/*
    *    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.lang.ref.ReferenceQueue;
  import java.lang.ref.WeakReference;
  import java.util.Deque;
  import java.util.LinkedList;
  
  import org.apache.ibatis.cache.Cache;
  
  /**
   * Weak Reference cache decorator.
   * Thanks to Dr. Heinz Kabutz for his guidance here.
   *
   * @author Clinton Begin
   */
  public class WeakCache implements Cache {
    private final Deque<Object> hardLinksToAvoidGarbageCollection;
    private final ReferenceQueue<Object> queueOfGarbageCollectedEntries;
    private final Cache delegate;
    private int numberOfHardLinks;
  
    public WeakCache(Cache delegate) {
      this.delegate = delegate;
      this.numberOfHardLinks = 256;
      this.hardLinksToAvoidGarbageCollection = new LinkedList<>();
      this.queueOfGarbageCollectedEntries = new ReferenceQueue<>();
    }
  
    @Override
    public String getId() {
      return delegate.getId();
    }
  
    @Override
    public int getSize() {
      removeGarbageCollectedItems();
      return delegate.getSize();
    }
  
    public void setSize(int size) {
      this.numberOfHardLinks = size;
    }
  
    @Override
    public void putObject(Object key, Object value) {
      removeGarbageCollectedItems();
      delegate.putObject(key, new WeakEntry(key, value, queueOfGarbageCollectedEntries));
    }
  
    @Override
    public Object getObject(Object key) {
      Object result = null;
      @SuppressWarnings("unchecked") // assumed delegate cache is totally managed by this cache
      WeakReference<Object> weakReference = (WeakReference<Object>) delegate.getObject(key);
      if (weakReference != null) {
        result = weakReference.get();
        if (result == null) {
          delegate.removeObject(key);
        } else {
          synchronized (hardLinksToAvoidGarbageCollection) {
            hardLinksToAvoidGarbageCollection.addFirst(result);
            if (hardLinksToAvoidGarbageCollection.size() > numberOfHardLinks) {
              hardLinksToAvoidGarbageCollection.removeLast();
            }
          }
        }
      }
      return result;
    }
  
    @Override
    public Object removeObject(Object key) {
      removeGarbageCollectedItems();
      @SuppressWarnings("unchecked")
      WeakReference<Object> weakReference = (WeakReference<Object>) delegate.removeObject(key);
      return weakReference == null ? null : weakReference.get();
    }
  
    @Override
    public void clear() {
      synchronized (hardLinksToAvoidGarbageCollection) {
        hardLinksToAvoidGarbageCollection.clear();
      }
      removeGarbageCollectedItems();
     delegate.clear();
   }
 
   private void removeGarbageCollectedItems() {
     WeakEntry sv;
     while ((sv = (WeakEntry) queueOfGarbageCollectedEntries.poll()) != null) {
       delegate.removeObject(sv.key);
     }
   }
 
   private static class WeakEntry extends WeakReference<Object> {
     private final Object key;
 
     private WeakEntry(Object key, Object value, ReferenceQueue<Object> garbageCollectionQueue) {
       super(value, garbageCollectionQueue);
       this.key = key;
     }
   }
 
 }