log: Implement --follow

The FollowFilter can be installed on a RevWalk to cause the path
to be updated through rename detection when the affected file is
found to be added to the project.

The filter works reasonably well, for example we can follow the
history of the fsck command in git-core:

  $ jgit log --name-status --follow builtin/fsck.c | grep ^R
  R100	builtin-fsck.c	builtin/fsck.c
  R099	fsck.c	builtin-fsck.c
  R099	fsck-objects.c	fsck.c
  R099	fsck-cache.c	fsck-objects.c

Change-Id: I4017bcfd150126aa342fdd423a688493ca660a1f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

2 files changed, 62 insertions, 11 deletions

diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Log.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Log.java
index 83ef6eb875..48a05915e4 100644
--- a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Log.java
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Log.java
@@ -51,6 +51,7 @@ import java.text.DateFormat;
 import java.text.MessageFormat;
 import java.text.SimpleDateFormat;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Locale;
@@ -65,11 +66,12 @@ import org.eclipse.jgit.diff.RawTextIgnoreLeadingWhitespace;
 import org.eclipse.jgit.diff.RawTextIgnoreTrailingWhitespace;
 import org.eclipse.jgit.diff.RawTextIgnoreWhitespaceChange;
 import org.eclipse.jgit.diff.RenameDetector;
+import org.eclipse.jgit.diff.DiffEntry.ChangeType;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.PersonIdent;
 import org.eclipse.jgit.lib.Ref;
-import org.eclipse.jgit.lib.TextProgressMonitor;
+import org.eclipse.jgit.revwalk.FollowFilter;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.treewalk.TreeWalk;
@@ -195,20 +197,26 @@ class Log extends RevWalkTextBuiltin {
 
 	private void showDiff(RevCommit c) throws IOException {
 		final TreeWalk tw = new TreeWalk(db);
-		tw.reset();
 		tw.setRecursive(true);
+		tw.reset();
 		tw.addTree(c.getParent(0).getTree());
 		tw.addTree(c.getTree());
-		tw.setFilter(AndTreeFilter.create(TreeFilter.ANY_DIFF, pathFilter));
+		tw.setFilter(AndTreeFilter.create(pathFilter, TreeFilter.ANY_DIFF));
 
 		List<DiffEntry> files = DiffEntry.scan(tw);
-		if (detectRenames) {
-			RenameDetector rd = new RenameDetector(db);
-			if (renameLimit != null)
-				rd.setRenameLimit(renameLimit.intValue());
-			rd.addAll(files);
-			files = rd.compute(new TextProgressMonitor());
-		}
+		if (pathFilter instanceof FollowFilter && isAdd(files)) {
+			// The file we are following was added here, find where it
+			// came from so we can properly show the rename or copy,
+			// then continue digging backwards.
+			//
+			tw.reset();
+			tw.addTree(c.getParent(0).getTree());
+			tw.addTree(c.getTree());
+			tw.setFilter(TreeFilter.ANY_DIFF);
+			files = updateFollowFilter(detectRenames(DiffEntry.scan(tw)));
+
+		} else if (detectRenames)
+			files = detectRenames(files);
 
 		if (showNameAndStatusOnly) {
 			Diff.nameStatus(out, files);
@@ -220,4 +228,39 @@ class Log extends RevWalkTextBuiltin {
 		}
 		out.println();
 	}
+
+	private List<DiffEntry> detectRenames(List<DiffEntry> files)
+			throws IOException {
+		RenameDetector rd = new RenameDetector(db);
+		if (renameLimit != null)
+			rd.setRenameLimit(renameLimit.intValue());
+		rd.addAll(files);
+		return rd.compute();
+	}
+
+	private boolean isAdd(List<DiffEntry> files) {
+		String oldPath = ((FollowFilter) pathFilter).getPath();
+		for (DiffEntry ent : files) {
+			if (ent.getChangeType() == ChangeType.ADD
+					&& ent.getNewName().equals(oldPath))
+				return true;
+		}
+		return false;
+	}
+
+	private List<DiffEntry> updateFollowFilter(List<DiffEntry> files) {
+		String oldPath = ((FollowFilter) pathFilter).getPath();
+		for (DiffEntry ent : files) {
+			if (isRename(ent) && ent.getNewName().equals(oldPath)) {
+				pathFilter = FollowFilter.create(ent.getOldName());
+				return Collections.singletonList(ent);
+			}
+		}
+		return Collections.emptyList();
+	}
+
+	private static boolean isRename(DiffEntry ent) {
+		return ent.getChangeType() == ChangeType.RENAME
+				|| ent.getChangeType() == ChangeType.COPY;
+	}
 }
diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/RevWalkTextBuiltin.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/RevWalkTextBuiltin.java
index beb961d99c..bf3924b70b 100644
--- a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/RevWalkTextBuiltin.java
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/RevWalkTextBuiltin.java
@@ -53,6 +53,7 @@ import org.kohsuke.args4j.Option;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.pgm.opt.PathTreeFilterHandler;
+import org.eclipse.jgit.revwalk.FollowFilter;
 import org.eclipse.jgit.revwalk.ObjectWalk;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevFlag;
@@ -110,6 +111,11 @@ abstract class RevWalkTextBuiltin extends TextBuiltin {
 		enableRevSort(RevSort.BOUNDARY, on);
 	}
 
+	@Option(name = "--follow", metaVar = "metaVar_path")
+	void follow(final String path) {
+		pathFilter = FollowFilter.create(path);
+	}
+
 	@Argument(index = 0, metaVar = "metaVar_commitish")
 	private final List<RevCommit> commits = new ArrayList<RevCommit>();
 
@@ -139,7 +145,9 @@ abstract class RevWalkTextBuiltin extends TextBuiltin {
 		for (final RevSort s : sorting)
 			walk.sort(s, true);
 
-		if (pathFilter != TreeFilter.ALL)
+		if (pathFilter instanceof FollowFilter)
+			walk.setTreeFilter(pathFilter);
+		else if (pathFilter != TreeFilter.ALL)
 			walk.setTreeFilter(AndTreeFilter.create(pathFilter,
 					TreeFilter.ANY_DIFF));