Change trending hashtags to not disappear instantly after midnight #11712
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When a hashtag is used, update its counters, and mark it down as used on that day for easy retrieval. Do not calculate the score right then, instead, add a regular scheduler. The scheduler goes over all used hashtags, and hashtags that were previously in the trends, and recalculates their scores. If a score peaks, record the time at which it peaked. Use the peak score, but decay it from the point at which it peaked with a half-life of 6 hours, for the sorting in the trends.
This removes the need to fully reset the trends at midnight, while ensuring that newer trends take precedence over older ones.