Consecutive bch.decode calls lead to wrong output

[Lukas113]

Using bchlib=1.0.0 & python=3.12, I've found that consecutive calls of bch.decode of the same bch instance results most likely in wrong outputs. Steps to reproduce:

Consecutive bch.decode calls:

>>> from bchlib import BCH
>>> from bitstring import Bits
>>> bch = BCH(t=16, prim_poly=8219)
>>> data = Bits(b"First").tobitarray()
>>> ecc = bch.encode(data)
>>> data2 = Bits(b"Second").tobitarray()
>>> ecc2 = bch.encode(data2)
>>> print(f"n-errors data: {bch.decode(data, ecc)}")
n-errors data: 0
>>> print(f"n-errors data2: {bch.decode(data2, ecc2)}")
n-errors data2: -1

I also added bch.correct in between the decoding steps in case some cleanup would be done there. However, the result is the same.

Same as above, but without the first bch.decode call:

>>> from bchlib import BCH
>>> from bitstring import Bits
>>> bch = BCH(t=16, prim_poly=8219)
>>> data = Bits(b"First").tobitarray()
>>> ecc = bch.encode(data)
>>> data2 = Bits(b"Second").tobitarray()
>>> ecc2 = bch.encode(data2)
>>> # print(f"n-errors data: {bch.decode(data, ecc)}")
>>> print(f"n-errors data2: {bch.decode(data2, ecc2)}")
n-errors data2: 0

Testing this using bchlib=0.14.0 worked fine as expected.

[jkent]

Hi Lukas113,

bchlib works with octets, not raw bits. This issue may be because you are using 'bitstring.Bits' instead of a bytes or bytearray object, and the C wrapper may need more error checking on the data type. To be honest, I've never used a Bits object. I'll check this out.

[jkent]

OK, reproduced this with bytes so there must be some state being kept between consecutive calls.

[jkent]

I understand why it's not working now. Before decoding, the encode function required you to set your message length: bch.data_len = 0. The first invocation, it was 0, and bch.data_len got set to len(data) on the first run. Amusingly, "First" and "Secon" would have worked as expected. It was the length that threw things off.

Since this is a very bad API decision, I've removed data_len.