content: /*
   * 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.calcite.adapter.kafka;
  
  import org.apache.calcite.linq4j.Enumerator;
  
  import org.apache.kafka.clients.consumer.Consumer;
  import org.apache.kafka.clients.consumer.ConsumerConfig;
  import org.apache.kafka.clients.consumer.ConsumerRecord;
  import org.apache.kafka.clients.consumer.ConsumerRecords;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.time.Duration;
  import java.util.ArrayDeque;
  import java.util.Deque;
  import java.util.concurrent.atomic.AtomicBoolean;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Enumerator to read data from {@link Consumer},
   * and converted into SQL rows with {@link KafkaRowConverter}.
   *
   * @param <K> Type for Kafka message key,
   *           refer to {@link ConsumerConfig#KEY_DESERIALIZER_CLASS_CONFIG};
   * @param <V> Type for Kafka message value,
   *           refer to {@link ConsumerConfig#VALUE_DESERIALIZER_CLASS_CONFIG};
   */
  public class KafkaMessageEnumerator<K, V> implements Enumerator<@Nullable Object[]> {
    final Consumer consumer;
    final KafkaRowConverter<K, V> rowConverter;
    private final AtomicBoolean cancelFlag;
  
    // runtime
    private final Deque<ConsumerRecord<K, V>> bufferedRecords = new ArrayDeque<>();
    private @Nullable ConsumerRecord<K, V> curRecord;
  
    KafkaMessageEnumerator(final Consumer consumer,
        final KafkaRowConverter<K, V> rowConverter,
        final AtomicBoolean cancelFlag) {
      this.consumer = consumer;
      this.rowConverter = rowConverter;
      this.cancelFlag = cancelFlag;
    }
  
    /**
     * It returns an Array of Object, with each element represents a field of row.
     */
    @Override public Object[] current() {
      return rowConverter.toRow(requireNonNull(curRecord, "curRecord"));
    }
  
    @Override public boolean moveNext() {
      if (cancelFlag.get()) {
        return false;
      }
  
      while (bufferedRecords.isEmpty()) {
        pullRecords();
      }
  
      curRecord = bufferedRecords.removeFirst();
      return true;
    }
  
    private void pullRecords() {
      ConsumerRecords<K, V> records = consumer.poll(Duration.ofMillis(100));
      for (ConsumerRecord record : records) {
        bufferedRecords.add(record);
      }
    }
  
    @Override public void reset() {
      this.bufferedRecords.clear();
      pullRecords();
    }
  
    @Override public void close() {
      consumer.close();
    }
  }
  