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

PHPSkills/TrueSkill/Layers/TeamPerformancesToTeamPerformanceDifferencesLayer.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace Moserware\Skills\TrueSkill\Layers;
+
+class TeamPerformancesToTeamPerformanceDifferencesLayer extends TrueSkillFactorGraphLayer
+{
+    public function __construct(TrueSkillFactorGraph $parentGraph)
+    {
+        parent::__construct($parentGraph);
+    }
+
+    public function buildLayer()
+    {
+        $inputVariablesGroup = $this->getInputVariablesGroups();
+        $inputVariablesGroupCount = count($inputVariablesGroup);
+
+        for ($i = 0; $i < $inputVariablesGroupCount - 1; $i++)
+        {
+            $strongerTeam = $inputVariablesGroups[$i][0];
+            $weakerTeam = $inputVariablesGroups[$i + 1][0];
+
+            $currentDifference = $this->createOutputVariable();
+            $this->addLayerFactor($this->createTeamPerformanceToDifferenceFactor($strongerTeam, $weakerTeam, currentDifference));
+
+            // REVIEW: Does it make sense to have groups of one?
+            $this->getOutputVariablesGroups()[] = $currentDifference;
+        }
+    }
+
+    private function createTeamPerformanceToDifferenceFactor(
+        Variable $strongerTeam, Variable $weakerTeam, Variable $output)
+    {
+        return new GaussianWeightedSumFactor($output, array($strongerTeam, $weakerTeam), array(1.0, -1.0));
+    }
+
+    private function createOutputVariable()
+    {
+        return $this->getParentFactorGraph()->getVariableFactory()->createBasicVariable("Team performance difference");
+    }
+}
+
+?>