LessParser: Add custom function `extract-variable-default`

library/Icinga/Util/LessParser.php

@@ -3,11 +3,66 @@
 
 namespace Icinga\Util;
 
+use Less_Tree_Anonymous;
+use Less_Tree_Expression;
+use Less_Tree_Quoted;
+use Less_Tree_Value;
 use lessc;
 
 require_once 'lessphp/lessc.inc.php';
 
 class LessParser extends lessc
 {
-    // Overwrite methods here
+    public function __construct()
+    {
+        $this->registerFunction('extract-variable-default', [$this, 'extractVariableDefault']);
+    }
+
+    /**
+     * Extract default from given variable call
+     *
+     * How to use:
+     *
+     *   color: extract-variable-default(@mixin-parameter);
+     *   color: @mixin-parameter;
+     *
+     *   border: extract-variable-default(1px solid @mixin-parameter);
+     *   border: 1px solid @mixin-parameter;
+     *
+     *   background: drop-shadow(5px 0 3px extract-variable-default(@mixin-parameter, true));
+     *   background: drop-shadow(5px 0 3px @mixin-parameter);
+     *
+     * @param mixed $value
+     * @param bool $valAsDefault
+     *
+     * @return mixed
+     */
+    public function extractVariableDefault($value, $valAsDefault = false)
+    {
+        $defaultValue = 'inherit';
+        if ($value instanceof Less_Tree_Quoted) {
+            $stripped = preg_replace(
+                '~var\s*\(\s*[-\w]+\s*,\s*([^)]+)\)~',
+                '$1',
+                $value->value,
+                -1,
+                $replacements
+            );
+            if ($replacements > 0) {
+                $defaultValue = $stripped;
+            }
+        } elseif ($value instanceof Less_Tree_Expression) {
+            foreach ($value->value as $i => $item) {
+                $value->value[$i] = $this->extractVariableDefault($item, true);
+            }
+
+            return $value;
+        }
+
+        if ($valAsDefault && $defaultValue === 'inherit') {
+            return $value;
+        }
+
+        return new Less_Tree_Value([new Less_Tree_Anonymous($defaultValue)]);
+    }
 }