http://lucene.apache.org/lucene-parent/lucene-core: Apache Lucene Java Core (The Apache Software Foundation) 
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 *     http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.lucene.index;


import org.apache.lucene.util.Version;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.HashMap;


This MergePolicy is used for upgrading all existing segments of an index when calling IndexWriter.forceMerge(int). All other methods delegate to the base MergePolicy given to the constructor. This allows for an as-cheap-as possible upgrade of an older index by only upgrading segments that are created by previous Lucene versions. forceMerge does no longer really merge; it is just used to "forceMerge" older segment versions away. 
In general one would use IndexUpgrader, but for a fully customizeable upgrade, you can use this like any other MergePolicy and call IndexWriter.forceMerge(int): 
 IndexWriterConfig iwc = new IndexWriterConfig(Version.LUCENE_XX, new KeywordAnalyzer());
 iwc.setMergePolicy(new UpgradeIndexMergePolicy(iwc.getMergePolicy()));
 IndexWriter w = new IndexWriter(dir, iwc);
 w.forceMerge(1);
 w.close();



Warning: This merge policy may reorder documents if the index was partially upgraded before calling forceMerge (e.g., documents were added). If your application relies on "monotonicity" of doc IDs (which means that the order in which the documents were added to the index is preserved), do a forceMerge(1) instead. Please note, the delegate MergePolicy may also reorder documents. 
See Also:
@lucene.experimental
/** This {@link MergePolicy} is used for upgrading all existing segments of
  * an index when calling {@link IndexWriter#forceMerge(int)}.
  * All other methods delegate to the base {@code MergePolicy} given to the constructor.
  * This allows for an as-cheap-as possible upgrade of an older index by only upgrading segments that
  * are created by previous Lucene versions. forceMerge does no longer really merge;
  * it is just used to "forceMerge" older segment versions away.
  * <p>In general one would use {@link IndexUpgrader}, but for a fully customizeable upgrade,
  * you can use this like any other {@code MergePolicy} and call {@link IndexWriter#forceMerge(int)}:
  * <pre class="prettyprint lang-java">
  *  IndexWriterConfig iwc = new IndexWriterConfig(Version.LUCENE_XX, new KeywordAnalyzer());
  *  iwc.setMergePolicy(new UpgradeIndexMergePolicy(iwc.getMergePolicy()));
  *  IndexWriter w = new IndexWriter(dir, iwc);
  *  w.forceMerge(1);
  *  w.close();
  * </pre>
  * <p><b>Warning:</b> This merge policy may reorder documents if the index was partially
  * upgraded before calling forceMerge (e.g., documents were added). If your application relies
  * on &quot;monotonicity&quot; of doc IDs (which means that the order in which the documents
  * were added to the index is preserved), do a forceMerge(1) instead. Please note, the
  * delegate {@code MergePolicy} may also reorder documents.
  * @lucene.experimental
  * @see IndexUpgrader
  */
public class UpgradeIndexMergePolicy extends FilterMergePolicy {

  
Wrap the given MergePolicy and intercept forceMerge requests to only upgrade segments written with previous Lucene versions. 
/** Wrap the given {@link MergePolicy} and intercept forceMerge requests to
   * only upgrade segments written with previous Lucene versions. */
  public UpgradeIndexMergePolicy(MergePolicy in) {
    super(in);
  }
  
  
Returns if the given segment should be upgraded. The default implementation will return !Version.LATEST.equals(si.getVersion()), so all segments created with a different version number than this Lucene version will get upgraded. 
/** Returns if the given segment should be upgraded. The default implementation
   * will return {@code !Version.LATEST.equals(si.getVersion())},
   * so all segments created with a different version number than this Lucene version will
   * get upgraded.
   */
  protected boolean shouldUpgradeSegment(SegmentCommitInfo si) {
    return !Version.LATEST.equals(si.info.getVersion());
  }

  @Override
  public MergeSpecification findMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos, MergeContext mergeContext) throws IOException {
    return in.findMerges(null, segmentInfos, mergeContext);
  }
  
  @Override
  public MergeSpecification findForcedMerges(SegmentInfos segmentInfos, int maxSegmentCount, Map<SegmentCommitInfo,Boolean> segmentsToMerge, MergeContext mergeContext) throws IOException {
    // first find all old segments
    final Map<SegmentCommitInfo,Boolean> oldSegments = new HashMap<>();
    for (final SegmentCommitInfo si : segmentInfos) {
      final Boolean v = segmentsToMerge.get(si);
      if (v != null && shouldUpgradeSegment(si)) {
        oldSegments.put(si, v);
      }
    }
    
    if (verbose(mergeContext)) {
      message("findForcedMerges: segmentsToUpgrade=" + oldSegments, mergeContext);
    }
      
    if (oldSegments.isEmpty())
      return null;

    MergeSpecification spec = in.findForcedMerges(segmentInfos, maxSegmentCount, oldSegments, mergeContext);
    
    if (spec != null) {
      // remove all segments that are in merge specification from oldSegments,
      // the resulting set contains all segments that are left over
      // and will be merged to one additional segment:
      for (final OneMerge om : spec.merges) {
        oldSegments.keySet().removeAll(om.segments);
      }
    }

    if (!oldSegments.isEmpty()) {
      if (verbose(mergeContext)) {
        message("findForcedMerges: " +  in.getClass().getSimpleName() +
        " does not want to merge all old segments, merge remaining ones into new segment: " + oldSegments, mergeContext);
      }
      final List<SegmentCommitInfo> newInfos = new ArrayList<>();
      for (final SegmentCommitInfo si : segmentInfos) {
        if (oldSegments.containsKey(si)) {
          newInfos.add(si);
        }
      }
      // add the final merge
      if (spec == null) {
        spec = new MergeSpecification();
      }
      spec.add(new OneMerge(newInfos));
    }

    return spec;
  }
  
}