trending

<?php

namespace App\Helpers;
/*
 * This class is an implementation of http://stackoverflow.com/questions/787496/what-is-the-best-way-to-compute-trending-topics-or-tags/826509#826509 in PHP.
 */
class ZScore {
    // The rate at which the historic data's effect will diminish
    private $decay;
    // The population (data)
    private $pop;
    private $avg;
    private $sqrAvg;
    
    public function __construct($decay, $pop) {
        $this->sqrAvg = $this->avg = 0;
        $this->decay = $decay;
        $this->pop = $pop;
        foreach($this->pop as $x) {
            $this->update($x);
        }
    }
    
    private function update($value) {
        
        if($this->avg == 0 && $this->sqrAvg == 0) {
            // Set initial averages to the first value in the sequence.
            $this->avg = $value;
            $this->sqrAvg = pow($value, 2);
        } else {
            // Calculate the average of the rest of the values using a floating average.
            $this->avg = $this->avg * $this->decay + $value * (1 - $this->decay);
            $this->sqrAvg = $this->sqrAvg * $this->decay + pow($value, 2) * (1 - $this->decay);
        }
        
        return $this;
    }
    
    private function std() {
        // Somewhat ad-hoc standard deviation calculation.
        return sqrt($this->sqrAvg - pow($this->avg, 2));
    }
    
    public function score($obs) {
        if($this->std() == 0) {
            return ($obs - $this->avg) * 1.8e308;
        } else {
            return ($obs - $this->avg) / $this->std();
        }
        
    }
        
}