/**
 * 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.hive.ql.exec.spark;

import java.io.IOException;
import java.io.Serializable;
import java.util.Iterator;
import java.util.NoSuchElementException;

import org.apache.hadoop.hive.ql.io.HiveKey;
import org.apache.hadoop.io.BytesWritable;
import org.apache.hadoop.mapred.OutputCollector;

import scala.Tuple2;

import com.google.common.base.Preconditions;

/**
 * Base class for
 *   - collecting Map/Reduce function output and
 *   - providing an Iterable interface for fetching output records. Input records
 *     are processed in lazy fashion i.e when output records are requested
 *     through Iterator interface.
 */
@SuppressWarnings("rawtypes")
public abstract class HiveBaseFunctionResultList<T>
  implements Iterator, OutputCollector<HiveKey, BytesWritable>, Serializable {
  private static final long serialVersionUID = -1L;
  private final Iterator<T> inputIterator;
  private boolean isClosed = false;

  // Contains results from last processed input record.
  private final HiveKVResultCache lastRecordOutput;

  public HiveBaseFunctionResultList(Iterator<T> inputIterator) {
    this.inputIterator = inputIterator;
    this.lastRecordOutput = new HiveKVResultCache();
  }

  @Override
  public void collect(HiveKey key, BytesWritable value) throws IOException {
    lastRecordOutput.add(SparkUtilities.copyHiveKey(key),
        SparkUtilities.copyBytesWritable(value));
  }

  /** Process the given record. */
  protected abstract void processNextRecord(T inputRecord) throws IOException;

  /**
   * @return true if current state of the record processor is done.
   */
  protected abstract boolean processingDone();

  /** Close the record processor. */
  protected abstract void closeRecordProcessor();

  @Override
  public boolean hasNext() {
    // Return remaining records (if any) from last processed input record.
    if (lastRecordOutput.hasNext()) {
      return true;
    }

    // Process the records in the input iterator until
    //  - new output records are available for serving downstream operator,
    //  - input records are exhausted or
    //  - processing is completed.
    while (inputIterator.hasNext() && !processingDone()) {
      try {
        processNextRecord(inputIterator.next());
        if (lastRecordOutput.hasNext()) {
          return true;
        }
      } catch (IOException ex) {
        throw new IllegalStateException("Error while processing input.", ex);
      }
    }

    // At this point we are done processing the input. Close the record processor
    if (!isClosed) {
      closeRecordProcessor();
      isClosed = true;
    }

    // It is possible that some operators add records after closing the processor, so make sure
    // to check the lastRecordOutput
    if (lastRecordOutput.hasNext()) {
      return true;
    }

    lastRecordOutput.clear();
    return false;
  }

  @Override
  public Tuple2<HiveKey, BytesWritable> next() {
    if (hasNext()) {
      return lastRecordOutput.next();
    }
    throw new NoSuchElementException("There are no more elements");
  }

  @Override
  public void remove() {
    throw new UnsupportedOperationException("Iterator.remove() is not supported");
  }

}