HDFS-4141. Support directory diff - the difference between the current state and a previous snapshot of an INodeDirectory.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1405250 13f79535-47bb-0310-9956-ffa450edef68

hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt

@@ -44,3 +44,6 @@ Branch-2802 Snapshot (Unreleased)
   HDFS-4116. Add auditlog for some snapshot operations. (Jing Zhao via suresh)
 
   HDFS-4095. Add some snapshot related metrics. (Jing Zhao via suresh)
+
+  HDFS-4141. Support directory diff - the difference between the current state
+  and a previous snapshot of an INodeDirectory. (szetszwo)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSDirectory.java

@@ -57,8 +57,6 @@ import org.apache.hadoop.hdfs.server.blockmanagement.BlockInfoUnderConstruction;
 import org.apache.hadoop.hdfs.server.blockmanagement.BlockManager;
 import org.apache.hadoop.hdfs.server.blockmanagement.DatanodeDescriptor;
 import org.apache.hadoop.hdfs.server.common.HdfsServerConstants.BlockUCState;
-import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeFileSnapshot;
-import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeFileWithLink;
 import org.apache.hadoop.hdfs.server.namenode.INodeDirectory.INodesInPath;
 import org.apache.hadoop.hdfs.util.ByteArray;
 

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INode.java

@@ -244,7 +244,7 @@ public abstract class INode implements Comparable<byte[]> {
    * @return null if the local name is null;
    *         otherwise, return the local name byte array.
    */
-  byte[] getLocalNameBytes() {
+  public byte[] getLocalNameBytes() {
     return name;
   }
 

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeDirectory.java

@@ -30,7 +30,7 @@ import org.apache.hadoop.hdfs.DFSUtil;
 import org.apache.hadoop.hdfs.protocol.Block;
 import org.apache.hadoop.hdfs.protocol.HdfsConstants;
 import org.apache.hadoop.hdfs.protocol.UnresolvedPathException;
-import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectorySnapshotRoot;
+import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectoryWithSnapshot;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectorySnapshottable;
 
 import com.google.common.annotations.VisibleForTesting;
@@ -56,7 +56,7 @@ public class INodeDirectory extends INode {
 
   private List<INode> children;
 
-  protected INodeDirectory(String name, PermissionStatus permissions) {
+  public INodeDirectory(String name, PermissionStatus permissions) {
     super(name, permissions);
     this.children = null;
   }
@@ -514,7 +514,7 @@ public class INodeDirectory extends INode {
      */
     private boolean isSnapshot;
     /**
-     * Index of {@link INodeDirectorySnapshotRoot} for snapshot path, else -1
+     * Index of {@link INodeDirectoryWithSnapshot} for snapshot path, else -1
      */
     private int snapshotRootIndex;
 
@@ -542,7 +542,7 @@ public class INodeDirectory extends INode {
     }
     
     /**
-     * @return index of the {@link INodeDirectorySnapshotRoot} in
+     * @return index of the {@link INodeDirectoryWithSnapshot} in
      *         {@link #inodes} for snapshot path, else -1.
      */
     int getSnapshotRootIndex() {

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeFile.java

@@ -69,6 +69,7 @@ public class INodeFile extends INode implements BlockCollection {
   protected INodeFile(INodeFile f) {
     this(f.getPermissionStatus(), f.getBlocks(), f.getFileReplication(),
         f.getModificationTime(), f.getAccessTime(), f.getPreferredBlockSize());
+    this.name = f.getLocalNameBytes();
   }
 
   /**

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectorySnapshotRoot.java

@@ -1,29 +0,0 @@
-/**
- * Licensed to the Apache Software Foundation (ASF) under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership.  The ASF licenses this file
- * to you under the Apache License, Version 2.0 (the
- * "License"); you may not use this file except in compliance
- * with the License.  You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.apache.hadoop.hdfs.server.namenode.snapshot;
-
-import org.apache.hadoop.hdfs.server.namenode.INodeDirectory;
-
-/** The root directory of a snapshot. */
-public class INodeDirectorySnapshotRoot extends INodeDirectory {
-  INodeDirectorySnapshotRoot(String name, INodeDirectory dir) {
-    super(name, dir.getPermissionStatus());
-    setLocalName(name);
-    parent = dir;
-  }
-}

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectorySnapshottable.java

@@ -57,8 +57,8 @@ public class INodeDirectorySnapshottable extends INodeDirectoryWithQuota {
   }
 
   /** A list of snapshots of this directory. */
-  private final List<INodeDirectorySnapshotRoot> snapshots
+  private final List<INodeDirectoryWithSnapshot> snapshots
-      = new ArrayList<INodeDirectorySnapshotRoot>();
+      = new ArrayList<INodeDirectoryWithSnapshot>();
   
   public INode getSnapshotINode(byte[] name) {
     if (snapshots == null || snapshots.size() == 0) {
@@ -98,7 +98,7 @@ public class INodeDirectorySnapshottable extends INodeDirectoryWithQuota {
   }
 
   /** Add a snapshot root under this directory. */
-  INodeDirectorySnapshotRoot addSnapshotRoot(final String name
+  INodeDirectoryWithSnapshot addSnapshotRoot(final String name
       ) throws SnapshotException {
     //check snapshot quota
     if (snapshots.size() + 1 > snapshotQuota) {
@@ -107,7 +107,7 @@ public class INodeDirectorySnapshottable extends INodeDirectoryWithQuota {
           + snapshotQuota);
     }
 
-    final INodeDirectorySnapshotRoot r = new INodeDirectorySnapshotRoot(name, this);
+    final INodeDirectoryWithSnapshot r = new INodeDirectoryWithSnapshot(name, this);
     snapshots.add(r);
 
     //set modification time

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeDirectoryWithSnapshot.java

@@ -0,0 +1,264 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hdfs.server.namenode.snapshot;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+import org.apache.hadoop.hdfs.server.namenode.INode;
+import org.apache.hadoop.hdfs.server.namenode.INodeDirectory;
+
+/** The directory with snapshots. */
+public class INodeDirectoryWithSnapshot extends INodeDirectory {
+  /**
+   * The difference between the current state and a previous snapshot
+   * of an INodeDirectory.
+   *
+   * Two lists are maintained in the algorithm:
+   * - c-list for newly created inodes
+   * - d-list for the deleted inodes
+   *
+   * Denote the state of an inode by the following
+   *   (0, 0): neither in c-list nor d-list
+   *   (c, 0): in c-list but not in d-list
+   *   (0, d): in d-list but not in c-list
+   *   (c, d): in both c-list and d-list
+   *
+   * For each case below, ( , ) at the end shows the result state of the inode.
+   *
+   * Case 1. Suppose the inode i is NOT in the previous snapshot.        (0, 0)
+   *   1.1. create i in current: add it to c-list                        (c, 0)
+   *   1.1.1. create i in current and then create: impossible
+   *   1.1.2. create i in current and then delete: remove it from c-list (0, 0)
+   *   1.1.3. create i in current and then modify: replace it in c-list  (c, 0)
+   *
+   *   1.2. delete i from current: impossible
+   *
+   *   1.3. modify i in current: impossible
+   *
+   * Case 2. Suppose the inode i is ALREADY in the previous snapshot.    (0, 0)
+   *   2.1. create i in current: impossible
+   *
+   *   2.2. delete i from current: add it to d-list                      (0, d)
+   *   2.2.1. delete i from current and then create: add it to c-list    (c, d)
+   *   2.2.2. delete i from current and then delete: impossible
+   *   2.2.2. delete i from current and then modify: impossible
+   *
+   *   2.3. modify i in current: put it in both c-list and d-list        (c, d)
+   *   2.3.1. modify i in current and then create: impossible
+   *   2.3.2. modify i in current and then delete: remove it from c-list (0, d)
+   *   2.3.3. modify i in current and then modify: replace it in c-list  (c, d)
+   */
+  static class Diff {
+    /**
+     * Search the inode from the list.
+     * @return -1 if the list is null; otherwise, return the insertion point
+     *    defined in {@link Collections#binarySearch(List, Object)}.
+     *    Note that, when the list is null, -1 is the correct insertion point.
+     */
+    static int search(final List<INode> inodes, final INode i) {
+      return search(inodes, i.getLocalNameBytes());
+    }
+    private static int search(final List<INode> inodes, final byte[] name) {
+      return inodes == null? -1: Collections.binarySearch(inodes, name);
+    }
+
+    /** The ID (e.g. snapshot ID) of this object. */
+    final int id;
+    /** c-list: inode(s) created in current. */
+    private List<INode> created;
+    /** d-list: inode(s) deleted from current. */
+    private List<INode> deleted;
+
+    Diff(int id) {
+      this.id = id;
+    }
+
+    /**
+     * Insert the inode to created.
+     * @param i the insertion point defined
+     *          in {@link Collections#binarySearch(List, Object)}
+     */
+    private void insertCreated(final INode inode, final int i) {
+      if (i >= 0) {
+        throw new AssertionError("Inode already exists: inode=" + inode
+            + ", created=" + created);
+      }
+      if (created == null) {
+        created = new ArrayList<INode>(DEFAULT_FILES_PER_DIRECTORY);
+      }
+      created.add(-i - 1, inode);
+    }
+
+    /**
+     * Insert the inode to deleted.
+     * @param i the insertion point defined
+     *          in {@link Collections#binarySearch(List, Object)}
+     */
+    private void insertDeleted(final INode inode, final int i) {
+      if (i >= 0) {
+        throw new AssertionError("Inode already exists: inode=" + inode
+            + ", deleted=" + deleted);
+      }
+      if (deleted == null) {
+        deleted = new ArrayList<INode>(DEFAULT_FILES_PER_DIRECTORY);
+      }
+      deleted.add(-i - 1, inode);
+    }
+
+    /** Create an inode in current state. */
+    void create(final INode inode) {
+      final int c = search(created, inode);
+      insertCreated(inode, c);
+    }
+
+    /** Delete an inode from current state. */
+    void delete(final INode inode) {
+      final int c = search(created, inode);
+      if (c >= 0) {
+        // remove a newly created inode
+        created.remove(c);
+      } else {
+        // not in c-list, it must be in previous
+        final int d = search(deleted, inode);
+        insertDeleted(inode, d);
+      }
+    }
+
+    /** Modify an inode in current state. */
+    void modify(final INode oldinode, final INode newinode) {
+      if (!oldinode.equals(newinode)) {
+        throw new AssertionError("The names do not match: oldinode="
+            + oldinode + ", newinode=" + newinode);
+      }
+      final int c = search(created, newinode);
+      if (c >= 0) {
+        // inode is already in c-list,
+        created.set(c, newinode);
+      } else {
+        final int d = search(deleted, oldinode);
+        if (d < 0) {
+          // neither in c-list nor d-list
+          insertCreated(newinode, c);
+          insertDeleted(oldinode, d);
+        }
+      }
+    }
+
+    /**
+     * Given an inode in current state, find the corresponding inode in previous
+     * snapshot. The inodes in current state and previous snapshot can possibly
+     * be the same.
+     *
+     * @param inodeInCurrent The inode, possibly null, in current state.
+     * @return null if the inode is not found in previous snapshot;
+     *         otherwise, return the corresponding inode in previous snapshot.
+     */
+    INode accessPrevious(byte[] name, INode inodeInCurrent) {
+      return accessPrevious(name, inodeInCurrent, created, deleted);
+    }
+
+    private static INode accessPrevious(byte[] name, INode inodeInCurrent,
+        final List<INode> clist, final List<INode> dlist) {
+      final int d = search(dlist, name);
+      if (d >= 0) {
+        // the inode was in previous and was once deleted in current.
+        return dlist.get(d);
+      } else {
+        final int c = search(clist, name);
+        // When c >= 0, the inode in current is a newly created inode.
+        return c >= 0? null: inodeInCurrent;
+      }
+    }
+
+    /**
+     * Given an inode in previous snapshot, find the corresponding inode in
+     * current state. The inodes in current state and previous snapshot can
+     * possibly be the same.
+     *
+     * @param inodeInPrevious The inode, possibly null, in previous snapshot.
+     * @return null if the inode is not found in current state;
+     *         otherwise, return the corresponding inode in current state.
+     */
+    INode accessCurrent(byte[] name, INode inodeInPrevious) {
+      return accessPrevious(name, inodeInPrevious, deleted, created);
+    }
+
+    /**
+     * Apply this diff to previous snapshot in order to obtain current state.
+     * @return the current state of the list.
+     */
+    List<INode> apply2Previous(final List<INode> previous) {
+      return apply2Previous(previous, created, deleted);
+    }
+
+    private static List<INode> apply2Previous(final List<INode> previous,
+        final List<INode> clist, final List<INode> dlist) {
+      final List<INode> current = new ArrayList<INode>(previous);
+      if (dlist != null) {
+        for(INode d : dlist) {
+          current.remove(d);
+        }
+      }
+      if (clist != null) {
+        for(INode c : clist) {
+          final int i = search(current, c);
+          current.add(-i - 1, c);
+        }
+      }
+      return current;
+    }
+
+    /**
+     * Apply the reverse of this diff to current state in order
+     * to obtain the previous snapshot.
+     * @return the previous state of the list.
+     */
+    List<INode> apply2Current(final List<INode> current) {
+      return apply2Previous(current, deleted, created);
+    }
+
+    /** Convert the inode list to a compact string. */
+    static String toString(List<INode> inodes) {
+      if (inodes == null) {
+        return null;
+      } else if (inodes.isEmpty()) {
+        return "[]";
+      }
+      final StringBuilder b = new StringBuilder("[")
+          .append(inodes.get(0).getLocalName());
+      for(int i = 1; i < inodes.size(); i++) {
+        b.append(", ").append(inodes.get(i).getLocalName());
+      }
+      return b.append("]").toString();
+    }
+
+    @Override
+    public String toString() {
+      return getClass().getSimpleName() + "_" + id
+          + ":\n  created=" + toString(created)
+          + "\n  deleted=" + toString(deleted);
+    }
+  }
+
+  INodeDirectoryWithSnapshot(String name, INodeDirectory dir) {
+    super(name, dir.getPermissionStatus());
+    parent = dir;
+  }
+}

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeFileWithLink.java

@@ -37,7 +37,6 @@ public class INodeFileWithLink extends INodeFile {
 
   public INodeFileWithLink(INodeFile f) {
     super(f);
-    setLocalName(f.getLocalName());
     next = this;
   }
 

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/SnapshotManager.java

@@ -104,7 +104,7 @@ public class SnapshotManager implements SnapshotStats {
     }
     
     void run(final String name) throws IOException {
-      final INodeDirectorySnapshotRoot root = srcRoot.addSnapshotRoot(name);
+      final INodeDirectoryWithSnapshot root = srcRoot.addSnapshotRoot(name);
       processRecursively(srcRoot, root);
     }
 

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/TestSnapshotPathINodes.java

@@ -28,7 +28,7 @@ import org.apache.hadoop.hdfs.DFSTestUtil;
 import org.apache.hadoop.hdfs.DistributedFileSystem;
 import org.apache.hadoop.hdfs.MiniDFSCluster;
 import org.apache.hadoop.hdfs.server.namenode.INodeDirectory.INodesInPath;
-import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectorySnapshotRoot;
+import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectoryWithSnapshot;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectorySnapshottable;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeFileSnapshot;
 import org.junit.After;
@@ -163,13 +163,13 @@ public class TestSnapshotPathINodes {
     // SnapshotRootIndex should be 3: {root, Testsnapshot, sub1, s1, file1}
     assertEquals(nodesInPath.getSnapshotRootIndex(), 3);
     assertTrue(inodes[nodesInPath.getSnapshotRootIndex()] instanceof 
-        INodeDirectorySnapshotRoot);
+        INodeDirectoryWithSnapshot);
     // Check the INode for file1 (snapshot file)
     INode snapshotFileNode = inodes[inodes.length - 1]; 
     assertEquals(snapshotFileNode.getLocalName(), file1.getName());
     assertTrue(snapshotFileNode instanceof INodeFileSnapshot);
     assertTrue(snapshotFileNode.getParent() instanceof 
-        INodeDirectorySnapshotRoot);
+        INodeDirectoryWithSnapshot);
     
     // Call getExistingPathINodes and request only one INode.
     nodesInPath = fsdir.rootDir.getExistingPathINodes(components, 1, false);
@@ -266,13 +266,13 @@ public class TestSnapshotPathINodes {
     // SnapshotRootIndex should be 3: {root, Testsnapshot, sub1, s1, file1}
     assertEquals(nodesInPath.getSnapshotRootIndex(), 3);
     assertTrue(inodes[nodesInPath.getSnapshotRootIndex()] instanceof 
-        INodeDirectorySnapshotRoot);
+        INodeDirectoryWithSnapshot);
     // Check the INode for file1 (snapshot file)
     INode snapshotFileNode = inodes[inodes.length - 1]; 
     assertEquals(snapshotFileNode.getLocalName(), file1.getName());
     assertTrue(snapshotFileNode instanceof INodeFileSnapshot);
     assertTrue(snapshotFileNode.getParent() instanceof 
-        INodeDirectorySnapshotRoot);
+        INodeDirectoryWithSnapshot);
   }
   
   /** 
@@ -326,11 +326,11 @@ public class TestSnapshotPathINodes {
     // SnapshotRootIndex should still be 3: {root, Testsnapshot, sub1, s1, null}
     assertEquals(nodesInPath.getSnapshotRootIndex(), 3);
     assertTrue(inodes[nodesInPath.getSnapshotRootIndex()] instanceof 
-        INodeDirectorySnapshotRoot);
+        INodeDirectoryWithSnapshot);
     // Check the last INode in inodes, which should be null
     assertNull(inodes[inodes.length - 1]);
     assertTrue(inodes[inodes.length - 2] instanceof 
-        INodeDirectorySnapshotRoot);
+        INodeDirectoryWithSnapshot);
   }
   
   /** 

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/snapshot/TestINodeDirectoryWithSnapshot.java

@@ -0,0 +1,231 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hdfs.server.namenode.snapshot;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+
+import org.apache.hadoop.fs.permission.FsPermission;
+import org.apache.hadoop.fs.permission.PermissionStatus;
+import org.apache.hadoop.hdfs.server.namenode.INode;
+import org.apache.hadoop.hdfs.server.namenode.INodeDirectory;
+import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectoryWithSnapshot.Diff;
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Test {@link INodeDirectoryWithSnapshot}, especially, {@link Diff}.
+ */
+public class TestINodeDirectoryWithSnapshot {
+  private static Random RANDOM = new Random();
+  private static final PermissionStatus PERM = PermissionStatus.createImmutable(
+      "user", "group", FsPermission.createImmutable((short)0));
+
+  static int nextStep(int n) {
+    return n == 0? 1: 10*n;
+  }
+
+  /** Test directory diff. */
+  @Test
+  public void testDiff() throws Exception {
+    for(int startSize = 0; startSize <= 1000; startSize = nextStep(startSize)) {
+      for(int m = 0; m <= 10000; m = nextStep(m)) {
+        runDiffTest(startSize, m, true);
+      }
+    }
+  }
+
+  /**
+   * The following are the step of the diff test:
+   * 1) Initialize the previous list and add s elements to it,
+   *    where s = startSize.
+   * 2) Initialize the current list by coping all elements from the previous list
+   * 3) Initialize an empty diff object.
+   * 4) Make m modifications to the current list, where m = numModifications.
+   *    Record the modifications in diff at the same time.
+   * 5) Test if current == previous + diff and previous == current - diff.
+   * 6) Test accessPrevious and accessCurrent.
+   *
+   * @param startSize
+   * @param numModifications
+   * @param computeDiff
+   */
+  void runDiffTest(int startSize, int numModifications, boolean computeDiff) {
+    final int width = findWidth(startSize + numModifications);
+    System.out.println("\nstartSize=" + startSize
+        + ", numModifications=" + numModifications
+        + ", width=" + width);
+
+    // initialize previous
+    final List<INode> previous = new ArrayList<INode>();
+    int n = 0;
+    for(; n < startSize; n++) {
+      previous.add(newINode(n, width));
+    }
+
+    // make modifications to current and record the diff
+    final List<INode> current = new ArrayList<INode>(previous);
+    final Diff diff = computeDiff? new Diff(0): null;
+
+    for(int m = 0; m < numModifications; m++) {
+      // if current is empty, the next operation must be create;
+      // otherwise, randomly pick an operation.
+      final int nextOperation = current.isEmpty()? 1: RANDOM.nextInt(3) + 1;
+      switch(nextOperation) {
+      case 1: // create
+      {
+        final INode i = newINode(n++, width);
+        create(i, current, diff);
+        break;
+      }
+      case 2: // delete
+      {
+        final INode i = current.get(RANDOM.nextInt(current.size()));
+        delete(i, current, diff);
+        break;
+      }
+      case 3: // modify
+      {
+        final INode i = current.get(RANDOM.nextInt(current.size()));
+        modify(i, current, diff);
+        break;
+      }
+      }
+    }
+
+    if (computeDiff) {
+      // check if current == previous + diff
+      final List<INode> c = diff.apply2Previous(previous);
+      if (!hasIdenticalElements(current, c)) {
+        System.out.println("previous = " + Diff.toString(previous));
+        System.out.println();
+        System.out.println("current  = " + Diff.toString(current));
+        System.out.println("c        = " + Diff.toString(c));
+        System.out.println();
+        System.out.println("diff     = " + diff);
+        throw new AssertionError("current and c are not identical.");
+      }
+
+      // check if previous == current - diff
+      final List<INode> p = diff.apply2Current(current);
+      if (!hasIdenticalElements(previous, p)) {
+        System.out.println("previous = " + Diff.toString(previous));
+        System.out.println("p        = " + Diff.toString(p));
+        System.out.println();
+        System.out.println("current  = " + Diff.toString(current));
+        System.out.println();
+        System.out.println("diff     = " + diff);
+        throw new AssertionError("previous and p are not identical.");
+      }
+    }
+
+    if (computeDiff) {
+      for(int m = 0; m < n; m++) {
+        final INode inode = newINode(m, width);
+        {// test accessPrevious
+          final int i = Diff.search(current, inode);
+          final INode inodeInCurrent = i < 0? null: current.get(i);
+          final INode computed = diff.accessPrevious(
+              inode.getLocalNameBytes(), inodeInCurrent);
+
+          final int j = Diff.search(previous, inode);
+          final INode expected = j < 0? null: previous.get(j);
+          // must be the same object (equals is not enough)
+          Assert.assertTrue(computed == expected);
+        }
+
+        {// test accessCurrent
+          final int i = Diff.search(previous, inode);
+          final INode inodeInPrevious = i < 0? null: previous.get(i);
+          final INode computed = diff.accessCurrent(
+              inode.getLocalNameBytes(), inodeInPrevious);
+
+          final int j = Diff.search(current, inode);
+          final INode expected = j < 0? null: current.get(j);
+          // must be the same object (equals is not enough)
+          Assert.assertTrue(computed == expected);
+        }
+      }
+    }
+  }
+
+  static boolean hasIdenticalElements(final List<INode> expected,
+      final List<INode> computed) {
+    if (expected == null) {
+      return computed == null;
+    }
+    if (expected.size() != computed.size()) {
+      return false;
+    }
+    for(int i = 0; i < expected.size(); i++) {
+      // must be the same object (equals is not enough)
+      if (expected.get(i) != computed.get(i)) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  static String toString(INode inode) {
+    return inode == null? null
+        : inode.getLocalName() + ":" + inode.getModificationTime();
+  }
+
+  static int findWidth(int max) {
+    int w = 1;
+    for(long n = 10; n < max; n *= 10, w++);
+    return w;
+  }
+
+  static INode newINode(int n, int width) {
+    return new INodeDirectory(String.format("n%0" + width + "d", n), PERM);
+  }
+
+  static void create(INode inode, final List<INode> current, Diff diff) {
+    final int i = Diff.search(current, inode);
+    Assert.assertTrue(i < 0);
+    current.add(-i - 1, inode);
+    if (diff != null) {
+      diff.create(inode);
+    }
+  }
+
+  static void delete(INode inode, final List<INode> current, Diff diff) {
+    final int i = Diff.search(current, inode);
+    Assert.assertTrue("i=" + i + ", inode=" + inode + "\ncurrent=" + current,
+        i >= 0);
+    current.remove(i);
+    if (diff != null) {
+      diff.delete(inode);
+    }
+  }
+
+  static void modify(INode inode, final List<INode> current, Diff diff) {
+    final int i = Diff.search(current, inode);
+    Assert.assertTrue(i >= 0);
+    final INodeDirectory oldinode = (INodeDirectory)current.get(i);
+    final INodeDirectory newinode = new INodeDirectory(oldinode);
+    newinode.setModificationTime(oldinode.getModificationTime() + 1);
+
+    current.set(i, newinode);
+    if (diff != null) {
+      diff.modify(oldinode, newinode);
+    }
+  }
+}