reddit-ranking-algorithms.php

<?php
// Zeuxis Lo 
// Created at 2011-03-23 05:05 PM

date_default_timezone_set("Asia/Hong_Kong");

function calculate_rank_sum($score, $created_at) {
	$order = log10(max(abs($score), 1));
	
	if ($score > 0) {
		$sign = 1;
	}elseif ($score < 0) {
		$sign = -1;	
	}else{
		$sign = 0;
	}

	$seconds = intval(($created_at - mktime(0, 0, 0, 1, 1, 1970))/86400);
	$long_number = $order + $sign * $seconds / 45000;
	
	return round($long_number, 7);
}

function confidence($ups, $downs) {
	if ($ups + $downs == 0) {
		return 0;	
	}else{
		$n = $ups + $downs;
		
		if ($n == 0) return 0;
		
		$z = 1.0;
		$phat = (float)$ups / $n;
		return sqrt($phat+$z*$z/(2*$n)-$z*(($phat*(1-$phat)+$z*$z/(4*$n))/$n))/(1+$z*$z/$n);
	}
}

echo calculate_rank_sum(10, time()), "\n";
echo confidence(10, 0), "\n";
?>