Reddit's Time Based Algorithm

gistfile1.py

def get_rating(objects):
    """
    Pseduo-python for calculating a rating.
    """
 
    upvotes = get_upvotes(object)
    downvotes = get_downvotes(object)

    vote_diff = upvotes - downvotes
    abs_vote_diff = abs(vote_diff)

    # y simply 1, 0 or -1 based upon whether the difference 
    # between votes was positive, equal or negative.

    if vote_diff > 0:
        y = 1
    elif vote_diff == 0:
        y = 0
    else:
        y = -1

    # z is the maximal value of the absolute value of x and 1

    if abs_vote_diff >= 1:
        z = abs_vote_diff
    else:
        z = 1

    # We need to compare a constant timestamp against the date 
    # of submission timestamp. Set this to your development time.

    constant_timestamp = 1287662400
    submission_timestamp = time.mktime(objects.date_added.timetuple())
    age = submission_timestamp - constant_timestamp

    # Create the log for weighting use later.

    log_z = math.log10(z)

    # The water down constant defines how long objects stay "new" for.
    # With 45000 (12.5 hours), a 1-hour-old object would have to improve
    # its vote differential 10x over the next 12.5 hours to maintain its
    # rating to compensate for elapsed time. If you change the constant
    # you will have to recalculate all ratings for all objects (with a 
    # management command).
    water_down_constant = getattr(settings, 'WATER_DOWN_CONSTANT', 45000)
    
    weighted_calc = (y*age) / water_down_constant

    return log_z + weighted_calc