Avoid redundant content reads when opening a compare editor

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/internal/CompareContentViewerSwitchingPane.java

@@ -60,6 +60,9 @@ public class CompareContentViewerSwitchingPane extends CompareViewerSwitchingPan
 
 	private ViewerDescriptor fSelectedViewerDescriptor;
 
+	/** Descriptors computed by getViewer, reused by setInput to avoid a second lookup. */
+	private ViewerDescriptor[] fViewerDescriptors;
+
 	private ToolBar toolBar;
 	private CLabel labelOptimized;
 	private Link recomputeLink;
@@ -79,9 +82,10 @@ private CompareConfiguration getCompareConfiguration() {
 	@Override
 	protected Viewer getViewer(Viewer oldViewer, Object input) {
 		if (fSelectedViewerDescriptor != null) {
-			ViewerDescriptor[] array = CompareUIPlugin.getDefault().findContentViewerDescriptor(
+			fViewerDescriptors = CompareUIPlugin.getDefault().findContentViewerDescriptor(
 					oldViewer, input, getCompareConfiguration());
-			List<ViewerDescriptor> list = array != null ? Arrays.asList(array) : Collections.emptyList();
+			List<ViewerDescriptor> list = fViewerDescriptors != null ? Arrays.asList(fViewerDescriptors)
+					: Collections.emptyList();
 			if (list.contains(fSelectedViewerDescriptor)) {
 				// use selected viewer only when appropriate for the new input
 				fCompareEditorInput
@@ -94,12 +98,19 @@ protected Viewer getViewer(Viewer oldViewer, Object input) {
 			fSelectedViewerDescriptor = null;
 		}
 		if (input instanceof ICompareInput) {
-			fCompareEditorInput.setContentViewerDescriptor(null);
+			fViewerDescriptors = CompareUIPlugin.getDefault().findContentViewerDescriptor(
+					oldViewer, input, getCompareConfiguration());
+			// Feeding the default descriptor avoids a second lookup inside findContentViewer.
+			ViewerDescriptor defaultDescriptor = fViewerDescriptors != null && fViewerDescriptors.length > 0
+					? fViewerDescriptors[0]
+					: null;
+			fCompareEditorInput.setContentViewerDescriptor(defaultDescriptor);
 			Viewer viewer =
 					fCompareEditorInput.findContentViewer(oldViewer, (ICompareInput) input, this);
 			fCompareEditorInput.setContentViewerDescriptor(fSelectedViewerDescriptor);
 			return viewer;
 		}
+		fViewerDescriptors = null;
 		return null;
 	}
 
@@ -203,8 +214,12 @@ public void setInput(Object input) {
 		if (getViewer() == null || !Utilities.okToUse(getViewer().getControl())) {
 			return;
 		}
-		ViewerDescriptor[] vd = CompareUIPlugin.getDefault()
-				.findContentViewerDescriptor(getViewer(), getInput(), getCompareConfiguration());
+		// Reuse the descriptors computed by getViewer; recompute only if the cache is empty.
+		ViewerDescriptor[] vd = fViewerDescriptors;
+		if (vd == null) {
+			vd = CompareUIPlugin.getDefault()
+					.findContentViewerDescriptor(getViewer(), getInput(), getCompareConfiguration());
+		}
 		boolean toolbarVisible = vd != null && vd.length > 1;
 		toolBar.setVisible(toolbarVisible);
 		((RowData) toolBar.getLayoutData()).exclude = !toolbarVisible;

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/internal/CompareStructureViewerSwitchingPane.java

@@ -52,6 +52,9 @@ public class CompareStructureViewerSwitchingPane extends
 
 	private ViewerDescriptor fSelectedViewerDescriptor;
 
+	/** Descriptors computed by getViewer, reused by setInput to avoid a second lookup. */
+	private ViewerDescriptor[] fViewerDescriptors;
+
 	private ToolBar toolBar;
 
 	public CompareStructureViewerSwitchingPane(Composite parent, int style,
@@ -68,9 +71,10 @@ private CompareConfiguration getCompareConfiguration() {
 	protected Viewer getViewer(Viewer oldViewer, Object input) {
 		if (input instanceof ICompareInput) {
 			if (fSelectedViewerDescriptor != null) {
-				ViewerDescriptor[] array = CompareUIPlugin.getDefault().findStructureViewerDescriptor(
+				fViewerDescriptors = CompareUIPlugin.getDefault().findStructureViewerDescriptor(
 						oldViewer, (ICompareInput)input, getCompareConfiguration());
-				List<ViewerDescriptor> list = array != null ? Arrays.asList(array) : Collections.emptyList();
+				List<ViewerDescriptor> list = fViewerDescriptors != null ? Arrays.asList(fViewerDescriptors)
+						: Collections.emptyList();
 				if (list.contains(fSelectedViewerDescriptor)) {
 					// use selected viewer only when appropriate for the new input
 					fCompareEditorInput
@@ -83,12 +87,15 @@ protected Viewer getViewer(Viewer oldViewer, Object input) {
 				fSelectedViewerDescriptor = null;
 			}
 
+			// The default path computes no descriptors; invalidate so setInput recomputes.
+			fViewerDescriptors = null;
 			fCompareEditorInput.setStructureViewerDescriptor(null);
 			Viewer viewer = fCompareEditorInput.findStructureViewer(oldViewer,
 					(ICompareInput) input, this);
 			fCompareEditorInput.setStructureViewerDescriptor(fSelectedViewerDescriptor);
 			return viewer;
 		}
+		fViewerDescriptors = null;
 		return null;
 	}
 
@@ -141,8 +148,9 @@ public void setInput(Object input) {
 		if (getViewer() == null || !Utilities.okToUse(getViewer().getControl())) {
 			return;
 		}
-		ViewerDescriptor[] vd = null;
-		if (getInput() instanceof ICompareInput) {
+		// Reuse the descriptors computed by getViewer; recompute only if the cache is empty.
+		ViewerDescriptor[] vd = fViewerDescriptors;
+		if (vd == null && getInput() instanceof ICompareInput) {
 			vd = CompareUIPlugin.getDefault().findStructureViewerDescriptor(
 					getViewer(), (ICompareInput) getInput(),
 					getCompareConfiguration());

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/internal/CompareUIPlugin.java

@@ -30,6 +30,7 @@
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Hashtable;
+import java.util.IdentityHashMap;
 import java.util.Iterator;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -261,6 +262,14 @@ Collection<T> getAll() {
 	// content type
 	private static final IContentTypeManager fgContentTypeManager= Platform.getContentTypeManager();
 
+	/**
+	 * Per-open memoization of content-type sniffing, keyed by element identity and
+	 * reset when a different input is examined. Only accessed on the UI thread.
+	 */
+	private static Object fSniffInput;
+	private static final Map<ITypedElement, IContentType> fContentTypeSniffCache= new IdentityHashMap<>();
+	private static final Map<ITypedElement, String> fGuessTypeSniffCache= new IdentityHashMap<>();
+
 	public static final int NO_DIFFERENCE = 10000;
 
 	/**
@@ -1060,6 +1069,7 @@ public IStreamMerger createStreamMerger(IContentType type) {
 
 	public ViewerDescriptor[] findStructureViewerDescriptor(Viewer oldViewer,
 			ICompareInput input, CompareConfiguration configuration) {
+		beginContentTypeSniffing(input);
 		// we don't show the structure of additions or deletions
 		if ((input == null) || input == null || input.getLeft() == null || input.getRight() == null) {
 			return null;
@@ -1245,6 +1255,7 @@ private Collection<Object> getContentTypes(Object in) {
 	}
 
 	public ViewerDescriptor[] findContentViewerDescriptor(Viewer oldViewer, Object in, CompareConfiguration cc) {
+		beginContentTypeSniffing(in);
 		LinkedHashSet<ViewerDescriptor> result = new LinkedHashSet<>();
 		if (in instanceof IStreamContentAccessor) {
 			String type= ITypedElement.TEXT_TYPE;
@@ -1469,10 +1480,28 @@ private static String[] getTypes(ICompareInput input) {
 		return tmp.toArray(new String[tmp.size()]);
 	}
 
+	/** Resets the sniffing caches when a different input is examined. */
+	private static void beginContentTypeSniffing(Object input) {
+		if (input != fSniffInput) {
+			fContentTypeSniffCache.clear();
+			fGuessTypeSniffCache.clear();
+			fSniffInput= input;
+		}
+	}
+
 	private static IContentType getContentType(ITypedElement element) {
 		if (element == null) {
 			return null;
 		}
+		if (fContentTypeSniffCache.containsKey(element)) {
+			return fContentTypeSniffCache.get(element);
+		}
+		IContentType ct= computeContentType(element);
+		fContentTypeSniffCache.put(element, ct);
+		return ct;
+	}
+
+	private static IContentType computeContentType(ITypedElement element) {
 		String name= element.getName();
 		IContentType ct= null;
 		if (element instanceof IResourceProvider) {
@@ -1603,6 +1632,18 @@ private static IContentType[] toFullPath(IContentType ct) {
 	 * Returns <code>null</code> if the input isn't an <code>IStreamContentAccessor</code>.
 	 */
 	private static String guessType(ITypedElement input) {
+		if (input == null) {
+			return null;
+		}
+		if (fGuessTypeSniffCache.containsKey(input)) {
+			return fGuessTypeSniffCache.get(input);
+		}
+		String guessed= computeGuessType(input);
+		fGuessTypeSniffCache.put(input, guessed);
+		return guessed;
+	}
+
+	private static String computeGuessType(ITypedElement input) {
 		if (input instanceof IStreamContentAccessor sca) {
 			InputStream is= null;
 			try {

team/tests/org.eclipse.compare.tests/src/org/eclipse/compare/tests/CompareOpenEfficiencyTest.java

@@ -55,11 +55,11 @@ public class CompareOpenEfficiencyTest {
 
 	/**
 	 * Upper bound for the {@code getContents()} calls per side during a single
-	 * compare editor open, caused by repeated content-type sniffing and viewer
-	 * descriptor lookups. The exact count is platform dependent (observed: 15 on
-	 * Linux and Windows, 9 on macOS), so only the worst case is asserted.
+	 * compare editor open: one read for content-type detection, one for the text
+	 * heuristic, and one for the document shown in the viewer. Asserted as a bound
+	 * because the exact count can vary by platform.
 	 */
-	private static final int MAX_GET_CONTENTS_PER_SIDE = 15;
+	private static final int MAX_GET_CONTENTS_PER_SIDE = 3;
 
 	private static final long TIMEOUT_MILLIS = 30_000;