流文件增量编码/解码

Question

Here's the problem - I want to generate the delta of a binary file (> 1 MB in size) on a server and send the delta to a memory-constrained (low on RAM and no dynamic memory) embedded device over HTTP. Deltas are preferred (as opposed to sending the full binary file from the server) because of the high cost involved in transmitting data over the wire.

Trouble is, the embedded device cannot decode deltas and create the contents of the new file in memory. I have looked into various binary delta encoding/decoding algorithms like bsdiff, VCDiff etc. but was unable to find libraries that supported streaming.

Perhaps, rather than asking if there are suitable libraries out there, are there alternate approaches I can take that will still solve the original problem (send minimal data over the wire)? Although it would certainly help if there are suitable delta libraries out there that support streaming decode (written in C or C++ without using dynamic memory).

Answer 1

Maintain a copy on the server of the current file as held by the embedded device. When you want to send an update, XOR the new version of the file with the old version and compress the resultant stream with any sensible compressor. (Algorithms which allow high-cost encoding to allow low-cost decoding would be particularly helpful here.) Send the compressed stream to the embedded device, which reads the stream, decompresses it on the fly and XORs directly (a copy of) the target file.

If your updates are such that the file content changes little over time and retains a fixed structure, the XOR stream will be predominantly zeroes, and will compress extremely well: number of bytes transmitted will be small, effort to decompress will be low, memory requirements on the embedded device will be minimal. The further your model is from these assumptions, the less this approach will gain you.

Answer 2

Since you said the delta could be arbitrarily random (from zero delta to a completely different file), compression of the delta may be a lost cause. Lossless compression of random binary data is theoretically impossible. Also, since the embedded device has limited memory anyway, using a sophisticated -and therefore computationally expensive- library for compression/decompression of the occasional "simple" delta will probably be infeasible.

I would recommend simply sending the new file to the device in raw byte format, and overwriting the existing old file.

Answer 3

As Kevin mentioned, compressing random data should not be your goal. A few more comments about the type of data your working with would be helpful. Context is key in compression.

You used the term image which makes it sound like the classic video codec challenge. If you've ever seen weird video aliasing effects that impact the portion of the frame that has changed, and then suddenly everything clears up. You've likely witnessed the notion of a key frame along with a series of delta frames. Where the delta frames were not properly applied.

In this model, the server decides what's cheaper:

• complete key frame
• delta commands

The delta commands are communicated as a series of write instructions that can overlay the clients existing buffer.

Example Format:

• [Address][Length][Repeat][Delta Payload]
• [Address][Length][Repeat][Delta Payload]
• [Address][Length][Repeat][Delta Payload]

There are likely a variety of methods for computing these delta commands. A brute force method would be:

• Perform Smith Waterman between two images.
• Compress the resulting transform into delta commands.