September 03, 2010

Friday Puzzles #68

For this week’s puzzle, I’ve decided to have a little play with Yajilin, and its presentation.

A problem I quickly ran into was with how the clues have an effect on the puzzle. It wasn’t too long into making this that I had a nice symmetric pattern laid out in the grid, and a few clues in place. As I continued putting in more and more clues, it became increasingly obvious that I wasn’t going to need all the marked squares for clues that I’d set aside. However, I also realised that I couldn’t simply throw them away (for one thing, think of the symmetry!). But more importantly, because the rules of Yajilin say that you have to use every square in the grid, these as yet clueless squares were still playing a crucial role in the solving process.

As such I’m going to throw you two versions of the same puzzle, presented slightly differently. For the first, I filled in the the clue squares I needed, sometimes ridiculously, and sometimes a little more obscurely, all the time trying not to introduce too many logical short-cuts into the puzzle.

I am not sure then, that despite having more information in the first version, whether actually the second is a nicer solve.

#083a Yajilin – rated hard

Note in this version that you can shade in a square directly adjacent to a blank grey square, although the loop definitely cannot enter these squares. Also note that the the grey squares aren’t included in the count of the to-be-shaded squares.

I’m hoping I’ve deleted the right clues, although I suppose it’s not the end of the world If I’ve gone too far. I think it should work out that there are probably still one or two technically surplus clues hiding in there.

#083b Yajilin – rated hard

All puzzles © Tom Collyer 2009-10


- 4 comments by 1 or more people Not publicly viewable

  1. Thomas Snyder

    It’s really impossible to test both styles, but having argued for the bottom presentation as an improvement on Yajilin when I was talking about them 2 months ago, I must say I really liked both the clue density and use of “squares” as logical path holders in the bottom example. It really seems this Akari-like style is exactly what Yajilin should be. Thanks for the fun puzzle.

    03 Sep 2010, 16:27

  2. Thomas Collyer

    Yeah, I recall your post and they were almost certainly a subconscious inspiration here. The disadvantage with the modified format is the slightly fiddly way you have to state the rules – presumably this is why nikoli settled on the format that currently exists – however having played with a few toy examples with more interesting grey patterns I think that is a price worth paying.

    Indeed, one of the more memorable yajilin on nikoli.com set up big walls with clues that were essentially trivial; it was the way the clues were set out that added interest…

    04 Sep 2010, 00:26

  3. Thomas Snyder

    I’d say the rules can be stated as simply in this format since “shaded squares” aren’t an essential part of the puzzle definition. Its also something I quibble with on Nikoli.com since just marking the loop is the puzzle in my mind.

    Instructions: Draw a single closed loop, without crossing, passing through the white cells of the grid with vertical/horizontal connections between cells. The numbered arrows indicate how many white cells are not a part of the loop in the indicated direction. Unused white cells cannot share an edge with each other.

    04 Sep 2010, 03:15

  4. MellowMelon

    I don’t think that in general it’s very hard to prevent the extra clues from making logical shortcuts. Usually the puzzle solves from a few starting points that spread out, and there’s one or two places you pretty much have to finish on. If you have extra clues near starting points, point them towards the end. If you have extra clues near the finish, point them towards starting points. It is highly unlikely either will result in any sequence breaking. Dealing with the case that they do is no worse than, say, trying to get clue symmetry in a Slitherlink puzzle to do what you want when you’re trying to lay some very specific logic.

    As for what that does to the puzzle as a whole, I don’t think it’s necessarily good or bad. Those extra clues are often fairly worthless. So it challenges the solver to identify which clues they should focus on to make progress. Whether this outweighs the “inelegance” of the extra clues depends on the puzzle and personal taste. I’ve personally always been okay with extra clues.

    This puzzle is the main one I made in which a serious amount of sequence breaking resulted from having to fill in all the extra squares: http://mellowmelon.wordpress.com/2009/10/27/puzzle-82/
    I still think it solves okay, even if it didn’t quite go down like I constructed it, which I pointed out in the post comment.

    I went with the bottom puzzle; it was a good solve.

    05 Sep 2010, 04:31


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Welcome to the blog of current UKPA sudoku champion, two-time Times national sudoku champion and general logic puzzle fan Tom Collyer.



Home of the original Friday Puzzles, each Friday I publish a 100% original and handmade logic puzzle, inspired by the world-famous Nikoli company.


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