Do not delta compress objects that have already tried to compress.

If an object is in a pack file already, delta compression will not
attempt to re-compress it. This assumes that the previous
packing already performed the optimal compression attempt, however,
the subclasses of StoredObjectRepresentation may use other heuristics
to determine if the stored format is optimal.

Change-Id: I403de522f4b0dd2667d54f6faed621f392c07786

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/DeltaWindow.java

@@ -141,7 +141,7 @@ class DeltaWindow {
 				}
 				res.set(toSearch[off]);
 
-				if (res.object.isEdge()) {
+				if (res.object.isEdge() || res.object.doNotAttemptDelta()) {
 					// We don't actually want to make a delta for
 					// them, just need to push them into the window
 					// so they can be read by other objects.

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/ObjectToPack.java

@@ -66,6 +66,10 @@ public class ObjectToPack extends PackedObjectInfo {
 
 	private static final int EDGE = 1 << 3;
 
+	private static final int DELTA_ATTEMPTED = 1 << 4;
+
+	private static final int ATTEMPT_DELTA_MASK = REUSE_AS_IS | DELTA_ATTEMPTED;
+
 	private static final int TYPE_SHIFT = 5;
 
 	private static final int EXT_SHIFT = 8;
@@ -88,7 +92,7 @@ public class ObjectToPack extends PackedObjectInfo {
 	 * <li>1 bit: canReuseAsIs</li>
 	 * <li>1 bit: doNotDelta</li>
 	 * <li>1 bit: edgeObject</li>
-	 * <li>1 bit: unused</li>
+	 * <li>1 bit: deltaAttempted</li>
 	 * <li>3 bits: type</li>
 	 * <li>4 bits: subclass flags (if any)</li>
 	 * <li>--</li>
@@ -265,6 +269,22 @@ public class ObjectToPack extends PackedObjectInfo {
 		flags |= EDGE;
 	}
 
+	boolean doNotAttemptDelta() {
+		// Do not attempt if delta attempted and object reuse.
+		return (flags & ATTEMPT_DELTA_MASK) == ATTEMPT_DELTA_MASK;
+	}
+
+	boolean isDeltaAttempted() {
+		return (flags & DELTA_ATTEMPTED) != 0;
+	}
+
+	void setDeltaAttempted(boolean deltaAttempted) {
+		if (deltaAttempted)
+			flags |= DELTA_ATTEMPTED;
+		else
+			flags &= ~DELTA_ATTEMPTED;
+	}
+
 	/** @return the extended flags on this object, in the range [0x0, 0xf]. */
 	protected int getExtendedFlags() {
 		return (flags >>> EXT_SHIFT) & EXT_MASK;

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackWriter.java

@@ -1870,6 +1870,7 @@ public class PackWriter {
 			otp.clearReuseAsIs();
 		}
 
+		otp.setDeltaAttempted(next.wasDeltaAttempted());
 		otp.select(next);
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/StoredObjectRepresentation.java

@@ -72,8 +72,9 @@ public class StoredObjectRepresentation {
 	}
 
 	/**
-	 * @return true if this is a delta against another object and this is stored
-	 *         in pack delta format.
+	 * @return the storage format type, which must be one of
+	 *         {@link #PACK_DELTA}, {@link #PACK_WHOLE}, or
+	 *         {@link #FORMAT_OTHER}.
 	 */
 	public int getFormat() {
 		return FORMAT_OTHER;
@@ -87,4 +88,13 @@ public class StoredObjectRepresentation {
 	public ObjectId getDeltaBase() {
 		return null;
 	}
+
+	/**
+	 * @return whether the current representation of the object has had delta
+	 *         compression attempted on it.
+	 */
+	public boolean wasDeltaAttempted() {
+		int fmt = getFormat();
+		return fmt == PACK_DELTA || fmt == PACK_WHOLE;
+	}
 }