/*
 * 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.accumulo.core.iterators.system;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.concurrent.atomic.AtomicBoolean;

import org.apache.accumulo.core.data.ByteSequence;
import org.apache.accumulo.core.data.Key;
import org.apache.accumulo.core.data.PartialKey;
import org.apache.accumulo.core.data.Range;
import org.apache.accumulo.core.data.Value;
import org.apache.accumulo.core.iterators.IteratorEnvironment;
import org.apache.accumulo.core.iterators.SortedKeyValueIterator;

/**
 * A SortedKeyValueIterator which presents a view over some section of data, regardless of whether or not it is backed by memory (InMemoryMap) or an RFile
 * (InMemoryMap that was minor compacted to a file). Clients reading from a table that has data in memory should not see interruption in their scan when that
 * data is minor compacted. This iterator is designed to manage this behind the scene.
 */
public class SourceSwitchingIterator implements InterruptibleIterator {

  public interface DataSource {
    boolean isCurrent();

    DataSource getNewDataSource();

    DataSource getDeepCopyDataSource(IteratorEnvironment env);

    SortedKeyValueIterator<Key,Value> iterator() throws IOException;

    void setInterruptFlag(AtomicBoolean flag);
  }

  private DataSource source;
  private SortedKeyValueIterator<Key,Value> iter;

  private Key key;
  private Value val;

  private Range range;
  private boolean inclusive;
  private Collection<ByteSequence> columnFamilies;

  private boolean onlySwitchAfterRow;

  // Synchronization on copies synchronizes operations across all deep copies of this instance.
  //
  // This implementation assumes that there is one thread reading data (a scan) from all deep copies
  // and that another thread may call switch at any point. A single scan may have multiple deep
  // copies of this iterator if other iterators above this one duplicate their source. For example,
  // if an IntersectingIterator over two columns was configured, `copies` would contain two SSIs
  // instead of just one SSI. The two instances in `copies` would both be at the same "level"
  // in the tree of iterators for the scan. If multiple instances of SSI are configure in the iterator
  // tree (e.g. priority 8 and priority 12), each instance would share their own `copies` e.g.
  // SSI@priority8:copies1[...], SSI@priority12:copies2[...]

  private final List<SourceSwitchingIterator> copies;

  private SourceSwitchingIterator(DataSource source, boolean onlySwitchAfterRow, List<SourceSwitchingIterator> copies) {
    this.source = source;
    this.onlySwitchAfterRow = onlySwitchAfterRow;
    this.copies = copies;
    copies.add(this);
  }

  public SourceSwitchingIterator(DataSource source, boolean onlySwitchAfterRow) {
    this(source, onlySwitchAfterRow, new ArrayList<SourceSwitchingIterator>());
  }

  public SourceSwitchingIterator(DataSource source) {
    this(source, false);
  }

  @Override
  public SortedKeyValueIterator<Key,Value> deepCopy(IteratorEnvironment env) {
    synchronized (copies) {
      return new SourceSwitchingIterator(source.getDeepCopyDataSource(env), onlySwitchAfterRow, copies);
    }
  }

  @Override
  public Key getTopKey() {
    return key;
  }

  @Override
  public Value getTopValue() {
    return val;
  }

  @Override
  public boolean hasTop() {
    return key != null;
  }

  @Override
  public void init(SortedKeyValueIterator<Key,Value> source, Map<String,String> options, IteratorEnvironment env) throws IOException {
    throw new UnsupportedOperationException();
  }

  @Override
  public void next() throws IOException {
    synchronized (copies) {
      readNext(false);
    }
  }

  private void readNext(boolean initialSeek) throws IOException {

    // check of initialSeek second is intentional so that it does not short
    // circuit the call to switchSource
    boolean seekNeeded = (!onlySwitchAfterRow && switchSource()) || initialSeek;

    if (seekNeeded)
      if (initialSeek)
        iter.seek(range, columnFamilies, inclusive);
      else
        iter.seek(new Range(key, false, range.getEndKey(), range.isEndKeyInclusive()), columnFamilies, inclusive);
    else {
      iter.next();
      if (onlySwitchAfterRow && iter.hasTop() && !source.isCurrent() && !key.getRowData().equals(iter.getTopKey().getRowData())) {
        switchSource();
        iter.seek(new Range(key.followingKey(PartialKey.ROW), true, range.getEndKey(), range.isEndKeyInclusive()), columnFamilies, inclusive);
      }
    }

    if (iter.hasTop()) {
      Key nextKey = iter.getTopKey();
      Value nextVal = iter.getTopValue();

      try {
        key = (Key) nextKey.clone();
      } catch (CloneNotSupportedException e) {
        throw new IOException(e);
      }
      val = nextVal;
    } else {
      key = null;
      val = null;
    }
  }

  private boolean switchSource() throws IOException {
    if (!source.isCurrent()) {
      source = source.getNewDataSource();
      iter = source.iterator();
      return true;
    }

    return false;
  }

  @Override
  public void seek(Range range, Collection<ByteSequence> columnFamilies, boolean inclusive) throws IOException {
    synchronized (copies) {
      this.range = range;
      this.inclusive = inclusive;
      this.columnFamilies = columnFamilies;

      if (iter == null)
        iter = source.iterator();

      readNext(true);
    }
  }

  private void _switchNow() throws IOException {
    if (onlySwitchAfterRow)
      throw new IllegalStateException("Can only switch on row boundries");

    if (switchSource()) {
      if (key != null) {
        iter.seek(new Range(key, true, range.getEndKey(), range.isEndKeyInclusive()), columnFamilies, inclusive);
      }
    }
  }

  public void switchNow() throws IOException {
    synchronized (copies) {
      for (SourceSwitchingIterator ssi : copies)
        ssi._switchNow();
    }
  }

  @Override
  public void setInterruptFlag(AtomicBoolean flag) {
    synchronized (copies) {
      if (copies.size() != 1)
        throw new IllegalStateException("setInterruptFlag() called after deep copies made " + copies.size());

      if (iter != null)
        ((InterruptibleIterator) iter).setInterruptFlag(flag);

      source.setInterruptFlag(flag);
    }
  }
}