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.runners.dataflow.worker.util.common.worker;
  
  import static org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Preconditions.checkNotNull;
  import static org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Preconditions.checkState;
  
  import java.util.ListIterator;
  import java.util.NoSuchElementException;
  import javax.annotation.concurrent.NotThreadSafe;
  import org.apache.beam.sdk.util.common.Reiterator;
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /** BatchingShuffleEntryReader provides a mechanism for reading entries from a shuffle dataset. */
  @NotThreadSafe
  @SuppressWarnings({
    "nullness" // TODO(https://github.com/apache/beam/issues/20497)
  })
  public final class BatchingShuffleEntryReader implements ShuffleEntryReader {
    private final ShuffleBatchReader batchReader;
  
    /**
     * Constructs a {@link BatchingShuffleEntryReader}.
     *
     * @param batchReader supplies the underlying {@link ShuffleBatchReader} to read batches of
     *     entries from
     */
    public BatchingShuffleEntryReader(ShuffleBatchReader batchReader) {
      this.batchReader = checkNotNull(batchReader);
    }
  
    @Override
    public Reiterator<ShuffleEntry> read(
        @Nullable ShufflePosition startPosition, @Nullable ShufflePosition endPosition) {
      return new ShuffleReadIterator(startPosition, endPosition);
    }
  
    /** ShuffleReadIterator iterates over a (potentially huge) sequence of shuffle entries. */
    private final class ShuffleReadIterator implements Reiterator<ShuffleEntry> {
      // Shuffle service returns entries in pages. If the response contains a
      // non-null nextStartPosition, we have to ask for more pages. The response
      // with null nextStartPosition signifies the end of stream.
      private final @Nullable ShufflePosition endPosition;
      private @Nullable ShufflePosition nextStartPosition;
  
      /** The most recently read batch. */
      ShuffleBatchReader.@Nullable Batch currentBatch;
      /** An iterator over the most recently read batch. */
      private @Nullable ListIterator<ShuffleEntry> entries;
  
      ShuffleReadIterator(
          @Nullable ShufflePosition startPosition, @Nullable ShufflePosition endPosition) {
        this.nextStartPosition = startPosition;
        this.endPosition = endPosition;
      }
  
      private ShuffleReadIterator(ShuffleReadIterator it) {
        this.endPosition = it.endPosition;
        this.nextStartPosition = it.nextStartPosition;
        this.currentBatch = it.currentBatch;
        // The idea here: if the iterator being copied was in the middle of a
        // batch (the typical case), create a new iteration state at the same
        // point in the same batch.
        this.entries =
            (it.entries == null
                ? null
                : it.currentBatch.entries.listIterator(it.entries.nextIndex()));
      }
  
      @Override
      public boolean hasNext() {
        fillEntriesIfNeeded();
        // TODO: Report API errors to the caller using checked
        // exceptions.
        return entries.hasNext();
      }
  
      @Override
      public ShuffleEntry next() throws NoSuchElementException {
        fillEntriesIfNeeded();
        return entries.next();
      }
  
      @Override
      public void remove() throws UnsupportedOperationException {
        throw new UnsupportedOperationException();
      }
  
      @Override
      public ShuffleReadIterator copy() {
        return new ShuffleReadIterator(this);
      }
  
      private void fillEntriesIfNeeded() {
        if (entries != null && entries.hasNext()) {
          // Has more records in the current page, or error.
          return;
        }
  
        if (entries != null && nextStartPosition == null) {
          // End of stream.
          checkState(!entries.hasNext());
          return;
        }
  
        do {
          fillEntries();
        } while (!entries.hasNext() && nextStartPosition != null);
      }
  
      private void fillEntries() {
        try {
          ShuffleBatchReader.Batch batch = batchReader.read(nextStartPosition, endPosition);
          nextStartPosition = batch.nextStartPosition;
          entries = batch.entries.listIterator();
          currentBatch = batch;
        } catch (RuntimeException e) {
          throw e;
        } catch (Throwable t) {
          throw new RuntimeException(t);
        }
  
        checkState(entries != null);
      }
    }
  
    @Override
    public void close() {}
  }
  