The NxM-Sudoku is a generalization of the standard 3x3 Sudoku with 9 squares of size 3x3 and a total of 81 cells. In the general case we have N*M rectangles of size NxM and a total of (N*M)2 cells.
Though there are standard Sudokus which are almost impossible to solve only with logical reasoning by humans, from a computational point of view solving a standard Sudoku is almost trivial. Since the general problem is NP-complete finding a solution gets more demanding for larger grids.
We choose an approach where we simplify the given NxM-Sudoku as far as possible using "human" methods like hidden and naked singles and tuples, block-line interaction etc. and transform the remaining problem into a boolean satisfiability problem. We then use Sat4J for solving.
Besides solving of Sudokus the program also includes features like generating Sudokus, testing for unique solutions etc.