presto-spi/src/main/java/io/prestosql/spi/block/RunLengthBlockEncoding.java/RunLengthBlockEncoding/readBlock
class RunLengthBlockEncoding:    @Override
    public RunLengthEncodedBlock readBlock(BlockEncodingSerde blockEncodingSerde, SliceInput sliceInput)
    {
        // read the run length
        int positionCount = sliceInput.readInt();

        // read the value
        Block value = blockEncodingSerde.readBlock(sliceInput);

        return new RunLengthEncodedBlock(value, positionCount);
    }