lucene - Solr segments.gen and segments_N file restore - Stack Overflow

would guess that it is unlikely that the segment_N and segments.gen files were the only things lost, by the sound of it, but you can try using CheckIndex.

You can run it from a command line something like:

java -ea:org.apache.lucene... org.apache.lucene.index.CheckIndex path/to/index -fix  

Or you can invoke methods of it in your own implementation, something like:

Directory directory = FSDirectory.open(new File("path/to/index"));  CheckIndex check = new CheckIndex(directory);  CheckIndex.Satus status = check.checkIndex();  check.fixIndex(status);

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