I cannot resist posting a few words about ALS (Almost Locked Sets), a very advanced solving technique with a lot of solving power. As I stated earlier, I just missed the deadline for 1.5.1 with these new solving techniques. They are now released in 1.5.1.1

One of them, long overdue, is Empty Rectanges. It does not solve anything new, but it is an easy to use alternative for multi-coloring and sometimes even for Nishio. Many other sudoku programs already supported ER, but having the most powerful techniques already implemented makes you lazy, I guess. The templates catch any single digit elimination, so I was not encouraged to implement lower-level single digit techniques. However, after the overwhelming success of my finned-fish implementation, it seemed the right time to do Empty Rectangles as well. Having restructured the solver in such a way that it is now very easy to add new techniques does also help.

The real thing I wanted to talk about is ALS. It is a bad acronym, but commonly known in the sudoku community. An Almost Locked Set is a situation where N cells have N+1 digits as their candidates. The simplest ALS is a single cell with only 2 candidates. Consider the following grid:

How many Almost Locked Set does this grid contain? 10? 20?

No, there are almost 200 ALS’s in this grid, with only 96 candidates left. Most of them are harmless, but when you can pick the right combinations, they can become incredible solving tools. My list of 50,000 sudokus that require tabling steps is reduced to half its size, now ALS can be used. There is no single technique that had such a tremendous impact.

I will expand my solving guide to tell more about the power of ALS.