add coallesce operator associativity experiment

experiments/CoallesceOperatorAssociativity.php

@@ -0,0 +1,56 @@
+<?php
+
+/*
+ * PURPOSE:
+ * Understand the associativity properties of the coallesce operator.
+ * This experiment compares the time required to evaluate the following groupings:
+ *   None  : (a() ?? b() ?? b());
+ *   Right : a() ?? (b() ?? (b()); // # comparisons = 1
+ *   Left  : (a() ?? b()) ?? b();  // # comparisons = # of ?? operators
+ *
+ * RESULTS:
+ * None  : 0.00024787425994873
+ * Right : 0.00025341987609863
+ * Left  : 0.042938680648804
+ *
+ * CONCLUSION:
+ * Confirmed that the coallesce operator matches performance properties of right-grouping,
+ * and is therefore likely right associative.
+ */
+
+function a () { return "a"; }
+
+$parenCount = 3000;
+
+$noGroupingStr = str_repeat('(', $parenCount). 'a() ?? ' . str_repeat('b() ?? ', $parenCount) . 'b()' . str_repeat(')', $parenCount) . ";";
+$rightGroupingStr = 'a() ?? ' . str_repeat('(b() ?? ', $parenCount) . 'b()' . str_repeat(')', $parenCount) . ";";
+$leftGroupingStr = str_repeat('(', $parenCount) . 'a() ?? ' . str_repeat('b()) ?? ', $parenCount) . 'b()' . ";";
+
+$runBenchmark = function ($str) {
+    $benchmark = <<< 'before'
+unset ($benchmark);
+$benchmark = function () {
+$average = 0;
+for ($j = 0; $j < 50; $j++) {
+    $startTime = microtime(true);
+    for ($i = 0; $i < 2000; $i++) {
+before;
+
+    $benchmark .= $str;
+
+    $benchmark .= <<< 'after'
+    }
+    $average += microtime(true) - $startTime;
+}
+return $average / 50;
+};
+
+echo $benchmark(), PHP_EOL;
+after;
+
+    eval($benchmark);
+};
+
+echo "None  : " , $runBenchmark($noGroupingStr);
+echo "Right : " , $runBenchmark($rightGroupingStr);
+echo "Left  : ", $runBenchmark($leftGroupingStr);