Friday 16 October 2020

There shined a shiny demon, in the middle of the road

And I made the first grid that came to my head
Just so happened to be
The best grid in the world
It was the best grid in the world

Look into my eyes, is it easy to see?
One and One go where?
Two and One must be?
It was destiny

More on this one later, especially for my dearest readers whose memory might not go back quite as far as 2008, but also for all my dearest readers who have enjoyed the recent discussions.  

And although this is just a tribute to a legend of, I also think it's one of the better Sudokus I've ever put together.  For whatever that's worth.  Enjoy!
   #351 Sudoku – rated 8/10 [Very Hard]

Edit: I'm having second thoughts about the rating of this puzzle.  I initially thought I wouldn't have the balls to put something like this into a sudoku competition, because people might end up guessing anyway.  But on second thoughts (see the comments for more thoughts), maybe I would.  In which case that 10 might get downgraded to an 8 or 9.


  1. Very clean execution of an idea!

    As you probably expect by now, I decided to take a look at the equivalent SK-Loop. Here it is:

    (35=89)r1c12 - (89=7)r23c1 - (7=12)r78c1 - (12=45)r9c23 - (45=8)r9c78 - (8=356)r78c9 - (356=129)r23c9 - (129=35) - SK Loop

    Clear as mud, I'm sure you'll agree.

    This puzzle, and others over the last week, have convinced me that Phistomefel's point of view is *the* correct way to see this technique. There are a few reasons for that:

    - An SK Loop is fire-and-forget, whereas Phistomefel gives you a global constraint that it is easy to keep in mind for the rest of the solve.

    - Phistomefel neatly avoids the hideous and pointless SK-Loop classification system (e.g. have a skim of For example, this puzzle has a 2-1-2-2-1-3-3-2 type loop, which isn't even mentioned on that page. The MSLS formulation of SK Loops also avoids this classification, but...

    - Both the loop formulation and the MSLS formulation of SK Loops need exhaustive pencil marks in the grid. On the other hand, the Phistomefel formulation makes it bleedin' obvious what to do before making any marks on the grid at all!

    In my opinion, this is probably the greatest advance in manual Sudoku solving since maybe 2007.

  2. So I don’t want to provide too many spoilers for the follow up post, but this is basically the same trick as a puzzle that featured on nikoli many years ago.

    Cracking the cryptic did a video on it and called it the schroedinger’s cell technique (although it took a little while for them to provide the correct attribution, something they’ve always been shaky on)

    Scanraid called this alternate inference chains. I did make sure that the opening wasn’t short cut by anything, and that once you got it the rest of the puzzle flowed nicely. I was quite pleased to set by hand a puzzle that got the diabolical rating, but is very plausibly solved on paper in less than 3-4 minutes by a good solver. Maybe less!

    Anyhow my interest was neither phistomefel/fred not skill loops, but rather a sort of MSLS view of things, which didn’t look much harder than spotting hidden pairs to me, and even cleaner than the schroedingers cell explanation. I’ll explain more in the next post!

    1. I see the "Schroedinger's Cell" here now, a very familiar setup having already seen the other puzzle although I didn't notice it initially having jumped straight for my sledgehammer.

      I eagerly await the follow-up post.

  3. On a side note: Sam are you aware of any equivalence of sk loops and MSLS? I guess it all depends on definitions, but it feels that they might by equivalent ways of looking at exactly the same thing

  4. SK Loops are a specific type of MSLS.

  5. By which I mean that you can always phrase an SK Loop in terms of MSLS logic, but the converse is not true.

  6. I don't think it's too hard for a competition. Of course lot of players could make a guess if they don't see the trick, but I'm not sure it's a real advantage to make a guess.
    I solved it by using a nonconventional/dirty technique (you know I love dirty sudoku techniques, haha !), I don't know if it is what you're speaking about. I don't know if it has a specific name. No need for pencilmark, very easy to understand. I remember I solved a classic sudoku created by Stefan Heine with this technique once (I don't remember if it was at a German championship or a German online qualification tournament).
    I'll not explain now, perhaps it's the intended solving path, I'll wait until your explanations first ;).
    Nice sudoku !


Contact Form


Email *

Message *