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#1: Rebecca Cary (rec3) on Feb 28, 2009

If you assume symmetry, which the title implies, there's no guessing required.

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LOL!!! :)

rofl #2! ;)

Found to require some guessing by jan.

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LOL at #7. Paul, Paul Paul... don't get overworked about the silliness of solvability ratings. Does it really matter what people rate them? Just feel fortunate that you are including yourself in the "smart enough" group. :-)

I am the webmaster (silly word, I like the Car Talk version: web-lacky).

(1) I didn't see how to solve this puzzle logically. I'd be interested in a description of how you got the smiley part.

(2) I have written an open source program, pbnsolve, that can solve most of the puzzles on this site. It is used to decide for nearly all puzzles whether or not they have a unique solution. However, it works by deep trial-and-error search, a solution method that is not fun for humans. So it marks puzzles that are solvable by line solving alone as "logically solvable" and leaves the rest up to vote. Some of the puzzles people vote as "requiring guessing" I check.

(3) I think it would be possible to teach the pbnsolve program to use more human-like solution techniques, so it can better rate a puzzles ability to be solved by such techniques. This is a complex programming problem though, and I don't know when or if I'll get around to attempting it.

(4) I am planning to eventually deputize a few people to help manually rate puzzles. I'll need to do some more programming to create the software that would enable them to do that, and I need to do some data mining to find people who rate a lot of peoples pretty well and see if some of them are willing to volunteer. Maybe some day.

(5) The whole "logically solvable" / "requires guessing" distinction is a fuzzy one. People will disagree. It's not something worth getting too worked up about.

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Found to be logically solvable by jan.

I'm not sure I buy your first sentence.

If I put the 4 clue on the far left in row 20, then the column clues for the first for columns tell me that the first four columns of row 19 would be red (which is OK since the clue there is a 6), and the first four columns of row 18 would be dots (which is OK too). And that's as far a ordinary edge logic takes me.

Ah, I see. This is quite ordinary edge logic, because the red 2 is the only clue left in columns 1, 2 and 3. So if red 4 in row 20 is in columns 1, 2 or 3, we get dots in those columns in row 17. In fact, dots in both column 2 and 3 are enough to make row 17 unsolvable, so the red four can't start at 1 or 4. Yeah, once I see that, it's obvious enough.

OK, I'm convinced. You can do it, but it's danged hard.

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