Redis sorted set ranking changes

1_redis_sorted_set_ranking_changes.rb

redis.zadd('scores:20120701', 10, 'alice')
redis.zadd('scores:20120701',  7, 'bob')
redis.zadd('scores:20120701',  5, 'carol')
# resulting ranking:
# 1 - alice
# 2 - carol
# 3 - bob

redis.zadd('scores:20120702',  7, 'alice')
redis.zadd('scores:20120702',  9, 'bob')
redis.zadd('scores:20120702', 15, 'carol')
# resulting ranking:
# 1 - carol
# 2 - bob
# 3 - alice

redis.zsomething('scores:changes:20120701-20120702', 'scores:20120701', 'scores:20120702')
# expected result:
# carol: +2
# bob: 0
# alice: -2

2_iterative_solution.rb

old_ranks = redis.zrevrangebyscore('scores:20120701', '+inf', '-inf')
new_ranks = redis.zrevrangebyscore('scores:20120702', '+inf', '-inf')

redis.multi do |redis|
  old_ranks.each_with_index do |user, i|
    redis.zadd('scores:ranking:20120701', i + 1, user)
  end
  new_ranks.each_with_index do |user, i|
    redis.zadd('scores:ranking:20120702', i + 1, user)
  end
end

redis.zinterstore('scores:changes:20120701-20120702', ['scores:ranking:20120701', 'scores:ranking:20120702'], :weights => [1, -1])

Use case: I want to show a ranking of users including a little icon that indicates how their ranking has changed compared to rankings of previous days/weeks/periods (up arrow = better ranking etc.). Is there a way to do this directly in Redis? I imagine I could store away the rankings two separate sorted sets and do a weighted zunionstore over those sets but I'd much prefer a direct approach. :)

I've added a simple iterative solution based on intersecting the two rankings and weighing the newer ranking as negative. It's simple and I guess it scales OK since it's a one-off calculation but I'm still curious if there's a better way. :)

Having the same problem, will probably use your solution ;). Unless you figured out something better since 2012 ^^?