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.linq4j;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.concurrent.atomic.AtomicInteger;
  
  /**
   * Enumerator that keeps some recent and some "future" values.
   *
   * @param <E> Row value
   */
  public class MemoryEnumerator<@Nullable E> implements Enumerator<MemoryFactory.Memory<E>> {
    private final Enumerator<E> enumerator;
    private final MemoryFactory<E> memoryFactory;
    private final AtomicInteger prevCounter;
    private final AtomicInteger postCounter;
  
    /**
     * Creates a MemoryEnumerator.
     *
     * <p>Use factory method {@link MemoryEnumerable#enumerator()}.
     *
     * @param enumerator The Enumerator that memory should be "wrapped" around
     * @param history Number of present steps to remember
     * @param future Number of future steps to "remember"
     */
    MemoryEnumerator(Enumerator<E> enumerator, int history, int future) {
      this.enumerator = enumerator;
      this.memoryFactory = new MemoryFactory<>(history, future);
      this.prevCounter = new AtomicInteger(future);
      this.postCounter = new AtomicInteger(future);
    }
  
    @Override public MemoryFactory.Memory<E> current() {
      return memoryFactory.create();
    }
  
    @Override public boolean moveNext() {
      if (prevCounter.get() > 0) {
        boolean lastMove = false;
        while (prevCounter.getAndDecrement() >= 0) {
          lastMove = moveNextInternal();
        }
        return lastMove;
      } else {
        return moveNextInternal();
      }
    }
  
    private boolean moveNextInternal() {
      final boolean moveNext = enumerator.moveNext();
      if (moveNext) {
        memoryFactory.add(enumerator.current());
        return true;
      } else {
        // Check if we have to add "history" additional values
        if (postCounter.getAndDecrement() > 0) {
          memoryFactory.add(null);
          return true;
        }
      }
      return false;
    }
  
    @Override public void reset() {
      enumerator.reset();
    }
  
    @Override public void close() {
      enumerator.close();
    }
  }
  