import socket


def find_available_port(
    preferred_port: int | None = None,
    host: str = "127.0.0.1",
) -> int:
    """Return an available TCP port on the given host.

    The preferred port is returned when available. Otherwise, the operating
    system selects an available ephemeral port.
    """
    if preferred_port is not None:
        if not 1 <= preferred_port <= 65535:
            raise ValueError("preferred_port must be between 1 and 65535")

        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
            try:
                sock.bind((host, preferred_port))
            except OSError:
                pass
            else:
                return preferred_port

    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
        sock.bind((host, 0))
        return int(sock.getsockname()[1])