Add Base.splice() to handle lists of items with _index referneces and use it for paper.documents and Item#children.

src/document/Document.js

@@ -50,12 +50,12 @@ var Document = this.Document = Base.extend({
 			this._size = Size.read(arguments) || new Size(1024, 768);
 			this.canvas = CanvasProvider.getCanvas(this._size);
 		}
-		// TODO: currently we don't do anything with Document#bounds.. What
-		// does it mean to change the bounds of the document? (JP)
+		// TODO: Currently we don't do anything with Document#bounds.
+		// What does it mean to change the bounds of the document? (JP)
 		this.bounds = Rectangle.create(0, 0, this._size.width,
 				this._size.height);
 		this.context = this.canvas.getContext('2d');
-		paper.documents.push(this);
+		Base.splice(paper.documents, [this]);
 		this.activate();
 		this.layers = [];
 		this.activeLayer = new Layer();
@@ -65,7 +65,7 @@ var Document = this.Document = Base.extend({
 		this.views = [this.activeView];
 		this._selectedItems = {};
 		this._selectedItemCount = 0;
-		// TODO: test this on IE:
+		// TODO: Test this on IE:
 		if (this.canvas.attributes.stats) {
 			this.stats = new Stats();
 			// Align top-left to the canvas
@@ -102,6 +102,7 @@ var Document = this.Document = Base.extend({
 	},
 
 	activate: function() {
+		// TODO: Remove indexOf()
 		var index = paper.documents.indexOf(this);
 		if (index != -1) {
 			paper.document = this;

src/item/Item.js

@@ -251,24 +251,21 @@ var Item = this.Item = Base.extend({
 	 * The next item on the same level as this item.
 	 */
 	getNextSibling: function() {
-		return this.parent && this.parent.children[this.getIndex() + 1] || null;
+		return this.parent && this.parent.children[this._index + 1] || null;
 	},
 
 	/**
 	 * The previous item on the same level as this item.
 	 */
 	getPreviousSibling: function() {
-		return this.parent && this.parent.children[this.getIndex() - 1] || null;
+		return this.parent && this.parent.children[this._index - 1] || null;
 	},
 
 	/**
 	 * The index of this item within the list of it's parent's children.
 	 */
 	getIndex: function() {
-		// TODO: Relying on indexOf() here is slow, especially since it is
-		// used for getPrevious/NextSibling(). 
-		// We need linked lists instead.
-		return this.parent && this.parent.children.indexOf(this) || null;
+		return this._index !== undefined ? this._index : null;
 	},
 
 	/**
@@ -276,8 +273,8 @@ var Item = this.Item = Base.extend({
 	*/
 	_removeFromParent: function() {
 		if (this.parent) {
-			var ok = !!this.parent.children.splice(this.getIndex(), 1).length;
-			// TODO: Reassign _index
+			var ok = !!Base.splice(this.parent.children, null,
+					this._index, 1).length;
 			this.parent = null;
 			return ok;
 		}
@@ -812,8 +809,7 @@ var Item = this.Item = Base.extend({
 		return function(item) {
 			item._removeFromParent();
 			if (this.children) {
-				this.children.splice(top ? this.children.length : 0, 0, item);
-				// TODO: Reassign _index
+				Base.splice(this.children, [item], top ? undefined : 0, 0);
 				item.parent = this;
 				item._setDocument(this._document);
 				return true;
@@ -826,9 +822,8 @@ var Item = this.Item = Base.extend({
 		return function(item) {
 			// first remove the item from its parent's children list
 			if (item.parent && this._removeFromParent()) {
-				item.parent.children.splice(item.getIndex()
-						+ (above ? 1 : -1), 0, this);
-				// TODO: Reassign _index
+				Base.splice(item.parent.children, [this],
+						item._index + (above ? 1 : -1), 0);
 				this.parent = item.parent;
 				this._setDocument(item._document);
 				return true;

src/paper.js

@@ -79,6 +79,38 @@ Base.inject({
 		return res;
 	},
 
+	/**
+	 * Utility function for adding and removing items from a list of which
+	 * each entry keeps a reference to its index in the list in the private
+	 * _index property. Used for paper.documents and Item#children.
+	 */
+	splice: function(list, items, index, remove) {
+		var amount = items && items.length,
+			append = index === undefined;
+		index = append ? list.length : index;
+		// Update _index on the items to be added first.
+		for (var i = 0; i < amount; i++) {
+			items[i]._index = index + i;
+		}
+		if (append) {
+			// Append them all at the end by using push
+			list.push.apply(list, items);
+			// Nothing removed, and nothing to adjust above
+			return [];
+		} else {
+			// Insert somewhere else and/or remove
+			var args = [index, remove];
+			if (items)
+				args.push.apply(args, items);
+			var removed = list.splice.apply(list, args);
+			// Adjust the indices of the items above.
+			for (var i = index + amount, l = list.length; i < l; i++) {
+				list[i]._index = i;
+			}
+			return removed;
+		}
+	},
+
 	capitalize: function(str) {
 		return str.replace(/\b[a-z]/g, function(match) {
 			return match.toUpperCase();

src/path/Path.js

@@ -145,7 +145,7 @@ var Path = this.Path = PathItem.extend({
 		var segments = this._segments,
 			curves = this._curves,
 			amount = segs.length,
-			append = index === undefined,
+			append = index == null,
 			index = append ? segments.length : index;
 		// Scan through segments to add first, convert if necessary and set
 		// _path and _index references on them.