perf: simplify DOM.compareDocumentPosition (xmldom#805)

PR contributes a more efficient coding for the `compareDocumentPosition`
method contributed in PR xmldom#488
. In detail:

* Removes the `parentChain` method and replaces it with a parent chain
computation that immediately compares target nodes, thus avoiding
unnecessary computations.
* Uses `push` and `reverse` instead of `unshift` to assemble chains as
this is considerably faster.

lib/dom.js

@@ -257,23 +257,6 @@ var DocumentPosition = conventions.freeze({
 });
 
 //helper functions for compareDocumentPosition
-/**
- * Constructs a parent chain for a node.
- *
- * @param {Node} node
- * The start node from which the parent chain will be constructed.
- * @returns {Node[]}
- * The array of nodes representing the parent chain from the root to the specified node.
- */
-function parentChain(node) {
-	var chain = [];
-	while (node.parentNode || node.ownerElement) {
-		node = node.parentNode || node.ownerElement;
-		chain.unshift(node);
-	}
-	return chain;
-}
-
 /**
  * Finds the common ancestor in two parent chains.
  *
@@ -1476,15 +1459,36 @@ Node.prototype = {
 					: DocumentPosition.DOCUMENT_POSITION_PRECEDING)
 			);
 		}
-		var chain1 = parentChain(node1);
-		var chain2 = parentChain(node2);
-		if ((!attr1 && chain2.indexOf(node1) >= 0) || (attr2 && node1 === node2)) {
+		if (attr2 && node1 === node2) {
 			return DocumentPosition.DOCUMENT_POSITION_CONTAINS + DocumentPosition.DOCUMENT_POSITION_PRECEDING;
 		}
-		if ((!attr2 && chain1.indexOf(node2) >= 0) || (attr1 && node1 === node2)) {
+		if (attr1 && node1 === node2) {
 			return DocumentPosition.DOCUMENT_POSITION_CONTAINED_BY + DocumentPosition.DOCUMENT_POSITION_FOLLOWING;
 		}
-		var ca = commonAncestor(chain2, chain1);
+
+		var chain1 = [];
+		var ancestor1 = node1.parentNode;
+		while (ancestor1) {
+			if (!attr2 && ancestor1 === node2) {
+				return DocumentPosition.DOCUMENT_POSITION_CONTAINED_BY + DocumentPosition.DOCUMENT_POSITION_FOLLOWING;
+			}
+			chain1.push(ancestor1);
+			ancestor1 = ancestor1.parentNode;
+		}
+		chain1.reverse();
+
+		var chain2 = [];
+		var ancestor2 = node2.parentNode;
+		while (ancestor2) {
+			if (!attr1 && ancestor2 === node1) {
+				return DocumentPosition.DOCUMENT_POSITION_CONTAINS + DocumentPosition.DOCUMENT_POSITION_PRECEDING;
+			}
+			chain2.push(ancestor2);
+			ancestor2 = ancestor2.parentNode;
+		}
+		chain2.reverse();
+
+		var ca = commonAncestor(chain1, chain2);
 		for (var n in ca.childNodes) {
 			var child = ca.childNodes[n];
 			if (child === node2) return DocumentPosition.DOCUMENT_POSITION_FOLLOWING;