/*
 * 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.hudi.common.util.collection;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Iterator flattening source {@link Iterator} holding other {@link Iterator}s
 */
public final class FlatteningIterator<T, I extends Iterator<T>> implements Iterator<T> {

  private final Iterator<I> sourceIterator;
  private Iterator<T> innerSourceIterator;

  public FlatteningIterator(Iterator<I> source) {
    this.sourceIterator = source;
  }

  public boolean hasNext() {
    while (innerSourceIterator == null || !innerSourceIterator.hasNext()) {
      if (sourceIterator.hasNext()) {
        innerSourceIterator = sourceIterator.next();
      } else {
        return false;
      }
    }

    return true;
  }

  @Override
  public T next() {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }

    return innerSourceIterator.next();
  }
}
