lib/compression: add simple python bindings

lib/compression: add simple python bindings

There are four functions, allowing compression and decompression in
the two formats we support so far. The functions will accept bytes or
unicode strings which are treated as utf-8.

The LZ77+Huffman decompression algorithm requires an exact target
length to decompress, so this is mandatory.

The plain decompression algorithm does not need an exact length, but
you can provide one to help it know how much space to allocate. As
currently written, you can provide a short length and it will often
succeed in decompressing to a different shorter string.

These bindings are intended to make ad-hoc investigation easier, not
for production use. This is reflected in the guesses about output size
that plain_decompress() makes if you don't supply one -- either they
are stupidly wasteful or ridiculously insufficient, depending on
whether or not you were trying to decompress a 20MB string.

>>> a = '12345678'
>>> import compression
>>> b = compression.huffman_compress(a)
>>> b
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00  #....
>>> len(b)
262
>>> c = compression.huffman_decompress(b, len(a))
>>> c
b'12345678'                                   # note, c is bytes, a is str
>>> a
'12345678'
>>> d = compression.plain_compress(a)
>>> d
b'\xff\xff\xff\x0012345678'
>>> compression.plain_decompress(d)           # no size specified, guesses
b'12345678'
>>> compression.plain_decompress(d,5)
b'12345'
>>> compression.plain_decompress(d,0)         # 0 for auto
b'12345678'
>>> compression.plain_decompress(d,1)
b'1'
>>> compression.plain_decompress(a,444)
Traceback (most recent call last):
   compression.CompressionError: unable to decompress data into a buffer of 444 bytes.
>>> compression.plain_decompress(b,444)
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00 #...

That last one decompresses the Huffman compressed file with the plain
compressor; pretty much any string is valid for plain decompression.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>