Digging into the post ranking code

The post ranking algorithm is included in the plugin and written here

function wpeddit_post_ranking($post_id){
	$x = get_post_meta($post_id, 'epicredvote', true );
	if($x == ""){
		$x = 0;
	}
	$ts = get_the_time("U",$post_id);
	if($x > 0){
		$y = 1;
	}elseif($x<0){ 		
                $y = -1; 	
        }else{ 		
                $y = 0; 	
        } 	$absx = abs($x); 	
        if($absx >= 1){
		$z = $absx;
	}else{
		$z = 1;
	}
	$rating = log10($z) + (($y * $ts)/45000);
	update_post_meta($post_id,'epicredrank',$rating);
	return $rating;
}

In mathematical notation the hot algorithm looks like this:

Effects of submission time

Following things can be said about submission time related to story ranking:

• Submission time has a big impact on the ranking and the algorithm will rank newer stories higher than older
• The score won’t decrease as time goes by, but newer stories will get a higher score than older. This is a different approach than the Hacker News’s algorithm which decreases the score as time goes by

Here is a visualization of the score for a story that has same amount of up and downvotes, but different submission time:

The logarithm scale

The hot ranking uses the logarithm function to weight the first votes higher than the rest. Generally this applies:

• The first 10 upvotes have the same weight as the next 100 upvotes which have the same weight as the next 1000 etc…

Here is a visualisation:

Without using the logarithm scale the score would look like this:

Effects of downvotes

Reddit is one of the few sites that has downvotes. As you can read in the code a story’s “score” is defined to be:

• up_votes – down_votes

The meaning of this can be visualised like this:

This has a big impact for stories that get a lot of upvotes and downvotes (e.g. controversial stories) as they will get a lower ranking than stories that just get upvotes. This could explain why kittens (and other non-controversial stories) rank so high 🙂

Conclusion of the algorithms

• Submission time is a very important parameter, generally newer stories will rank higher than older
• The first 10 upvotes count as high as the next 100. E.g. a story that has 10 upvotes and a story that has 50 upvotes will have a similar ranking
• Controversial stories that get similar amounts of upvotes and downvotes will get a low ranking compared to stories that mainly get upvotes