A sudoku solver is one of programs, each programmer should write at least once in their lives. I’ve created mine back in 2004 to test two things - the first thing was, how much can techniques from constraint programming (propagation of possible values) improve efficiency in searching. The other thing I wanted to try was using Eclipse IDE as a replacement for NetBeans.

As for the efficiency - the solver is based on exhaustive search, but it tracks the list of possible values for each cell, and maintains it throughout the search. This list is based on the “fixed” cells in the row, column and square of the cell, and it is updated each time, a value of a cell is fixed. Moreover, if there is a single possible value left in a cell, it is fixed “immediately”, outside the search algorithm.

It shows, that for simple problems, backtracking is never really used. The solution of difficult problems from the Minimal sudoku site takes a few hundreds of milliseconds on a common PC.

If you’re interested in the source code, or want to try it out, checkout the repository on GitHub.