Define DiffAlgorithm as an abstract function

This makes it easier to parametrize DiffFormatter with a different
implementation, as we later plan to add PatienceDiff to JGit.

Change-Id: Id35ef478d5fa20fe10a1ba297f9436fd7adde9ce
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.iplog/src/org/eclipse/jgit/iplog/IpLogGenerator.java

@@ -384,9 +384,9 @@ public class IpLogGenerator {
 					else
 						oldImage = new byte[0];
 
-					EditList edits = new MyersDiff<RawText>(
+					EditList edits = MyersDiff.INSTANCE.diff(
 							RawTextComparator.DEFAULT, new RawText(oldImage),
-							new RawText(openBlob(1))).getEdits();
+							new RawText(openBlob(1)));
 					for (Edit e : edits)
 						addedLines += e.getEndB() - e.getBeginB();
 				}

org.eclipse.jgit.test/tst/org/eclipse/jgit/diff/MyersDiffPerformanceTest.java

@@ -164,14 +164,14 @@ public class MyersDiffPerformanceTest extends TestCase {
 		CharArray ac = new CharArray(a);
 		CharArray bc = new CharArray(b);
 		CharCmp cmp = new CharCmp();
-		MyersDiff<CharArray> myersDiff = null;
+		int D = 0;
 		int cpuTimeChanges = 0;
 		long lastReadout = 0;
 		long interimTime = 0;
 		int repetitions = 0;
 		stopwatch.start();
 		while (cpuTimeChanges < minCPUTimerTicks && interimTime < longTaskBoundary) {
-			myersDiff = new MyersDiff<CharArray>(cmp, ac, bc);
+			D = MyersDiff.INSTANCE.diff(cmp, ac, bc).size();
 			repetitions++;
 			interimTime = stopwatch.readout();
 			if (interimTime != lastReadout) {
@@ -181,7 +181,7 @@ public class MyersDiffPerformanceTest extends TestCase {
 		}
 		ret.runningTime = stopwatch.stop() / repetitions;
 		ret.N = ac.size() + bc.size();
-		ret.D = myersDiff.getEdits().size();
+		ret.D = D;
 
 		return ret;
 	}

org.eclipse.jgit.test/tst/org/eclipse/jgit/diff/MyersDiffTest.java

@@ -63,9 +63,9 @@ public class MyersDiffTest extends TestCase {
 	}
 
 	public void assertDiff(String a, String b, String edits) {
-		MyersDiff diff = new MyersDiff<CharArray>(new CharCmp(),
+		EditList editList = MyersDiff.INSTANCE.diff(new CharCmp(),
 				toCharArray(a), toCharArray(b));
-		assertEquals(edits, toString(diff.getEdits()));
+		assertEquals(edits, toString(editList));
 	}
 
 	private static String toString(EditList list) {

org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffAlgorithm.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright (C) 2010, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.diff;
+
+/**
+ * Compares two {@link Sequence}s to create an {@link EditList} of changes.
+ *
+ * An algorithm's {@code diff} method must be callable from concurrent threads
+ * without data collisions. This permits some algorithms to use a singleton
+ * pattern, with concurrent invocations using the same singleton. Other
+ * algorithms may support parameterization, in which case the caller can create
+ * a unique instance per thread.
+ */
+public interface DiffAlgorithm {
+	/**
+	 * Compare two sequences and identify a list of edits between them.
+	 * 
+	 * @param <S>
+	 *            type of sequence being compared.
+	 * @param <C>
+	 *            type of comparator to evaluate the sequence elements.
+	 * @param cmp
+	 *            the comparator supplying the element equivalence function.
+	 * @param a
+	 *            the first (also known as old or pre-image) sequence. Edits
+	 *            returned by this algorithm will reference indexes using the
+	 *            'A' side: {@link Edit#getBeginA()}, {@link Edit#getEndA()}.
+	 * @param b
+	 *            the second (also known as new or post-image) sequence. Edits
+	 *            returned by this algorithm will reference indexes using the
+	 *            'B' side: {@link Edit#getBeginB()}, {@link Edit#getEndB()}.
+	 * @return a modifiable edit list comparing the two sequences. If empty, the
+	 *         sequences are identical according to {@code cmp}'s rules. The
+	 *         result list is never null.
+	 */
+	public <S extends Sequence, C extends SequenceComparator<? super S>> EditList diff(
+			C cmp, S a, S b);
+}

org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java

@@ -118,6 +118,8 @@ public class DiffFormatter {
 
 	private int abbreviationLength = 7;
 
+	private DiffAlgorithm diffAlgorithm = MyersDiff.INSTANCE;
+
 	private RawTextComparator comparator = RawTextComparator.DEFAULT;
 
 	private int binaryFileThreshold = DEFAULT_BINARY_FILE_THRESHOLD;
@@ -206,6 +208,17 @@ public class DiffFormatter {
 		abbreviationLength = count;
 	}
 
+	/**
+	 * Set the algorithm that constructs difference output.
+	 *
+	 * @param alg
+	 *            the algorithm to produce text file differences.
+	 * @see MyersDiff#INSTANCE
+	 */
+	public void setDiffAlgorithm(DiffAlgorithm alg) {
+		diffAlgorithm = alg;
+	}
+
 	/**
 	 * Set the line equivalence function for text file differences.
 	 *
@@ -893,7 +906,7 @@ public class DiffFormatter {
 	}
 
 	private EditList diff(RawText a, RawText b) {
-		return new MyersDiff<RawText>(comparator, a, b).getEdits();
+		return diffAlgorithm.diff(comparator, a, b);
 	}
 
 	private void assertHaveRepository() {

org.eclipse.jgit/src/org/eclipse/jgit/diff/MyersDiff.java

@@ -106,13 +106,21 @@ import org.eclipse.jgit.util.LongList;
  *            type of sequence.
  */
 public class MyersDiff<S extends Sequence> {
+	/** Singleton instance of MyersDiff. */
+	public static final DiffAlgorithm INSTANCE = new DiffAlgorithm() {
+		public <S extends Sequence, C extends SequenceComparator<? super S>> EditList diff(
+				C cmp, S a, S b) {
+			return new MyersDiff<S>(cmp, a, b).getEdits();
+		}
+	};
+
 	/**
 	 * The list of edits found during the last call to {@link #calculateEdits()}
 	 */
 	protected EditList edits;
 
 	/** Comparison function for sequences. */
-	protected SequenceComparator<S> cmp;
+	protected SequenceComparator<? super S> cmp;
 
 	/**
 	 * The first text to be compared. Referred to as "Text A" in the comments
@@ -124,14 +132,7 @@ public class MyersDiff<S extends Sequence> {
 	 */
 	protected S b;
 
-	/**
-	 * The only constructor
-	 *
-	 * @param cmp comparison method for this execution.
-	 * @param a   the text A which should be compared
-	 * @param b   the text B which should be compared
-	 */
-	public MyersDiff(SequenceComparator<S> cmp, S a, S b) {
+	private MyersDiff(SequenceComparator<? super S> cmp, S a, S b) {
 		this.cmp = cmp;
 		this.a = a;
 		this.b = b;

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

@@ -90,9 +90,9 @@ public final class MergeAlgorithm {
 		sequences.add(ours);
 		sequences.add(theirs);
 		MergeResult result = new MergeResult<S>(sequences);
-		EditList oursEdits = new MyersDiff<S>(cmp, base, ours).getEdits();
+		EditList oursEdits = MyersDiff.INSTANCE.diff(cmp, base, ours);
 		Iterator<Edit> baseToOurs = oursEdits.iterator();
-		EditList theirsEdits = new MyersDiff<S>(cmp, base, theirs).getEdits();
+		EditList theirsEdits = MyersDiff.INSTANCE.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