Existing algorithms for mining frequent patterns are facing challenges to handle databases (a) of increasingly large sizes, (b) consisting of variable-length, irregularly-spaced data, and (c) with mixed or even unknown properties. In this paper, we propose a novel self-adaptive algorithm D-CLUB that thoroughly addresses these issues by progressively clustering the database into condensed association bitmaps, applying a differential technique to digest and remove dense patterns, and then mining the remaining tiny bitmaps directly through fast aggregate bit operations. The bitmaps are well organized into rectangular two-dimensional matrices and adaptively refined in regions that necessitate further computation. We show that this approach not only drastically cuts down the original database size but also largely reduces and simplifies the mining computation for a wide variety of datasets and parameters. We compare D-CLUB with various state-of-the-art algorithms and show significant performance improvement in all cases.