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>

2 files changed, 41 insertions, 1 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));
+
 	}
 
 	/**