First TwoPlayerTrueSkillCalculator unit test passed

PHPSkills/Rating.php

@@ -0,0 +1,79 @@
+<?php
+
+namespace Moserware\Skills;
+
+// Container for a player's rating.
+class Rating 
+{
+    const CONSERVATIVE_STANDARD_DEVIATION_MULTIPLIER = 3;
+    
+    private $_conservativeStandardDeviationMultiplier;
+    private $_mean;
+    private $_standardDeviation;
+
+    /**
+     * Constructs a rating.
+     * @param double $mean The statistical mean value of the rating (also known as mu).
+     * @param double $standardDeviation The standard deviation of the rating (also known as s).
+     * @param double $conservativeStandardDeviationMultiplier optional The number of standardDeviations to subtract from the mean to achieve a conservative rating.
+     */
+    public function __construct($mean, $standardDeviation, $conservativeStandardDeviationMultiplier = self::CONSERVATIVE_STANDARD_DEVIATION_MULTIPLIER)
+    {
+        $this->_mean = $mean;
+        $this->_standardDeviation = $standardDeviation;
+        $this->_conservativeStandardDeviationMultiplier = $conservativeStandardDeviationMultiplier;
+    }
+
+    /**
+     * The statistical mean value of the rating (also known as <20>).
+     */
+    public function getMean()
+    {
+        return $this->_mean;        
+    }
+
+    /**
+     * The standard deviation (the spread) of the rating. This is also known as s.
+     */
+    public function getStandardDeviation()
+    {
+        return $this->_standardDeviation;
+    }
+
+    /**
+     * A conservative estimate of skill based on the mean and standard deviation.
+     */
+    public function getConservativeRating()
+    {
+        return $this->_mean - $this->_conservativeStandardDeviationMultiplier*$this->_standardDeviation;
+    }
+
+    public function getPartialUpdate(Rating $prior, Rating $fullPosterior, $updatePercentage)
+    {
+        $priorGaussian = new GaussianDistribution($prior->getMean(), $prior->getStandardDeviation());
+        $posteriorGaussian = new GaussianDistribution($fullPosterior->getMean(), $fullPosterior.getStandardDeviation());
+
+        // From a clarification email from Ralf Herbrich:
+        // "the idea is to compute a linear interpolation between the prior and posterior skills of each player 
+        //  ... in the canonical space of parameters"
+
+        $precisionDifference = $posteriorGaussian->getPrecision() - $priorGaussian->getPrecision();
+        $partialPrecisionDifference = $updatePercentage*$precisionDifference;
+
+        $precisionMeanDifference = $posteriorGaussian->getPrecisionMean() - $priorGaussian.getPrecisionMean();
+        $partialPrecisionMeanDifference = $updatePercentage*$precisionMeanDifference;
+
+        $partialPosteriorGaussion = GaussianDistribution::fromPrecisionMean(
+            $priorGaussian->getPrecisionMean() + $partialPrecisionMeanDifference,
+            $priorGaussian->getPrecision() + $partialPrecisionDifference);
+
+        return new Rating($partialPosteriorGaussion->getMean(), $partialPosteriorGaussion->getStandardDeviation(), $prior->_conservativeStandardDeviationMultiplier);
+    }
+
+    public function __toString()
+    {
+        return 'mean=' . $this->_mean . ' standardDeviation=' . $this->_standardDeviation;
+    }  
+}
+
+?>