add some simple functions

Author: marmar8x

src/Helpers.php

@@ -173,4 +173,16 @@ public static function hasDirection(string $str): bool
     {
         return preg_match('/\b(top|bottom|right|left)\b/i', $str);
     }
+
+    /**
+     * check if string include other string
+     *
+     * @param string $str
+     * @param string $needle
+     * @return  bool
+     */
+    public static function strIncludes(string $str, string $needle): bool
+    {
+        return strpos($str, $needle) !== false;
+    }
 }

src/Manipulate.php

@@ -43,14 +43,21 @@ public static function complement($value): void
      */
     public static function negate($value): void
     {
-        if ($value instanceof ValueList) {
+        if ($value instanceof CSSFunction && Helpers::strIncludes($value->getName(), 'calc')) {
+            $arguments = implode($value->getListSeparator(), $value->getArguments());
+            $arguments = "-1*($arguments)";
+            $value->setListComponents([$arguments]);
+
+        } else if ($value instanceof ValueList) {
             foreach ($value->getListComponents() as $part) {
                 self::negate($part);
             }
+
         } else if ($value instanceof Size) {
             if ($value->getSize() != 0) {
                 $value->setSize(-$value->getSize());
             }
+
         }
     }
 }