final int blockSize = BLOCK_SIZES[random().nextInt(BLOCK_SIZES.length)];
final DocsFreqBlockIndexOutput out = this.getIndexOutput(blockSize);
final DocsFreqBlockWriter writer = out.getBlockWriter();
writer.setNodeBlockIndex(out.index());
writer.setPosBlockIndex(out.index());
// generate doc ids
final Set<Integer> docIds = new TreeSet<Integer>();
final int lenght = (int) this.nextLong(128000, 512000);
for (int i = 0; i < lenght; i++) {