add tail recursion

composer.json

@@ -8,7 +8,8 @@
     "files": [
       "src/Nerd/Common/Arrays.php",
       "src/Nerd/Common/Classes.php",
-      "src/Nerd/Common/Strings.php"
+      "src/Nerd/Common/Strings.php",
+      "src/Nerd/Common/Functional.php"
     ]
   },
   "require-dev": {

src/Nerd/Common/Functional.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace Nerd\Common\Functional;
+
+/**
+ * Optimize given function for tail recursion.
+ *
+ * @param callable $fn
+ * @return \Closure
+ */
+function tail(callable $fn)
+{
+    $underCall = false;
+    $pool = [];
+    return function (...$args) use (&$fn, &$underCall, &$pool) {
+        $result = null;
+        $pool[] = $args;
+        if (!$underCall) {
+            $underCall = true;
+            while ($pool) {
+                $head = array_shift($pool);
+                $result = $fn(...$head);
+            }
+            $underCall = false;
+        }
+        return $result;
+    };
+}

tests/FunctionalTest.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace tests;
+
+use function Nerd\Common\Functional\tail;
+use PHPUnit\Framework\TestCase;
+
+class FunctionalTest extends TestCase
+{
+    public function testTailRecursion()
+    {
+        $func = tail(function ($max, $acc = 0) use (&$func) {
+            if ($max == $acc) {
+                return $acc;
+            }
+            return $func($max, $acc + 1);
+        });
+        $this->assertEquals(10000, $func(10000));
+    }
+}