content: /*
   * The Alluxio Open Foundation licenses this work under the Apache License, version 2.0
   * (the "License"). You may not use this work except in compliance with the License, which is
   * available at www.apache.org/licenses/LICENSE-2.0
   *
   * This software is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
   * either express or implied, as more fully set forth in the License.
   *
   * See the NOTICE file distributed with this work for information regarding copyright ownership.
   */
  
  package alluxio.conf;
  
  import org.apache.commons.codec.binary.Hex;
  
  import java.security.MessageDigest;
  import java.security.NoSuchAlgorithmException;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.function.Supplier;
  import java.util.stream.Stream;
  import javax.annotation.concurrent.ThreadSafe;
  
  /**
   * A hex encoded MD5 hash of the cluster or path configurations.
   */
  @ThreadSafe
  public final class Hash {
    private final MessageDigest mMD5;
    private final Supplier<Stream<byte[]>> mProperties;
    private final AtomicBoolean mShouldUpdate;
    private volatile String mVersion;
    private volatile long mLastUpdateTime;
  
    /**
     * @param properties a stream of encoded properties
     */
    public Hash(Supplier<Stream<byte[]>> properties) {
      try {
        mMD5 = MessageDigest.getInstance("MD5");
      } catch (NoSuchAlgorithmException e) {
        throw new RuntimeException(e);
      }
      mProperties = properties;
      mShouldUpdate = new AtomicBoolean(true);
    }
  
    /**
     * Called when the version needs to be updated.
     * Internally, it just sets a flag to indicate that the version should be updated,
     * but version will not be recomputed until {@link #get()} is called.
     */
    public void markOutdated() {
      mShouldUpdate.set(true);
    }
  
    private String compute() {
      mMD5.reset();
      mProperties.get().forEach(property -> mMD5.update(property));
      return Hex.encodeHexString(mMD5.digest());
    }
  
    /**
     * If {@link #markOutdated()} is called since last {@link #get()}, then the version will be
     * recomputed, otherwise, the internally cached version is returned.
     *
     * @return the latest version
     */
    public String get() {
      if (mShouldUpdate.get()) {
        synchronized (this) {
          // If another thread has recomputed the version, no need to recompute again.
          if (mShouldUpdate.get()) {
            mVersion = compute();
            mLastUpdateTime = System.currentTimeMillis();
            mShouldUpdate.set(false);
          }
        }
      }
      return mVersion;
    }
  
    /**
     * @return the latest update time
     */
    public synchronized long getLastUpdateTime() {
      return mLastUpdateTime;
    }
  }
  