def count_and_log(message, i, total, log_every_n):
    """Print a message to reflect the progress of processing once a while.

    Parameters
    ----------
    message : str
        Message to print.
    i : int
        Current index.
    total : int
        Total count.
    log_every_n : None or int
        Molecule related computation can take a long time for a large dataset and we want
        to learn the progress of processing. This can be done by printing a message whenever
        a batch of ``log_every_n`` molecules have been processed. If None, no messages will
        be printed.
    """
    if log_every_n is not None and (i + 1) % log_every_n == 0:
        print('{} {:d}/{:d}'.format(message, i + 1, total))
