HDFS-4346. Add a file missed in r1471598

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1471602 13f79535-47bb-0310-9956-ffa450edef68

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/SequentialNumber.java

@@ -0,0 +1,83 @@
+/**
+ * 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.util;
+
+import java.util.concurrent.atomic.AtomicLong;
+
+import org.apache.hadoop.classification.InterfaceAudience;
+
+/**
+ * Sequential number generator.
+ * 
+ * This class is thread safe.
+ */
+@InterfaceAudience.Private
+public abstract class SequentialNumber {
+  private final AtomicLong currentValue;
+
+  /** Create a new instance with the given initial value. */
+  protected SequentialNumber(final long initialValue) {
+    currentValue = new AtomicLong(initialValue);
+  }
+
+  /** @return the current value. */
+  public long getCurrentValue() {
+    return currentValue.get();
+  }
+
+  /** Set current value. */
+  public void setCurrentValue(long value) {
+    currentValue.set(value);
+  }
+
+  /** Increment and then return the next value. */
+  public long nextValue() {
+    return currentValue.incrementAndGet();
+  }
+
+  /** Skip to the new value. */
+  public void skipTo(long newValue) throws IllegalStateException {
+    for(;;) {
+      final long c = getCurrentValue();
+      if (newValue < c) {
+        throw new IllegalStateException(
+            "Cannot skip to less than the current value (="
+            + c + "), where newValue=" + newValue);
+      }
+
+      if (currentValue.compareAndSet(c, newValue)) {
+        return;
+      }
+    }
+  }
+
+  @Override
+  public boolean equals(final Object that) {
+    if (that == null || this.getClass() != that.getClass()) {
+      return false;
+    }
+    final AtomicLong thatValue = ((SequentialNumber)that).currentValue;
+    return currentValue.equals(thatValue);
+  }
+
+  @Override
+  public int hashCode() {
+    final long v = currentValue.get();
+    return (int)v ^ (int)(v >>> 32);
+  }
+}