trunk/gora-core/src/main/java/org/apache/gora/query/impl/ResultBase.java

/**
 * 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.gora.query.impl;

import java.io.IOException;

import org.apache.gora.persistency.Persistent;
import org.apache.gora.query.Query;
import org.apache.gora.query.Result;
import org.apache.gora.store.DataStore;

/**
 * Base class for {@link Result} implementations.
 */
public abstract class ResultBase<K, T extends Persistent> 
  implements Result<K, T> {

  protected final DataStore<K,T> dataStore;
  
  protected final Query<K, T> query;
  
  protected K key;
  
  protected T persistent;
  
  /** Query limit */
  protected long limit;
  
  /** How far we have proceeded*/
  protected long offset = 0;
  
  public ResultBase(DataStore<K,T> dataStore, Query<K,T> query) {
    this.dataStore = dataStore;
    this.query = query;
    this.limit = query.getLimit();
  }
  
  @Override
  public DataStore<K, T> getDataStore() {
    return dataStore;
  }
  
  @Override
  public Query<K, T> getQuery() {
    return query;
  }
  
  @Override
  public T get() {
    return persistent;
  }
  
  @Override
  public K getKey() {
    return key;
  }
    
  @Override
  public Class<K> getKeyClass() {
    return getDataStore().getKeyClass();
  }
  
  @Override
  public Class<T> getPersistentClass() {
    return getDataStore().getPersistentClass();
  }
  
  /**
   * Returns whether the limit for the query is reached. 
   */
  protected boolean isLimitReached() {
    if(limit > 0 && offset >= limit) {
      return true;
    }
    return false;
  }
  
  protected void clear() {
    if(persistent != null) {
      persistent.clear();
    }
    if(key != null && key instanceof Persistent) {
      ((Persistent)key).clear();
    }
  }
  
  @Override
  public final boolean next() throws IOException {
    if(isLimitReached()) {
      return false;
    }
    
    clear();
    persistent = getOrCreatePersistent(persistent);
    
    boolean ret = nextInner();
    if(ret) ++offset;
    return ret;
  }
  
  @Override
  public long getOffset() {
    return offset;
  }
  
  /**
   * {@link ResultBase#next()} calls this function to read the 
   * actual results. 
   */
  protected abstract boolean nextInner() throws IOException; 
  
  protected T getOrCreatePersistent(T persistent) throws IOException {
    if(persistent != null) {
      return persistent;
    }
    return dataStore.newPersistent();
  }
}