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.beam.sdk.fn.stream;
  
  import java.util.Arrays;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  
  public class PrefetchableIterators {
  
    private static final PrefetchableIterator<Object> EMPTY_ITERATOR =
        new PrefetchableIterator<Object>() {
          @Override
          public boolean isReady() {
            return true;
          }
  
          @Override
          public void prefetch() {}
  
          @Override
          public boolean hasNext() {
            return false;
          }
  
          @Override
          public Object next() {
            throw new NoSuchElementException();
          }
        };
  
    /** Returns an empty {@link PrefetchableIterator}. */
    public static <T> PrefetchableIterator<T> emptyIterator() {
      return (PrefetchableIterator<T>) EMPTY_ITERATOR;
    }
  
    /**
     * Returns a {@link PrefetchableIterator} over the specified values.
     *
     * <p>{@link PrefetchableIterator#prefetch()} is a no-op and {@link
     * PrefetchableIterator#isReady()} always returns true.
     */
    public static <T> PrefetchableIterator<T> fromArray(T... values) {
      if (values.length == 0) {
        return emptyIterator();
      }
      return new PrefetchableIterator<T>() {
        int currentIndex;
  
        @Override
        public boolean isReady() {
          return true;
        }
  
        @Override
        public void prefetch() {}
  
        @Override
        public boolean hasNext() {
          return currentIndex < values.length;
        }
  
        @Override
        public T next() {
          if (!hasNext()) {
            throw new NoSuchElementException();
          }
          return values[currentIndex++];
        }
      };
    }
  
    /**
     * If the {@link Iterator} is not a {@link PrefetchableIterator} then one is constructed that
     * ensures {@link PrefetchableIterator#prefetch} is a no-op and {@link
     * PrefetchableIterator#isReady} always returns true.
     */
    // package private for PrefetchableIterables.
    static <T> PrefetchableIterator<T> maybePrefetchable(Iterator<T> iterator) {
      if (iterator == null) {
        throw new IllegalArgumentException("Expected non-null iterator.");
      }
      if (iterator instanceof PrefetchableIterator) {
        return (PrefetchableIterator<T>) iterator;
      }
      return new PrefetchableIterator<T>() {
        @Override
        public boolean isReady() {
          return true;
        }
  
        @Override
        public void prefetch() {}
  
        @Override
        public boolean hasNext() {
          return iterator.hasNext();
        }
  
        @Override
        public T next() {
          return iterator.next();
        }
      };
    }
  
    public static <T> PrefetchableIterator<T> concatIterators(Iterator<Iterator<T>> iterators) {
      if (!iterators.hasNext()) {
        return emptyIterator();
      }
      return new PrefetchableIterator<T>() {
        PrefetchableIterator<T> delegate = maybePrefetchable(iterators.next());
  
        @Override
        public boolean isReady() {
          // Ensure that we advance from iterators that don't have the next
          // element to an iterator that supports prefetch or does have an element
          for (; ; ) {
            // If the delegate isn't ready then we aren't ready.
            // We assume that non prefetchable iterators are always ready.
            if (!delegate.isReady()) {
              return false;
            }
  
            // If the delegate has a next and is ready then we are ready
            if (delegate.hasNext()) {
              return true;
            }
  
            // Otherwise we should advance to the next index since we know this iterator is empty
            // and re-evaluate whether we are ready
            if (!iterators.hasNext()) {
              return true;
            }
            delegate = maybePrefetchable(iterators.next());
          }
        }
  
        @Override
        public void prefetch() {
          if (!isReady()) {
            delegate.prefetch();
          }
        }
  
        @Override
        public boolean hasNext() {
          for (; ; ) {
            if (delegate.hasNext()) {
              return true;
            }
            if (!iterators.hasNext()) {
              return false;
            }
            delegate = maybePrefetchable(iterators.next());
          }
        }
  
        @Override
        public T next() {
          if (!hasNext()) {
            throw new NoSuchElementException();
          }
          return delegate.next();
        }
      };
    }
  
    /**
     * Concatentates the {@link Iterator}s.
     *
     * <p>{@link Iterable}s are first converted into a {@link PrefetchableIterable} via {@link
     * #maybePrefetchable}.
     *
     * <p>The returned {@link PrefetchableIterable} ensures that iterators which are returned
     * guarantee that {@link PrefetchableIterator#isReady} always advances till it finds an {@link
     * Iterable} that is not {@link PrefetchableIterator#isReady}. {@link
     * PrefetchableIterator#prefetch} is also guaranteed to advance past empty iterators till it finds
     * one that is not ready.
     */
    public static <T> PrefetchableIterator<T> concat(Iterator<T>... iterators) {
      for (int i = 0; i < iterators.length; ++i) {
        if (iterators[i] == null) {
          throw new IllegalArgumentException("Iterator at position " + i + " was null.");
        }
      }
      if (iterators.length == 0) {
        return emptyIterator();
      } else if (iterators.length == 1) {
        return maybePrefetchable(iterators[0]);
      }
      return concatIterators(Arrays.asList(iterators).iterator());
    }
  }
  