Sorting.php

<?php
 
//////////////////////////////////
// Reddit "hot" story algorithm //
//////////////////////////////////
function hot($ups, $downs, $date)
{
	if (is_string($date)) $date = strtotime($date);
 
	$s = $ups - $downs;
	$order = log10(max(abs($s), 1));
 
	if ($s > 0)
		$sign = 1;
	elseif ($s < 0)
		$sign = -1;
	else
		$sign = 0;
 
	$seconds = $date - 1134028003;
 
	return round($sign * $order + $seconds / 45000, 7);
}
 
/////////////////////////////////////
// Reddit "Best" comment algorithm //
/////////////////////////////////////
function confidence($ups, $downs)
{
	$n = $ups + $downs;
 
	if ($n == 0) return 0;
 
	$z    = 1.281551565545; // 80% confidence
	$phat = $ups / $n;
 
	$left  = $phat + 1/(2*$n)*$z*$z;
	$right = $z*sqrt($phat*(1-$phat)/$n + $z*$z/(4*$n*$n));
	$under = 1+1/$n*$z*$z;
 
	return ($left - $right) / $under;
}
 
//////////////////////////////////////////////
// Hacker News front page ranking algorithm //
//////////////////////////////////////////////
function calculateScore($votes, $hourAge, $gravity = 1.8)
{
	return ($votes-1) / pow(($hourAge+2), $gravity);
}
 
//////////////////////////////////////
// Zite "Popular on Zite" algorithm //
//////////////////////////////////////
function popular($ups, $downs, $hourAge, $clicks)
{
	$averageRating = confidence($ups, $downs);
	$interest = log10((1+$clicks) / (2*$hourAge/6.0));
 
	return $averageRating * $interest;
}
 
// @see http://stackoverflow.com/questions/11653545/hot-content-algorithm-score-with-time-decay
function decay($score, $weekNo)
{
	$score = 30;
	$baseScore = log10(max($score,1));
 
	if ($weekNo > 1) {
		$x = $weekNo - 1;
		$baseScore = $baseScore * exp(-8*$x*$x);
	}
	
	return $baseScore;
}