Mocking a bit stream reader with Mockito

I am currently debugging a rather complicated algorithm that fixes errors in a bit stream. A BitReader interface is quite simple, and the main reading method is like this:

/**
  Reads bits from the stream.
  @param length number of bits to read (<= 64)
  @return read bits in the least significant bits
*/
long read(int length) throws IOException;

The objective is to test whether BitStreamFixer actually fixes the stream (in a way that is too hard to describe here). Basically I need to provide “broken” inputs to it and test whether its output is as correct as it can be (some inputs can't be fixed completely), like this:

BitStreamFixer fixer = new BitStreamFixer(input);
int word1 = fixer.readWord();
int word2 = fixer.readWord();
// possibly a loop here
assertEquals(VALID_WORD1, word1);
assertEquals(VALID_WORD2, word2);
// maybe a loop here too

Now, the BitStreamFixer class accepts an instance of BitReader. When unit testing the fixer, I obviously need one such instance. But where do I get one? I have two obvious options: either give it a real implementation of BitReader or mock it.

The former option is not really appealing because it would create a dependency on another object which has nothing to do with the class being tested. Moreover, it's not that easy because existing BitReader implementations read form input streams, so I'll need either a file or somehow prepared byte array, which is a tedious thing to do.

The latter option looks better and fits the usual unit testing approach. However, since I'm not even supposed to know what arguments the fixer will give to read, mocking it is not easy. I'll have to go with when(bitReader.read(anyInt())).thenAnswer(...) approach, implementing a custom answer that will create a lot of bit-fiddling logic to spoon-feed the object under test with proper bits in chunks of whatever size it asks for. Considering that bit streams I'm working with have rather complicated higher-level structure, it's not easy. And introducing logic in unit tests also doesn't smell good.

What do you think, is there any other option? Or maybe one of these can be improved in a way I fail to notice?