package com.trendmicro.grid.acl.ds.cache.commands;
   
   import com.trendmicro.grid.acl.ds.cache.CacheSettings;
   import com.trendmicro.grid.acl.ds.cache.CacheSource;
   
   import java.util.Collection;
   import java.util.Iterator;
   import java.util.Map;
   
  /**
   * Retains only the keys that do not exist inside the cache.
   * <p/>
   * The purpose of this command is to differentiate between negative cache and un-cached values.
   * Disabling the command disables negative caching.
   *
   * @author juergen_kellerer, 2011-03-24
   * @version 1.0
   */
  public class RetainMissingKeysCommand<K, V> extends AbstractCommand<K, V> {
  
  	private CacheSource<K, V> cache;
  
  	/**
  	 * Creates the command.
  	 *
  	 * @param keys  the keys to check.
  	 * @param cache the cache source to check the keys against.
  	 */
  	public RetainMissingKeysCommand(Collection<K> keys, CacheSource<K, V> cache) {
  		super(keys);
  		this.cache = cache;
  	}
  
  	/**
  	 * Retains only the keys that do not exist inside the cache.
  	 * <p/>
  	 * Use this to reduce the key set by all keys that are negatively cached with a null value.
  	 * <p/>
  	 * A typical implementation is:<code><pre>
  	 * for (Iterator i = keys.iterator(); i.hasNext();)
  	 *   if (cached.containsKey(i.next()))
  	 *     i.remove();
  	 * </pre></code>
  	 *
  	 * @param keys the keys to filter.
  	 */
  	protected void retainMissingKeys(Collection<K> keys) {
  		for (Iterator<K> iterator = keys.iterator(); iterator.hasNext();) {
  			if (cache.containsKey(iterator.next())) iterator.remove();
  		}
  	}
  
  	/**
  	 * {@inheritDoc}
  	 */
  	@Override
  	public Map<K, V> call() throws Exception {
  		if (!CacheSettings.NO_NEGATIVE_CACHING) retainMissingKeys(keys);
  
  		return keysToCallResult();
  	}
  }