ProcessExtends: Use map instead of list for selectors and parents

* Convert Less\Tree\Extend#parent_ids from a list to a map.

* Maintain a copy of Less\Tree\Selector#_oelements list in map form.
  Keep the list as well for now because there is a logical order to this
  information, which callers outside ProcessExtends depend on.

These change bring an performance improvement.
About 4% speedup for Magento2.

Co-authored-by: Timo Tijhof <krinklemail@gmail.com>

lib/Less/Tree/Extend.php

@@ -42,8 +42,13 @@ public function __construct($selector, $option, $index){
 			break;
 		}
 
-		$this->object_id = $i++;
-		$this->parent_ids = array($this->object_id);
+		// This must use a string (instead of int) so that array_merge()
+		// preserves keys on arrays that use IDs in their keys.
+		$this->object_id = 'id_' . $i++;
+
+		$this->parent_ids = array(
+			$this->object_id => true
+		);
 	}
 
     public function accept( $visitor ){
@@ -74,4 +79,4 @@ public function findSelfSelectors( $selectors ){
 		$this->selfSelectors = array(new Less_Tree_Selector($selfElements));
 	}
 
-}
+}

lib/Less/Tree/Selector.php

@@ -22,6 +22,7 @@ class Less_Tree_Selector extends Less_Tree{
 	public $elements_len = 0;
 
 	public $_oelements;
+	public $_oelements_assoc;
 	public $_oelements_len;
 	public $cacheable = true;
 
@@ -83,6 +84,8 @@ public function match( $other ){
 	public function CacheElements(){
 
 		$this->_oelements = array();
+		$this->_oelements_assoc = array();
+
 		$css = '';
 
 		foreach($this->elements as $v){
@@ -108,6 +111,8 @@ public function CacheElements(){
 				array_shift($this->_oelements);
 				$this->_oelements_len--;
 			}
+
+			$this->_oelements_assoc = array_fill_keys($this->_oelements, true);
 		}
 	}
 

lib/Less/Visitor/processExtends.php

@@ -53,8 +53,9 @@ private function doExtendChaining( $extendsList, $extendsListTarget, $iterationC
 				$extend = $extendsList[$extendIndex];
 				$targetExtend = $extendsListTarget[$targetExtendIndex];
 
-				// look for circular references
-				if( in_array($targetExtend->object_id, $extend->parent_ids,true) ){
+				// Optimisation: Explicit reference, <https://github.com/wikimedia/less.php/pull/14>
+				if( \array_key_exists($targetExtend->object_id, $extend->parent_ids) ){
+					// ignore circular references
 					continue;
 				}
 
@@ -269,7 +270,8 @@ private function HasMatches($extend, $haystackSelectorPath){
 				return true;
 			}
 
-			if( in_array($first_el, $hackstackSelector->_oelements) ){
+			// Optimisation: Explicit reference, <https://github.com/wikimedia/less.php/pull/14>
+			if( \array_key_exists($first_el, $hackstackSelector->_oelements_assoc) ){
 				return true;
 			}
 		}
@@ -466,4 +468,4 @@ protected function visitDirectiveOut(){
 		array_pop($this->allExtendsStack);
 	}
 
-}
+}

test/index.php

@@ -690,7 +690,7 @@ function obj($mixed, $objects = array() ) {
 
 
 	$exclude_keys = array('originalRuleset','currentFileInfo','lookups','index','ruleset_id','type','_rulesets','_variables','allowImports','_css','cache_string','elements_len',
-					'_oelements','first_oelements','_oelements_len','cacheable', ); //'variable','combinator'
+					'_oelements','_oelements_assoc','first_oelements','_oelements_len','cacheable', ); //'variable','combinator'
 	//$exclude_keys = array();
 
 	$type = gettype($mixed);