/*
    *    Copyright 2009-2022 the original author or authors.
    *
    *    Licensed 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.ibatis.cache.decorators;
  
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.TimeUnit;
  
  import org.apache.ibatis.cache.Cache;
  import org.apache.ibatis.cache.CacheException;
  
  /**
   * <p>Simple blocking decorator
   *
   * <p>Simple and inefficient version of EhCache's BlockingCache decorator.
   * It sets a lock over a cache key when the element is not found in cache.
   * This way, other threads will wait until this element is filled instead of hitting the database.
   *
   * <p>By its nature, this implementation can cause deadlock when used incorrectly.
   *
   * @author Eduardo Macarron
   *
   */
  public class BlockingCache implements Cache {
  
    private long timeout;
    private final Cache delegate;
    private final ConcurrentHashMap<Object, CountDownLatch> locks;
  
    public BlockingCache(Cache delegate) {
      this.delegate = delegate;
      this.locks = new ConcurrentHashMap<>();
    }
  
    @Override
    public String getId() {
      return delegate.getId();
    }
  
    @Override
    public int getSize() {
      return delegate.getSize();
    }
  
    @Override
    public void putObject(Object key, Object value) {
      try {
        delegate.putObject(key, value);
      } finally {
        releaseLock(key);
      }
    }
  
    @Override
    public Object getObject(Object key) {
      acquireLock(key);
      Object value = delegate.getObject(key);
      if (value != null) {
        releaseLock(key);
      }
      return value;
    }
  
    @Override
    public Object removeObject(Object key) {
      // despite its name, this method is called only to release locks
      releaseLock(key);
      return null;
    }
  
    @Override
    public void clear() {
      delegate.clear();
    }
  
    private void acquireLock(Object key) {
      CountDownLatch newLatch = new CountDownLatch(1);
      while (true) {
        CountDownLatch latch = locks.putIfAbsent(key, newLatch);
        if (latch == null) {
          break;
        }
        try {
          if (timeout > 0) {
            boolean acquired = latch.await(timeout, TimeUnit.MILLISECONDS);
            if (!acquired) {
             throw new CacheException(
                 "Couldn't get a lock in " + timeout + " for the key " + key + " at the cache " + delegate.getId());
           }
         } else {
           latch.await();
         }
       } catch (InterruptedException e) {
         throw new CacheException("Got interrupted while trying to acquire lock for key " + key, e);
       }
     }
   }
 
   private void releaseLock(Object key) {
     CountDownLatch latch = locks.remove(key);
     if (latch == null) {
       throw new IllegalStateException("Detected an attempt at releasing unacquired lock. This should never happen.");
     }
     latch.countDown();
   }
 
   public long getTimeout() {
     return timeout;
   }
 
   public void setTimeout(long timeout) {
     this.timeout = timeout;
   }
 }