Encode binary data as ASCII in Java

Question

I have a bitset of binary data that I wish to encode compactly as an ASCII string. I intend to initially compress the data using run-length encoding to give a sequence of integers; e.g.

111110001000000000000111

becomes:

5o3z1o12z3o

(e.g. 5 ones, 3 zeros, 1 one, 12 zeros, 3 ones).

However, I wish to then compress this further into a compact ASCII string (i.e. a string using the full range of ASCII characters rather than the digits plus 'o' and 'z'). Can anyone recommend a suitable approach and / or 3rd party library to do this in Java?

Sean Owen · Accepted Answer · 2011-09-16 09:12:39Z

3

If your goal is compression, just gzip the stream. It's going to do better than your run-length encoding.

Then if you need it to be text for some reason, like to safely pass through old mail gateways, I'd also turn to a standard encoding like Base64, rather than make up your own.

But if you want to roll your own: first I'd note that you don't need the 'o' and 'z'. You already know those values since they alternate. Assume it starts on 0 (and if it doesn't, encode an initial 0 to show that there are 0 0s).

Encoding the numbers textually is possible but probably inefficient. Look into a variable-length encoding for integer values, then encode those bytes. Then 'escape' them into ASCII somehow.

But then we're back to Base64-like encoding, and the first suggestion to gzip + Base64 is probably easier than all of this.

answered Sep 16, 2011 at 9:12

Sean Owen

67k23 gold badges144 silver badges175 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Adamski Over a year ago

Thanks - I'll check it out. I notice there are non-public base64 encoders shipped in sun.misc. BTW I included the 'o' and 'z' delimiters assuming that the sequence of 0s or 1s was of arbitrary length rather than a fixed size integer.

Sean Owen Over a year ago

Commons Codec (commons.apache.org/codec) has code to encode Base64. You can still have arbitrary-length integers without needing a full byte of delimiter between integers.

Adamski Over a year ago

@Sean, if you were to encode arbitrary length integers without delimiters how would you know where one integer ended and the next one began? Only thing I can think of is that you'd need to use a variable length encoding scheme (e.g. using the top bit of each byte to denote "has more bytes").

Sean Owen Over a year ago

That's exactly how it's usually done. Google protobufs do this for instance, all the way back to the MIDI protocol.

Collectives™ on Stack Overflow

Encode binary data as ASCII in Java

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related