Make diff algorithm configurable

The diff algorithm which is used by Merge, Cherry-Pick, Rebase
should be configurable. A new configuration parameter "diff.algorithm"
is introduced which currently accepts the values "myers" or
"histogram". Based on this parameter for example the ResolveMerger
will choose a diff algorithm. The reason for this is bug 331078.
This bug shows that JGit is more compatible with C Git when
histogram diff is in place. But since histogram diff is quite new we
need an easy way to fall back to Myers diff.

Bug: 331078
Change-Id: I2549c992e478d991c61c9508ad826d1a9e539ae3
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Philipp Thun <philipp.thun@sap.com>

5 files changed, 78 insertions, 10 deletions

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffAlgorithm.java b/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffAlgorithm.java
index f6859a29c3..b20e3258b6 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffAlgorithm.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffAlgorithm.java
@@ -54,6 +54,38 @@ package org.eclipse.jgit.diff;
  */
 public abstract class DiffAlgorithm {
 	/**
+	 * Supported diff algorithm
+	 */
+	public enum SupportedAlgorithm {
+		/**
+		 * Myers diff algorithm
+		 */
+		MYERS,
+
+		/**
+		 * Histogram diff algorithm
+		 */
+		HISTOGRAM
+	}
+
+	/**
+	 * @param alg
+	 *            the diff algorithm for which an implementation should be
+	 *            returned
+	 * @return an implementation of the specified diff algorithm
+	 */
+	public static DiffAlgorithm getAlgorithm(SupportedAlgorithm alg) {
+		switch (alg) {
+		case MYERS:
+			return MyersDiff.INSTANCE;
+		case HISTOGRAM:
+			return new HistogramDiff();
+		default:
+			throw new IllegalArgumentException();
+		}
+	}
+
+	/**
 	 * Compare two sequences and identify a list of edits between them.
 	 *
 	 * @param <S>
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java b/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java
index be9a86eda9..fa0cb9c473 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java
@@ -63,6 +63,7 @@ import java.util.Collections;
 import java.util.List;
 
 import org.eclipse.jgit.JGitText;
+import org.eclipse.jgit.diff.DiffAlgorithm.SupportedAlgorithm;
 import org.eclipse.jgit.diff.DiffEntry.ChangeType;
 import org.eclipse.jgit.errors.AmbiguousObjectException;
 import org.eclipse.jgit.errors.CorruptObjectException;
@@ -70,6 +71,7 @@ import org.eclipse.jgit.errors.LargeObjectException;
 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.lib.AbbreviatedObjectId;
 import org.eclipse.jgit.lib.AnyObjectId;
+import org.eclipse.jgit.lib.ConfigConstants;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.FileMode;
 import org.eclipse.jgit.lib.ObjectId;
@@ -118,7 +120,7 @@ public class DiffFormatter {
 
 	private int abbreviationLength = 7;
 
-	private DiffAlgorithm diffAlgorithm = new HistogramDiff();
+	private DiffAlgorithm diffAlgorithm;
 
 	private RawTextComparator comparator = RawTextComparator.DEFAULT;
 
@@ -178,6 +180,12 @@ public class DiffFormatter {
 			setNewPrefix("");
 		}
 		setDetectRenames(dc.isRenameDetectionEnabled());
+
+		diffAlgorithm = DiffAlgorithm.getAlgorithm(db.getConfig().getEnum(
+				ConfigConstants.CONFIG_DIFF_SECTION, null,
+				ConfigConstants.CONFIG_KEY_ALGORITHM,
+				SupportedAlgorithm.HISTOGRAM));
+
 	}
 
 	/**
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
index 4ca11d0faf..9f742062b3 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
@@ -57,6 +57,12 @@ public class ConfigConstants {
 	/** The "remote" section */
 	public static final String CONFIG_REMOTE_SECTION = "remote";
 
+	/** The "diff" section */
+	public static final String CONFIG_DIFF_SECTION = "diff";
+
+	/** The "algorithm" key */
+	public static final String CONFIG_KEY_ALGORITHM = "algorithm";
+
 	/** The "autocrlf" key */
 	public static final String CONFIG_KEY_AUTOCRLF = "autocrlf";
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/merge/MergeAlgorithm.java b/org.eclipse.jgit/src/org/eclipse/jgit/merge/MergeAlgorithm.java
index aee0ba940a..62febd62c2 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/merge/MergeAlgorithm.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/merge/MergeAlgorithm.java
@@ -47,6 +47,7 @@ import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
 
+import org.eclipse.jgit.diff.DiffAlgorithm;
 import org.eclipse.jgit.diff.Edit;
 import org.eclipse.jgit.diff.EditList;
 import org.eclipse.jgit.diff.MyersDiff;
@@ -56,15 +57,27 @@ import org.eclipse.jgit.merge.MergeChunk.ConflictState;
 
 /**
  * Provides the merge algorithm which does a three-way merge on content provided
- * as RawText. Makes use of {@link MyersDiff} to compute the diffs.
+ * as RawText. By default {@link MyersDiff} is used as diff algorithm.
  */
 public final class MergeAlgorithm {
+	private final DiffAlgorithm diffAlg;
 
 	/**
-	 * Since this class provides only static methods I add a private default
-	 * constructor to prevent instantiation.
+	 * Creates a new MergeAlgorithm which uses {@link MyersDiff} as diff
+	 * algorithm
 	 */
-	private MergeAlgorithm() {
+	public MergeAlgorithm() {
+		this(MyersDiff.INSTANCE);
+	}
+
+	/**
+	 * Creates a new MergeAlgorithm
+	 *
+	 * @param diff
+	 *            the diff algorithm used by this merge
+	 */
+	public MergeAlgorithm(DiffAlgorithm diff) {
+		this.diffAlg = diff;
 	}
 
 	// An special edit which acts as a sentinel value by marking the end the
@@ -83,16 +96,16 @@ public final class MergeAlgorithm {
 	 * @param theirs the second sequence to be merged
 	 * @return the resulting content
 	 */
-	public static <S extends Sequence> MergeResult<S> merge(
+	public <S extends Sequence> MergeResult<S> merge(
 			SequenceComparator<S> cmp, S base, S ours, S theirs) {
 		List<S> sequences = new ArrayList<S>(3);
 		sequences.add(base);
 		sequences.add(ours);
 		sequences.add(theirs);
-		MergeResult result = new MergeResult<S>(sequences);
-		EditList oursEdits = MyersDiff.INSTANCE.diff(cmp, base, ours);
+		MergeResult<S> result = new MergeResult<S>(sequences);
+		EditList oursEdits = diffAlg.diff(cmp, base, ours);
 		Iterator<Edit> baseToOurs = oursEdits.iterator();
-		EditList theirsEdits = MyersDiff.INSTANCE.diff(cmp, base, theirs);
+		EditList theirsEdits = diffAlg.diff(cmp, base, theirs);
 		Iterator<Edit> baseToTheirs = theirsEdits.iterator();
 		int current = 0; // points to the next line (first line is 0) of base
 		                 // which was not handled yet
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/merge/ResolveMerger.java b/org.eclipse.jgit/src/org/eclipse/jgit/merge/ResolveMerger.java
index a6119edfae..a1bda41284 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/merge/ResolveMerger.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/merge/ResolveMerger.java
@@ -58,6 +58,8 @@ import java.util.List;
 import java.util.Map;
 
 import org.eclipse.jgit.JGitText;
+import org.eclipse.jgit.diff.DiffAlgorithm;
+import org.eclipse.jgit.diff.DiffAlgorithm.SupportedAlgorithm;
 import org.eclipse.jgit.diff.RawText;
 import org.eclipse.jgit.diff.RawTextComparator;
 import org.eclipse.jgit.diff.Sequence;
@@ -71,6 +73,7 @@ import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.IndexWriteException;
 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.errors.NoWorkTreeException;
+import org.eclipse.jgit.lib.ConfigConstants;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.FileMode;
 import org.eclipse.jgit.lib.ObjectId;
@@ -136,6 +139,7 @@ public class ResolveMerger extends ThreeWayMerger {
 
 	private WorkingTreeIterator workingTreeIterator;
 
+	private MergeAlgorithm mergeAlgorithm;
 
 	/**
 	 * @param local
@@ -143,6 +147,11 @@ public class ResolveMerger extends ThreeWayMerger {
 	 */
 	protected ResolveMerger(Repository local, boolean inCore) {
 		super(local);
+		SupportedAlgorithm diffAlg = local.getConfig().getEnum(
+				ConfigConstants.CONFIG_DIFF_SECTION, null,
+				ConfigConstants.CONFIG_KEY_ALGORITHM,
+				SupportedAlgorithm.HISTOGRAM);
+		mergeAlgorithm = new MergeAlgorithm(DiffAlgorithm.getAlgorithm(diffAlg));
 		commitNames = new String[] { "BASE", "OURS", "THEIRS" };
 		oi = getObjectInserter();
 		this.inCore = inCore;
@@ -459,7 +468,7 @@ public class ResolveMerger extends ThreeWayMerger {
 		MergeFormatter fmt = new MergeFormatter();
 
 		// do the merge
-		MergeResult<RawText> result = MergeAlgorithm.merge(
+		MergeResult<RawText> result = mergeAlgorithm.merge(
 				RawTextComparator.DEFAULT,
 				getRawText(base.getEntryObjectId(), db),
 				getRawText(ours.getEntryObjectId(), db),