Gazelle/sections/torrents/ranking_funcs.php

<?php
function inverse_ncdf($p) {
/***************************************************************************
 *																inverse_ncdf.php
 *														-------------------
 *	 begin								: Friday, January 16, 2004
 *	 copyright						: (C) 2004 Michael Nickerson
 *	 email								: nickersonm@yahoo.com
 *
 ***************************************************************************/

	//Inverse ncdf approximation by Peter John Acklam, implementation adapted to
	//PHP by Michael Nickerson, using Dr. Thomas Ziegler's C implementation as
	//a guide.	http://home.online.no/~pjacklam/notes/invnorm/index.html
	//I have not checked the accuracy of this implementation.	Be aware that PHP
	//will truncate the coeficcients to 14 digits.

	//You have permission to use and distribute this function freely for
	//whatever purpose you want, but please show common courtesy and give credit
	//where credit is due.

	//Input paramater is $p - probability - where 0 < p < 1.

	//Coefficients in rational approximations
	$a = array(1 => -3.969683028665376e+01, 2 => 2.209460984245205e+02,
			   3 => -2.759285104469687e+02, 4 => 1.383577518672690e+02,
			   5 => -3.066479806614716e+01, 6 => 2.506628277459239e+00);

	$b = array(1 => -5.447609879822406e+01, 2 => 1.615858368580409e+02,
			   3 => -1.556989798598866e+02, 4 => 6.680131188771972e+01,
			   5 => -1.328068155288572e+01);

	$c = array(1 => -7.784894002430293e-03, 2 => -3.223964580411365e-01,
			   3 => -2.400758277161838e+00, 4 => -2.549732539343734e+00,
			   5 => 4.374664141464968e+00,  6 => 2.938163982698783e+00);

	$d = array(1 => 7.784695709041462e-03, 2 => 3.224671290700398e-01,
			   3 => 2.445134137142996e+00, 4 => 3.754408661907416e+00);

	//Define break-points.
	$p_low  = 0.02425;									 //Use lower region approx. below this
	$p_high = 1 - $p_low;								 //Use upper region approx. above this

	//Define/list variables (doesn't really need a definition)
	//$p (probability), $sigma (std. deviation), and $mu (mean) are user inputs
	$q = NULL; $x = NULL; $y = NULL; $r = NULL;

	//Rational approximation for lower region.
	if (0 < $p && $p < $p_low) {
		$q = sqrt(-2 * log($p));
		$x = ((((($c[1] * $q + $c[2]) * $q + $c[3]) * $q + $c[4]) * $q + $c[5]) *
	 	 	 	 $q + $c[6]) / (((($d[1] * $q + $d[2]) * $q + $d[3]) * $q + $d[4]) *
		 		 $q + 1);
	}

	//Rational approximation for central region.
	elseif ($p_low <= $p && $p <= $p_high) {
		$q = $p - 0.5;
		$r = $q * $q;
		$x = ((((($a[1] * $r + $a[2]) * $r + $a[3]) * $r + $a[4]) * $r + $a[5]) *
	 	 	 	 $r + $a[6]) * $q / ((((($b[1] * $r + $b[2]) * $r + $b[3]) * $r +
		 		 $b[4]) * $r + $b[5]) * $r + 1);
	}

	//Rational approximation for upper region.
	elseif ($p_high < $p && $p < 1) {
		$q = sqrt(-2 * log(1 - $p));
		$x = -((((($c[1] * $q + $c[2]) * $q + $c[3]) * $q + $c[4]) * $q +
	 	 	 	 $c[5]) * $q + $c[6]) / (((($d[1] * $q + $d[2]) * $q + $d[3]) *
		 		 $q + $d[4]) * $q + 1);
	}

	//If 0 < p < 1, return a null value
	else {
		$x = NULL;
	}

	return $x;
	//END inverse ncdf implementation.
}

// Confidence level for binomial scoring.  Just compute this once.
define(Z_VAL, inverse_ncdf(1-(1-.95)/2));

// Implementation of the algorithm described at http://www.evanmiller.org/how-not-to-sort-by-average-rating.html
function binomial_score($Ups, $Total) {
	if (($Total <= 0) || ($Ups < 0)) {
		return 0;
	}
	$phat = $Ups/$Total;
	return ($phat + Z_VAL*Z_VAL/(2*$Total) - Z_VAL*sqrt(($phat*(1-$phat)+Z_VAL*Z_VAL/(4*$Total))/$Total))/(1+Z_VAL*Z_VAL/$Total);
}
?>