Compare View for big files with Line Spacing freezes eclipse

For big files with many differences, the compare view freezed eclipse,
if the line spacing was set. The reason were the necessary resources to
calculate the spacing in the method getHeightBetweenLines. We have to
save the heights in a list and use them to increase the performance.
Clear line heights cache at property changes.

Fixes eclipse-platform/eclipse.platform.ui#2143

Signed-off-by: Denis Ungemach <[email protected]>

Author: DenisUngemach

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/contentmergeviewer/TextMergeViewer.java

@@ -29,6 +29,7 @@
 import java.lang.reflect.InvocationTargetException;
 import java.text.MessageFormat;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
@@ -474,6 +475,12 @@ private static RGB interpolate(RGB fg, RGB bg, double scale) {
 	private IUndoableOperation copyUndoable = null;
 	private IOperationHistoryListener operationHistoryListener;
 
+	// https://github.com/eclipse-platform/eclipse.platform.ui/issues/2143
+	// calling w.getLineHeight(line) + w.getLineSpacing() +
+	// w.getLineVerticalIndent(line); again and again for the same line is too slow
+	// we therefore introduce a line height cache by viewer
+	private final Map<MergeSourceViewer, List<Integer>> lineHeightsByViewer = new HashMap<>();
+
 	/**
 	 * Preference key for highlighting current line.
 	 */
@@ -4001,6 +4008,16 @@ && getCompareConfiguration().getContainer().getActionBars() != null) {
 
 	@Override
 	protected void handlePropertyChangeEvent(PropertyChangeEvent event) {
+
+		// Property changes might change the height of lines,
+		// which means the heights cache should be cleared.
+		// But the actual height change of lines in StyledText does not happen in this
+		// method.
+		// There are other property change listeners for example:
+		// AbstractTextEditor.PropertyChangeListener,
+		// and these handle the height changes of StyledText.
+		lineHeightsByViewer.clear();
+
 		String key= event.getProperty();
 
 		if (key.equals(CompareConfiguration.IGNORE_WHITESPACE)
@@ -4206,9 +4223,29 @@ private void updateLines(IDocument d) {
 	 */
 	private int getHeightBetweenLines(MergeSourceViewer tp, int fromLine, int toLine) {
 		StyledText w = tp.getSourceViewer().getTextWidget();
+		List<Integer> lineHeights = this.lineHeightsByViewer.get(tp);
+		if (lineHeights == null) {
+			lineHeights = new ArrayList<>();
+			lineHeights.addAll(Collections.nCopies(w.getLineCount(), null));
+			this.lineHeightsByViewer.put(tp, lineHeights);
+			tp.getSourceViewer().addTextListener(event -> {
+				List<Integer> lineHeightsList = lineHeightsByViewer.get(tp);
+				if (lineHeightsList != null) {
+					lineHeightsList.clear();
+					lineHeightsList.addAll(Collections.nCopies(w.getLineCount(), null));
+				}
+			});
+		}
+		int lineSpacing = w.getLineSpacing();
 		int height = 0;
 		for (int i = fromLine; i < toLine; i++) {
-			height += w.getLineHeight(i) + w.getLineSpacing() + w.getLineVerticalIndent(i);
+			Integer heightAtLine = lineHeights.get(i);
+			if (heightAtLine == null) {
+				int lineOffset = w.getOffsetAtLine(i);
+				heightAtLine = w.getLineHeight(lineOffset) + lineSpacing + w.getLineVerticalIndent(i);
+				lineHeights.set(i, heightAtLine);
+			}
+			height += heightAtLine;
 		}
 		return height;
 	}