RegionSplitCalculator.java

/**
 * Copyright 2011 The Apache Software Foundation
 *
 * 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.hadoop.hbase.util;

import java.util.Collection;
import java.util.Comparator;
import java.util.Map.Entry;
import java.util.TreeSet;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.hbase.util.Bytes.ByteArrayComparator;

import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Multimap;
import com.google.common.collect.TreeMultimap;

/**
 * This is a generic region split calculator. It requires Ranges that provide
 * start, end, and a comparator. It works in two phases -- the first adds ranges
 * and rejects backwards ranges. Then one calls calcRegions to generate the
 * multimap that has a start split key as a key and possibly multiple Ranges as
 * members.
 * 
 * To traverse, one normally would get the split set, and iterate through the
 * calcRegions. Normal regions would have only one entry, holes would have zero,
 * and any overlaps would have multiple entries.
 * 
 * The interface is a bit cumbersome currently but is exposed this way so that
 * clients can choose how to iterate through the region splits.
 * 
 * @param <R>
 */
public class RegionSplitCalculator<R extends KeyRange> {
  final static Log LOG = LogFactory.getLog(RegionSplitCalculator.class);

  private final Comparator<R> rangeCmp;
  /**
   * This contains a sorted set of all the possible split points
   * 
   * Invariant: once populated this has 0 entries if empty or at most n+1 values
   * where n == number of added ranges.
   */
  private final TreeSet<byte[]> splits = new TreeSet<byte[]>(BYTES_COMPARATOR);

  /**
   * This is a map from start key to regions with the same start key.
   * 
   * Invariant: This always have n values in total
   */
  private final Multimap<byte[], R> starts = ArrayListMultimap.create();

  /**
   * SPECIAL CASE
   */
  private final static byte[] ENDKEY = null;

  public RegionSplitCalculator(Comparator<R> cmp) {
    rangeCmp = cmp;
  }

  public final static Comparator<byte[]> BYTES_COMPARATOR = new ByteArrayComparator() {
    @Override
    public int compare(byte[] l, byte[] r) {
      if (l == null && r == null)
        return 0;
      if (l == null)
        return 1;
      if (r == null)
        return -1;
      return super.compare(l, r);
    }
  };

  /**
   * SPECIAL CASE wrapper for empty end key
   * 
   * @return ENDKEY if end key is empty, else normal endkey.
   */
  private byte[] specialEndKey(R range) {
    byte[] end = range.getEndKey();
    if (end.length == 0) {
      return ENDKEY;
    }
    return end;
  }

  /**
   * Adds an edge to the split calculator
   * 
   * @return true if is included, false if backwards/invalid
   */
  public boolean add(R range) {
    byte[] start = range.getStartKey();
    byte[] end = specialEndKey(range);

    if (end != ENDKEY && Bytes.compareTo(start, end) > 0) {
      // don't allow backwards edges
      LOG.debug("attempted to add backwards edge: " + Bytes.toString(start)
          + " " + Bytes.toString(end));
      return false;
    }

    splits.add(start);
    splits.add(end);
    starts.put(start, range);
    return true;
  }

  /**
   * Generates a coverage multimap from split key to Regions that start with the
   * split key.
   * 
   * @return coverage multimap
   */
  public Multimap<byte[], R> calcCoverage() {
    // This needs to be sorted to force the use of the comparator on the values,
    // otherwise byte array comparison isn't used
    Multimap<byte[], R> regions = TreeMultimap.create(BYTES_COMPARATOR,
        rangeCmp);

    // march through all splits from the start points
    for (Entry<byte[], Collection<R>> start : starts.asMap().entrySet()) {
      byte[] key = start.getKey();
      for (R r : start.getValue()) {
        regions.put(key, r);

        for (byte[] coveredSplit : splits.subSet(r.getStartKey(),
            specialEndKey(r))) {
          regions.put(coveredSplit, r);
        }
      }
    }
    return regions;
  }

  public TreeSet<byte[]> getSplits() {
    return splits;
  }

  public Multimap<byte[], R> getStarts() {
    return starts;
  }

}