String indexRootPath = conf.get(Constants.IndexPath);
String collection = conf.get(Constants.CollectionPath);
RetrievalEnvironment env = new RetrievalEnvironment(indexRootPath, fs);
Path mappingFile = env.getDocnoMappingData();
new TrecDocnoMappingBuilder().build(new Path(collection), mappingFile, conf);
conf.set(Constants.DocnoMappingClass, TrecDocnoMapping.class.getCanonicalName());
conf.set(Constants.DocnoMappingFile, env.getDocnoMappingData().toString());
conf.setInt(Constants.DocnoOffset, 0); // docnos start at 1
conf.setInt(Constants.MinDf, 2); // toss away singleton terms