Algorithm to determine thread "hotness"

Question

I'm trying to come up with a way to determine how "hot" certain threads are in a forum. What criteria would you use and why? How would these come together to give a hotness score?

The criteria I'm thinking of include:

• how many replies
• how long since the last reply
• average time between replies

The problems this algorithm must solve:

• A thread which has 500 replies is clearly hot, unless the last reply was over a year ago.
• A thread with 500 replies that was replied to a second ago is clearly hot, unless it's taken 4 years to reach 500 replies.
• A thread with 15 replies in the last 4 minutes is really hot!

Any ideas, thoughts or complete solutions out there?

Answer 1

this might be what you are looking for:

http://stackoverflow.com/questions/32397/popularity-algorithm

Answer 2

I was thinking you could probably model it with diminishing waves here, using amplitude (or root mean square) to measure hotness. As time goes, the wave diminishes, and so a late reply will only cause a little stir.

In practice, I think this requires a lot of calculation. You could make good use of caching to speed up the calculation.

Just my two cents.

Answer 3

Simplest algorithm: If there have been greater than X replies since Y, it is hot.

If you prefer something that scales, just count how many replies since time y. More replies means more hotness.

Answer 4

In short I've found logarithmic decay of "hotness" to be the most natural.

Answer 5

Jeff Atwood has a nice question about this with a ton of information on other "hot" algorithms. I suggest using one of those and adapting it to your liking.

Answer 6

Thanks to those who posted the links to the other questions/answers. Unfortunately, those equations take a lot more things into consideration than what's possible with my setup (eg: voting, reputation of author, etc)

After playing around with it, I've come up with this equation which I'll use for the time being:

log10($numOfReplies * 20000 / pow($timeSinceLastPost, 1.3))

It still could use some work. For example, if there's a really really popular but old thread, it'll be low on the hotness, but if one person replies to it that'll put it right back to the top for a few days/weeks.

Answer 7

Why not just use a sort of exponential decay model. Hotness of thread = sum( k^(time since posting) ) for all posts. This has the advantage of being really easy to update and calculate. You'd have to play around with k and your unit of time measurement (k should be < 1, but fairly close to it)

Current hotness = hotness at time of last post * k^(time since last post).
Hotness after new post = current hotness + 1

Answer 8

One thing you should pay some attention to is whether people might want to "game" the algorithm in order to make/keep their threads "hot". Actually, you can pretty much assume that they will.

The minimum you should do to discourage this is to only consider replies from different people.