I've been out of this forum for quite some time -- has it been two years? Maybe. In any case, I've recently gotten back to some Sudoku programming and am interested in discussing a few things.
Recent news:
-- The Sudoku Assistant
http://www.stolaf.edu/people/hansonr/sudoku did have a bug that was preventing it from finding these locked candidates. That's fixed now.
-- The Sudoku Assistant was only minimally finding almost-locked sets. This bugged me, so I went back to this and redesigned the search engine to do that right. To my great delight it now finds LOADS of almost-locked sets, many of which are very simple.
-- Especially noteworthy (I think) is my recent finding that there is something I am now referring to as "bent naked subsets". One subset of these is related to almost-locked sets. These are now being found by the Sudoku Assistant, and they are very interesting. The basic idea is that what people originally referred to as "mutually doubly-linked almost-locked sets" are often just naked subsets that turn a corner.
I would greatly appreciate someone critiquing my analysis of these at
http://www.stolaf.edu/people/hansonr/sudoku/explain.htm#bentQuestions:
1) Have people discussed these? They are incredibly easy to find, and it looks like quite a number of named methods reduce to them (XY-Wing, XYZ-Wing, for instance)
2) Has anyone developed my original idea of a couple of years ago of grid-based analysis (X-Wings, Swordfish, etc.) as simple naked pairs/triples "turned on their side"? (
http://www.stolaf.edu/people/hansonr/sudoku/explain.htm#grid )
3) It turns out that the almost-locked business also extends to grids, and there the idea is incredibly powerful and easy to implement. Sudoku Assistant was sort of doing this, but now it is really doing it right. (
http://www.stolaf.edu/people/hansonr/sudoku/explain.htm#almostgrid )
4) What other solvers handle almost-locked sets -- regular or grid-based?
Finally, I've put together a site with over 4500 method examples. Very interested in comments and suggestions on that. This is just out today. I'm already learning a lot simply scanning it for interesting patterns. See
http://www.stolaf.edu/people/hansonr/sudoku/ex_Sh2.htm and associated pages.
please feel free to contact me at
hansonr@stolaf.edu, where I am more likely to respond than here.
Bob Hanson
hansonr@stolaf.edu