How to parse bit fields from a byte array in Java?

Question

I've been given the arduous task of parsing some incoming UDP packets from a source into an appropriate Java representation. The kicker is the data held within the packets are not byte aligned. In an effort to make the protocol as tight as possible, there are a number of bit fields indicating the presence or absence of data fields.

For example, at bit index 34 you may find a 24 bit field that should be converted to a float. Bit index 110 may be a flag indicating that the next 3 fields are each 5 and 6 bit values containing the hour, minute, and second of the day. (These are just made up by me but are representative of what the spec says). These packets are probably a few hundred bits long.

The spec is not likely to change, but it's completely possible that I'll be asked to decode other similar packet formats.

I can certainly bit shift and mask as needed, but I'm worried about ending up in bit shift hell and then drowning in it when more packet formats are discovered.

I'd love to hear any suggestions on best practices or Java libraries that may make the task more manageable.

Answer 1

Decoding QR codes is much the same exercise in reading a couple bits at a time, completely unaligned. Here's what I wrote to handle it -- maybe you can just reuse this.

http://code.google.com/p/zxing/source/browse/trunk/core/src/com/google/zxing/common/BitSource.java