Added "no direct initialization" logic

Author: WalterWoshid

src/CodeTransformerKernel.php

@@ -2,6 +2,7 @@
 
 namespace Okapi\CodeTransformer;
 
+use Okapi\CodeTransformer\Exception\Kernel\DirectKernelInitializationException;
 use Okapi\Singleton\Singleton;
 
 /**
@@ -41,5 +42,31 @@ public static function init(
         ?string $cacheDir,
         ?int    $cacheFileMode = null,
         bool    $debug = false,
-    ): void {}
+    ): void {
+        self::ensureNotKernelNamespace();
+
+        $instance = self::getInstance();
+        $instance->ensureNotAlreadyInitialized();
+
+        if ($instance->transformers) {
+            // TODO: Register services
+        }
+
+        $instance->setInitialized();
+    }
+
+    /**
+     * Make sure that the kernel is not called from this class.
+     *
+     * @return void
+     */
+    private static function ensureNotKernelNamespace(): void
+    {
+        // Get current namespace and class name
+        $namespace = get_called_class();
+
+        if ($namespace === CodeTransformerKernel::class) {
+            throw new DirectKernelInitializationException;
+        }
+    }
 }

src/Exception/Kernel/DirectKernelInitializationException.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Okapi\CodeTransformer\Exception\Kernel;
+
+use Okapi\CodeTransformer\Exception\KernelException;
+
+/**
+ * # Initialize Kernel Exception
+ *
+ * This exception is thrown when the CodeTransformerKernel is initialized
+ * directly.
+ */
+class DirectKernelInitializationException extends KernelException
+{
+    /**
+     * DirectKernelInitializationException constructor.
+     */
+    public function __construct()
+    {
+        parent::__construct(
+            "Cannot initialize CodeTransformerKernel directly. " .
+            "Please extend from CodeTransformerKernel and call the init() method.",
+        );
+    }
+}

src/Exception/KernelException.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace Okapi\CodeTransformer\Exception;
+
+/**
+ * # Kernel Exception
+ *
+ * Base exception for all `CodeTransformerKernel` exceptions.
+ */
+abstract class KernelException extends CodeTransformerException
+{
+}