Friday, January 22, 2010

800 lines of code for a bencode serializer?!

Wow, my Taipei-Torrent post made it to Hacker News and reddit.com/r/programming. I'm thrilled!

One comment on Hacker News was "800 lines for a bencode serializer?! It only took me 50 lines of Smalltalk!" That was a good comment! I replied:
A go bencode serialization library could be smaller if it just serialized the bencode data to and from a generic dictionary container. I think another go BitTorrent client, gobit, takes this approach.

But it's convenient to be able to serialize arbitrary application types. That makes the bencode library larger because of the need to use go's reflection APIs to analyze and access the application types. Go's reflection APIs are pretty verbose, which is why the line count is so high.

To make things even more complicated, the BitTorrent protocol has a funny "compute-the-sha1-of-the-info-dictionary" requirement that forces BitTorrent clients to parse that particular part of the BitTorrent protocol using a generic parser.

So in the end, the go bencode serializer supports both a generic dictionary parser and an application type parser, which makes it even larger.

In general, Taipei-Torrent was not written to minimize lines-of-code. (If anything, I was trying to maximize functionality per unit of coding time.) One example of not minimizing lines-of-code is that I used a verbose error handling idiom:

a, err := f()
if err != nil {
return
}

There are alternative ways to handle errors in go. Some of the alternatives take fewer lines of code in some situations. The above idiom is my preference because it works correctly in all cases. But it has the drawback of adding 3 lines of code to every function call that might return an error.
The go authors are considering adding exceptions to the go language. If they do so it will probably dramatically improve the line count of Taipei-Torrent.

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