copied file inherits the same TTL duration as the source.
        - Raise RuntimeError if the source file does not exist or is expired at the given timestamp.
        - Overwrite the destination if it already exists.
        """
        # TODO: implement copy with ttl

    def FILE_SEARCH_AT(self, timestamp: int, prefix: str):
        """
        Return up to 10 alive files starting with prefix, sorted by size desc, name asc.
        """
        # TODO: implement search with ttl filter