A little shameless plug:

Ive just released a game built around this project on play store - https://play.google.com/store/apps/details?id=com.noobgiraff...

Has 4x4, 5x5 and 6x6 puzzles. Generating 7x7 puzzles is long process and i think i may have to do it on cloud, but am planning to release them soon.