presto-spi/src/main/java/io/prestosql/spi/block/EncoderUtil.java/EncoderUtil/decodeNullBits
class EncoderUtil:    public static Optional<boolean[]> decodeNullBits(SliceInput sliceInput, int positionCount)
    {
        if (!sliceInput.readBoolean()) {
            return Optional.empty();
        }

        // read null bits 8 at a time
        boolean[] valueIsNull = new boolean[positionCount];
        for (int position = 0; position < (positionCount & ~0b111); position += 8) {
            byte value = sliceInput.readByte();
            valueIsNull[position] = ((value & 0b1000_0000) != 0);
            valueIsNull[position + 1] = ((value & 0b0100_0000) != 0);
            valueIsNull[position + 2] = ((value & 0b0010_0000) != 0);
            valueIsNull[position + 3] = ((value & 0b0001_0000) != 0);
            valueIsNull[position + 4] = ((value & 0b0000_1000) != 0);
            valueIsNull[position + 5] = ((value & 0b0000_0100) != 0);
            valueIsNull[position + 6] = ((value & 0b0000_0010) != 0);
            valueIsNull[position + 7] = ((value & 0b0000_0001) != 0);
        }

        // read last null bits
        if ((positionCount & 0b111) > 0) {
            byte value = sliceInput.readByte();
            int mask = 0b1000_0000;
            for (int position = positionCount & ~0b111; position < positionCount; position++) {
                valueIsNull[position] = ((value & mask) != 0);
                mask >>>= 1;
            }
        }

        return Optional.of(valueIsNull);
    }