Blogged by buffi

Bittorrent uses a quite decent algorithm for serialization of it’s data for the .torrent files called bencoding. This page has a pretty good description of it.

I wanted to be able to parse this data to get information out of them for a thing I’m working on and played around a bit in python. Since it was late at night and I was rather bored I wanted to see how short I could get the decoder without writing extremely ugly hacks.
I ended up with a fully functional module that clocks in at 30 lines of code.

class Decoder(object):
    def __init__(self, data): self.data, self.ptr = data, 0
    def _cur(self): return self.data[self.ptr]
    def _get(self, x):
        self.ptr += x
        return self.data[self.ptr-x:self.ptr]
    def _get_int_until(self, c):
        num = int(self._get(self.data.index(c, self.ptr)-self.ptr))
        self._get(1) # kill extra char
        return num
    def _get_str(self): return self._get(self._get_int_until(":"))
    def _get_int(self): return self._get_int_until("e")
    def decode(self):
        i = self._get(1)
        if i == "d":
            r = {}
            while self._cur() != "e":
                key = self._get_str()
                val = self.decode()
                r[key] = val
            self._get(1)
        elif i == "l":
            r = []
            while self._cur() != "e": r.append(self.decode())
            self._get(1)
        elif i == "i": r = self._get_int()
        elif i.isdigit():
            self._get(-1) # reeeeewind
            r = self._get_str()
        return r

Download it here

Runtime example for a random torrent I downloaded to test it on:

>>> from decode import Decoder
>>> f=open("stuff.torrent")
>>> data=f.read()
>>> d=Decoder(data)
>>> d.decode()
{'comment': '', 'comment.utf-8': '', 'azureus_properties': {'dht_backup_enable': 1}, 'encoding': 'UTF-8',
    'creation date': 1185571081, 'info': {'piece length': 32768, 'name': 'SHAVER.mp3', 'private': 0,
    'pieces': '\x1ee\xb4\xa0X\xe2 ... I removed characters here since the line is too long ... \xc3v\xc2\x00\xc9',
    'length': 229374, 'name.utf-8': 'SHAVER.mp3'}, 'created by': 'Azureus/2.5.0.4',
    'announce': 'http://tpb.tracker.thepiratebay.org/announce'}

Hopefully this might be useful to someone.

And yeah, this code is not very “pythonic” but that word is rather stupid anyways so… whatever
If I did a C++ (or whatever) implementation it would probably look pretty much the same.

buffi Programming & scripting, Python