Decayed ranking basic implementation

decay_ranking.py

import math
import operator

class DecayScore(object):
  """
  Class for calculating basic decayable score over keys (items) and there score values.
  It's not thread safe and not doesn't store anykind of timestamps against items stored.
  The caller is responsible of keeping track of time.
  """
  
  def __init__(me, init_score, half_life):
    """
    Initializes the instance of decaying score class for set of items
    :param init_score: The initial score assigned to new item.
    :param half_life: The half life time of an item.
    """
    me.__score = {}
    me.__n0 = init_score
    me.__lambda = math.log(2) / half_life

  def decay(me, key, time_elapsed):
    """
    Decays a given key (item identifier) with a given time elapsed.
    The function does not store any kind of time stamp or last update value.
    Caller is responsible for calculating and passing in correct delta.
    :param key: A string or key value for dictionary of item. Usually this will be your item identifier.
    :param time_elapsed: The amount of time elapsed. This must be in same units as half_life passed in constructor.
    """
    s = me.__n0 
    if key in me.__score:
      s = me.__score[key] 
    me.__score[key] = s - me.__lambda * time_elapsed

  def update(me, key, score, time_elapsed = 0):
    """
    Updates the score of an item (key) with given score and decays it if the time elapsed is passed in.
    The function does not store any kind of time stamp or last update value.
    Caller is responsible for calculating and passing in correct delta.
    :param key: A string or key value for dictionary of item. Usually this will be your item identifier.
    :param score: The amount of score to increment.
    :param time_elapsed: The amount of time elapsed. This must be in same units as half_life passed in constructor.
    """
    me.__score[key] = me.__score.get(key, me.__n0) - (me.__lambda * time_elapsed) + score

  def get(me):
    """
    Returns copy of the unsorted score dictionary.
    """
    return me.__score.copy()
  
  def get_ranked(me):
    """
    Returns sorted set of tuples containing key and score of items
    """
    x = me.__score
    return sorted(x.iteritems(), key=operator.itemgetter(1))
    

if __name__ == "__main__":
  x = DecayScore(math.log(10), 24)

  for i in range(24):
    x.decay('foo', 2)
    x.decay('bar', 1)
    print x.get_ranked()