/**
    *
    * 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.hbase.monitoring;
  
  import java.io.PrintWriter;
  import java.text.SimpleDateFormat;
  import java.util.Date;
  import java.util.LinkedList;
  import java.util.List;
  
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  
  import com.google.common.base.Charsets;
  import com.google.common.base.Preconditions;
  import com.google.common.collect.Lists;
  
  /**
   * A size-bounded repository of alerts, which are kept
   * in a linked list. Alerts can be added, and they will
   * automatically be removed one by one when the specified heap
   * usage is exhausted.
   */
  @InterfaceAudience.Private
  public class MemoryBoundedLogMessageBuffer {
    private final long maxSizeBytes;
    private long usage = 0;
    private LinkedList<LogMessage> messages;
    
    public MemoryBoundedLogMessageBuffer(long maxSizeBytes) {
      Preconditions.checkArgument(
          maxSizeBytes > 0);
      this.maxSizeBytes = maxSizeBytes;
      this.messages = Lists.newLinkedList();
    }
    
    /**
     * Append the given message to this buffer, automatically evicting
     * older messages until the desired memory limit is achieved.
     */
    public synchronized void add(String messageText) {
      LogMessage message = new LogMessage(messageText, System.currentTimeMillis());
      
      usage += message.estimateHeapUsage();
      messages.add(message);
      while (usage > maxSizeBytes) {
        LogMessage removed = messages.remove();
        usage -= removed.estimateHeapUsage();
        assert usage >= 0;
      }
    }
    
    /**
     * Dump the contents of the buffer to the given stream.
     */
    public synchronized void dumpTo(PrintWriter out) {
      SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
  
      for (LogMessage msg : messages) {
        out.write(df.format(new Date(msg.timestamp)));
        out.write(" ");
        out.println(new String(msg.message, Charsets.UTF_8));
      }
    }
    
    synchronized List<LogMessage> getMessages() {
      // defensive copy
      return Lists.newArrayList(messages);
    }
   
    /**
     * Estimate the number of bytes this buffer is currently
     * using.
     */
    synchronized long estimateHeapUsage() {
      return usage;
    }
    
    private static class LogMessage {
      /** the error text, encoded in bytes to save memory */
      public final byte[] message;
      public final long timestamp;
      
      /**
      * Completely non-scientific estimate of how much one of these
      * objects takes, along with the LinkedList overhead. This doesn't
      * need to be exact, since we don't expect a ton of these alerts.
      */
     private static final long BASE_USAGE=100;
     
     public LogMessage(String message, long timestamp) {
       this.message = message.getBytes(Charsets.UTF_8);
       this.timestamp = timestamp;
     }
     
     public long estimateHeapUsage() {
       return message.length + BASE_USAGE;
     }
   }
 
 }