Optimize MergeAlgorithm if ours or theirs is empty

Previously when merging two contents with a non-empty base and one of
the contents was empty (size == 0) and the other was modified there
was a potentially expensive calculation until we finally always come
to the same result -> the complete non-deleted content should collide
with the empty content.

This proposal adds an optimization to detect empty input content and
to produce the appropriate result immediatly.

Change-Id: Ie6a837260c19d808f0e99173f570ff96dd22acd3
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/merge/MergeAlgorithmTest.java

@@ -198,6 +198,25 @@ public class MergeAlgorithmTest {
 
 	}
 
+	/**
+	 * Test situations where (at least) one input value is the empty text
+	 *
+	 * @throws IOException
+	 */
+	@Test
+	public void testEmptyTexts() throws IOException {
+		// test modification against deletion
+		assertEquals(t("<AB=>"), merge("A", "AB", ""));
+		assertEquals(t("<=AB>"), merge("A", "", "AB"));
+
+		// test unmodified against deletion
+		assertEquals(t(""), merge("AB", "AB", ""));
+		assertEquals(t(""), merge("AB", "", "AB"));
+
+		// test deletion against deletion
+		assertEquals(t(""), merge("AB", "", ""));
+	}
+
 	private String merge(String commonBase, String ours, String theirs) throws IOException {
 		MergeResult r = new MergeAlgorithm().merge(RawTextComparator.DEFAULT,
 				T(commonBase), T(ours), T(theirs));
@@ -231,5 +250,4 @@ public class MergeAlgorithmTest {
 	public static RawText T(String text) {
 		return new RawText(Constants.encode(t(text)));
 	}
-
 }

org.eclipse.jgit/src/org/eclipse/jgit/merge/MergeAlgorithm.java

@@ -103,6 +103,37 @@ public final class MergeAlgorithm {
 		sequences.add(ours);
 		sequences.add(theirs);
 		MergeResult<S> result = new MergeResult<S>(sequences);
+
+		if (ours.size() == 0) {
+			if (theirs.size() != 0) {
+				EditList theirsEdits = diffAlg.diff(cmp, base, theirs);
+				if (!theirsEdits.isEmpty()) {
+					// we deleted, they modified -> Let their complete content
+					// conflict with empty text
+					result.add(1, 0, 0, ConflictState.FIRST_CONFLICTING_RANGE);
+					result.add(2, 0, theirs.size(),
+							ConflictState.NEXT_CONFLICTING_RANGE);
+				} else
+					// we deleted, they didn't modify -> Let our deletion win
+					result.add(1, 0, 0, ConflictState.NO_CONFLICT);
+			} else
+				// we and they deleted -> return a single chunk of nothing
+				result.add(1, 0, 0, ConflictState.NO_CONFLICT);
+			return result;
+		} else if (theirs.size() == 0) {
+			EditList oursEdits = diffAlg.diff(cmp, base, ours);
+			if (!oursEdits.isEmpty()) {
+				// we modified, they deleted -> Let our complete content
+				// conflict with empty text
+				result.add(1, 0, ours.size(),
+						ConflictState.FIRST_CONFLICTING_RANGE);
+				result.add(2, 0, 0, ConflictState.NEXT_CONFLICTING_RANGE);
+			} else
+				// they deleted, we didn't modify -> Let their deletion win
+				result.add(2, 0, 0, ConflictState.NO_CONFLICT);
+			return result;
+		}
+
 		EditList oursEdits = diffAlg.diff(cmp, base, ours);
 		Iterator<Edit> baseToOurs = oursEdits.iterator();
 		EditList theirsEdits = diffAlg.diff(cmp, base, theirs);