Amortize quadratic execution time of traversal eclipse-platform#882

TreeItem.getItem(int) had linear complexity, full traversal had
quadratic complexity. To alleviate this, we cache the result.

Note: this only speeds up breadth-first traversal as cache operates on "item of interest" basis, dropping contents if another item is requested.

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Tree.java

@@ -111,6 +111,8 @@ public class Tree extends Composite {
 
 	private long headerCSSProvider;
 
+	private TreeItemCache itemCache = null;
+
 	static final int ID_COLUMN = 0;
 	static final int CHECKED_COLUMN = 1;
 	static final int GRAYED_COLUMN = 2;
@@ -4346,4 +4348,11 @@ public void dispose() {
 		headerCSSProvider = 0;
 	}
 }
+
+TreeItemCache getItemCache(TreeItem treeItem) {
+	if (itemCache == null || itemCache.owner != treeItem) {
+		itemCache = new TreeItemCache(treeItem);
+	}
+	return itemCache;
+}
 }

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/TreeItem.java

@@ -45,7 +45,6 @@ public class TreeItem extends Item {
 	Font[] cellFont;
 	String [] strings;
 	boolean cached, grayed, isExpanded, updated, settingData;
-	int itemCountCache = -1;
 	static final int EXPANDER_EXTRA_PADDING = 4;
 
 /**
@@ -167,7 +166,9 @@ public TreeItem (TreeItem parentItem, int style, int index) {
 	this.parent = parent;
 	if (create) {
 		parent.createItem (this, parentIter, index);
-		resetParentCache();
+		if (parentIter != 0) {
+			parent._getItem(parentIter).resetCache();
+		}
 	} else {
 		handle = OS.g_malloc (GTK.GtkTreeIter_sizeof ());
 		GTK.gtk_tree_model_iter_nth_child (parent.modelHandle, handle, parentIter, index);
@@ -770,7 +771,7 @@ Rectangle getImageBoundsInPixels (int index) {
 public int getItemCount () {
 	checkWidget();
 	if (!parent.checkData (this)) error (SWT.ERROR_WIDGET_DISPOSED);
-	return getItemCountImpl();
+	return getCache().getItemCount();
 }
 
 /**
@@ -793,9 +794,12 @@ public int getItemCount () {
 public TreeItem getItem (int index) {
 	checkWidget();
 	if (index < 0) error (SWT.ERROR_INVALID_RANGE);
-	int itemCount = getItemCountImpl();
-	if (index >= itemCount)  error (SWT.ERROR_INVALID_RANGE);
-	return  parent._getItem (handle, index);
+	// It may seem that requesting GTK for an item by index is quicker,
+	// but it has O(N) execution time and cache of all children amortizes that.
+	// This avoids quadratic execution time on traversals
+	TreeItem[] items = getCache().getItems();
+	if (index >= items.length)  error (SWT.ERROR_INVALID_RANGE);
+	return items[index];
 }
 
 /**
@@ -1100,7 +1104,7 @@ public void dispose () {
  */
 public void removeAll () {
 	checkWidget ();
-	itemCountCache = 0;
+	resetCache();
 	long modelHandle = parent.modelHandle;
 	int length = GTK.gtk_tree_model_iter_n_children (modelHandle, handle);
 	if (length == 0) return;
@@ -1631,7 +1635,7 @@ public void setImage (Image [] images) {
 public void setItemCount (int count) {
 	checkWidget ();
 	count = Math.max (0, count);
-	itemCountCache = count;
+	resetCache();
 	parent.setItemCount (handle, count);
 }
 
@@ -1712,18 +1716,13 @@ public void setText (String [] strings) {
 	}
 }
 
-private void resetParentCache() {
-	TreeItem parentItem = getParentItem();
-	if (parentItem != null) {
-		parentItem.itemCountCache = -1;
-	}
+private void resetCache() {
+	getCache().reset();
 }
 
-private int getItemCountImpl() {
-	if (itemCountCache < 0) {
-		itemCountCache = GTK.gtk_tree_model_iter_n_children (parent.modelHandle, handle); 
-	}
-	return itemCountCache;
+private TreeItemCache getCache() {
+	return parent.getItemCache(this);
 }
 
+
 }

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/TreeItemCache.java

@@ -0,0 +1,50 @@
+/*******************************************************************************
+ * Copyright (c) 2023, 2023 IBM Corporation and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Vasili Gulevich - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.swt.widgets;
+
+import java.util.*;
+
+import org.eclipse.swt.internal.gtk.*;
+
+/** Volatile information about a TreeItem, that takes long to compute
+ * @since 3.125**/
+class TreeItemCache {
+	final TreeItem owner;
+	private TreeItem[] children;
+	private int itemCount = -1;
+
+	TreeItemCache(TreeItem owner) {
+		this.owner = Objects.requireNonNull(owner);
+	}
+
+	int getItemCount() {
+		if (itemCount < 0) {
+			itemCount = GTK.gtk_tree_model_iter_n_children (owner.parent.modelHandle, owner.handle);
+		}
+		return itemCount;
+	}
+
+	TreeItem[] getItems() {
+		if (children == null) {
+			children = owner.parent.getItems(owner.handle);
+			itemCount = children.length;
+		}
+		return children;
+	}
+
+	public void reset() {
+		itemCount = -1;
+		children = null;
+	}
+}