these files are modified from the compax version. So the framework are the same. The function name is the same too. Sometimes you may see functions with name "compax", but actually its an aplwah function.
A detailed survey is present in the following URL:
[1] Zhen Chen et al., “A Survey of Bitmap Index Compression Algorithms for Big Data,” Tsinghua Science and Technology, 20(1), pp.100-115, February 2015.
URL: http://ieeexplore.ieee.org/xpls/articleDetails.jsp?arnumber=7040519.