Start of factor graph port. Things don't work yet, but a lot of syntax updates towards PHP

PHPSkills/TrueSkill/Factors/GaussianWithinFactor.php

@@ -0,0 +1,72 @@
+<?php
+
+namespace Moserware\Skills\TrueSkill\Factors;
+
+/// <summary>
+/// Factor representing a team difference that has not exceeded the draw margin.
+/// </summary>
+/// <remarks>See the accompanying math paper for more details.</remarks>
+class GaussianWithinFactor extends GaussianFactor
+{
+    private $_epsilon;
+
+    public function __construct($epsilon, Variable $variable)
+    {
+        $this->_epsilon = $epsilon;
+        $this->createVariableToMessageBinding($variable);
+    }
+
+    public function getLogNormalization()
+    {
+        $variables = $this->getVariables();
+        $marginal = $variables[0]->getValue();
+
+        $messages = $this->getMessages();
+        $message = $messages[0]->getValue();
+        $messageFromVariable = GaussianDistribution::divide($marginal, $message);
+        $mean = $messageFromVariable->getMean();
+        $std = $messageFromVariable->getStandardDeviation();
+        $z = GaussianDistribution::cumulativeTo(($this->_epsilon - $mean)/$std)
+             -
+             GaussianDistribution::cumulativeTo((-$this->_epsilon - $mean)/$std);
+
+        return -GaussianDistribution::logProductNormalization($messageFromVariable, $message) + log($z);
+    }
+
+    protected function updateMessage(Message $message, Variable $variable)
+    {
+        $oldMarginal = clone $variable->getValue();
+        $oldMessage = clone $message->getValue();
+        $messageFromVariable = GaussianDistribution::divide($oldMarginal, $oldMessage);
+
+        $c = $messageFromVariable->getPrecision();
+        $d = $messageFromVariable->getPrecisionMean();
+
+        $sqrtC = sqrt($c);
+        $dOnSqrtC = $d/$sqrtC;
+
+        $epsilonTimesSqrtC = $this->_epsilon*$sqrtC;
+        $d = $messageFromVariable->getPrecisionMean();
+
+        $denominator = 1.0 - TruncatedGaussianCorrectionFunctions::wWithinMargin($dOnSqrtC, $epsilonTimesSqrtC);
+        $newPrecision = $c/$denominator;
+        $newPrecisionMean = ($d +
+                                   $sqrtC*
+                                   TruncatedGaussianCorrectionFunctions::vWithinMargin($dOnSqrtC, $epsilonTimesSqrtC))/
+                                  $denominator;
+
+        $newMarginal = GaussianDistribution::fromPrecisionMean($newPrecisionMean, $newPrecision);
+        $newMessage = GaussianDistribution::divide(
+                        GaussianDistribution::multiply($oldMessage, $newMarginal),
+                        $oldMarginal);
+
+        /// Update the message and marginal
+        $message->setValue($newMessage);
+        $variable->setValue($newMarginal);
+
+        /// Return the difference in the new marginal
+        return GaussianDistribution::subtract($newMarginal, $oldMarginal);
+    }
+}
+
+?>