Bitstring compression

What is currently the most efficient algorithm to compress bit strings.
special requirements:
1.NO algorithms which deal with bytes/ints or some fixed chunks of N bits. Must operate on variable-length substrings of bitstring.
2.The encoding must not take more than x8 memory of bitstring memory size(either less than x8 or using a buffer).
3.encoding should be as fast as possible(no complex modeling, data context switching or similar techniques)

Well since PPM and arithmetic coding are out based on your requirements, it's going to be a lempel-ziv variant, but which totally depends on the properties of the data your are trying to compress.