Introduce own `Call` class for less tree calls

library/Icinga/Less/Call.php

@@ -0,0 +1,70 @@
+<?php
+
+namespace Icinga\Less;
+
+use Less_Tree_Call;
+use Less_Tree_Color;
+use Less_Tree_Value;
+use Less_Tree_Variable;
+
+class Call extends Less_Tree_Call
+{
+    public static function fromCall(Less_Tree_Call $call)
+    {
+        return new static($call->name, $call->args, $call->index, $call->currentFileInfo);
+    }
+
+    public function compile($env = null)
+    {
+        if (! $env) {
+            // Not sure how to trigger this, but if there is no $env, there is nothing we can do
+            return parent::compile($env);
+        }
+
+        foreach ($this->args as $arg) {
+            $name = null;
+            if ($arg->value[0] instanceof Less_Tree_Variable) {
+                // This is the case when defining a variable with a callable LESS rules such as fade, fadeout..
+                // Example: `@foo: #fff; @foo-bar: fade(@foo, 10);`
+                $name = $arg->value[0]->name;
+            } elseif ($arg->value[0] instanceof ColorPropOrVariable) {
+                // This is the case when defining a CSS rule using the LESS functions and passing
+                // a variable as an argument to them. Example: `... { color: fade(@foo, 10%); }`
+                $name = $arg->value[0]->getVariable()->name;
+            }
+
+            if ($name) {
+                foreach (array_reverse($env->frames) as $frame) {
+                    if (($v = $frame->variable($name))) {
+                        // Variables from the frame stack are always of type LESS Tree Rule
+                        $vr = $v->value;
+                        if ($vr instanceof Less_Tree_Value) {
+                            // Get the actual color prop, otherwise this may cause an invalid argument error
+                            $vr = $vr->compile($env);
+                        }
+
+                        if ($vr instanceof DeferredColorProp) {
+                            if (! $vr->hasReference()) {
+                                // Should never happen, though just for safety's sake
+                                $vr->compile($env);
+                            }
+
+                            // Get the uppermost variable of the variable references
+                            while (! $vr instanceof ColorProp) {
+                                $vr = $vr->getRef();
+                            }
+                        } elseif ($vr instanceof Less_Tree_Color) {
+                            $vr = ColorProp::fromColor($vr);
+                            $vr->setName($name);
+                        }
+
+                        $arg->value[0] = $vr;
+                        break;
+                    }
+                }
+            }
+        }
+
+        return parent::compile($env);
+    }
+}