lemmy/docs/src/about_ranking.md

# Trending / Hot / Best Sorting algorithm
## Goals
- During the day, new posts and comments should be near the top, so they can be voted on.
- After a day or so, the time factor should go away.
- Use a log scale, since votes tend to snowball, and so the first 10 votes are just as important as the next hundred.

## Implementations

### Reddit
Does not take the lifetime of the thread into account, [giving early comments an overwhelming advantage over later ones,](https://minimaxir.com/2016/11/first-comment/) with the effect being even worse in small communities. New comments pool at the bottom of the thread, effectively killing off discussion and making each thread a race to comment early.  This lowers the quality of conversation and rewards comments that are repetitive and spammy.

### Hacker News

While far superior to Reddit's implementation for its decay of scores over time, [Hacker News' ranking algorithm](https://medium.com/hacking-and-gonzo/how-hacker-news-ranking-algorithm-works-1d9b0cf2c08d) does not use a logarithmic scale for scores.

### Lemmy

Counterbalances the snowballing effect of votes over time with a logarithmic scale.  Negates the inherent advantage of early comments while still ensuring that votes still matter in the long-term, not nuking older popular comments.

```
Rank = ScaleFactor * log(Max(1, 3 + Score)) / (Time + 2)^Gravity

Score = Upvotes - Downvotes
Time = time since submission (in hours)
Gravity = Decay gravity, 1.8 is default
```
- Lemmy uses the same `Rank` algorithm above, in two sorts: `Active`, and `Hot`.
  - `Active` uses the post votes, and latest comment time (limited to two days).
  - `Hot` uses the post votes, and the post published time.
- Use Max(1, score) to make sure all comments are affected by time decay.
- Add 3 to the score, so that everything that has less than 3 downvotes will seem new. Otherwise all new comments would stay at zero, near the bottom.
- The sign and abs of the score are necessary for dealing with the log of negative scores.
- A scale factor of 10k gets the rank in integer form.

A plot of rank over 24 hours, of scores of 1, 5, 10, 100, 1000, with a scale factor of 10k.

![](https://raw.githubusercontent.com/LemmyNet/lemmy/main/docs/img/rank_algorithm.png)