Get the last n lines of a file.

tail.py

from pathlib import Path


def tail(filename: str | Path, n: int) -> str:
    """
    Get the last n lines of a file.

    Parameters:
    - filename (str | Path): The path to the file.
    - n (int): The number of lines to retrieve from the end of the file.

    Returns:
    - str: The last n lines of the file.

    Algorithm Explanation:
    - We move the cursor to the end of the file using `file.seek(0, 2)`.
    - We iterate backward through the file character by character to find newline characters.
    - We keep track of the number of newline characters found (`count`).
    - Once we reach the desired number of newline characters (n + 1), we read from the last newline character found.

    Note:
    - This approach efficiently retrieves the last n lines without reading the entire file into memory.

    """
    if Path(filename).stat().st_size == 0:
        raise IOError("File is empty")
    idx = 0
    count = 0
    with open(filename, "rb") as file:
        # Move cursor to the end of the file
        file.seek(0, 2)
        # Start search for new lines
        while count < n + 1:
            idx += 1
            file.seek(-idx, 2)
            if file.read(1) == b"\n":
                count += 1
            if file.tell() == 0:
                break
        data = file.read()
    return data.decode("utf-8")