What's wrong with XY-loops?

Advanced methods and approaches for solving Sudoku puzzles

Re: What's wrong with XY-loops?

Postby eleven » Sun Oct 08, 2017 3:23 pm

Oh yes, most of my mistakes i make, when the hard part of a puzzle is already solved. That's natural, because i always double-check an elimination/placement from more complex steps.

I have tried different helper marks, but neither with consequence, nor with success. After years i was so used to stare at (printed) pencilmark grids, that i did not think about it anymore. Of course i am also using solvers, where i can highlight the single digits, which is rather tiring without (i can spot x-chains, but hardly a finned swordfish).
For harder grids i also used a kind of Medusa, where i just filled in 2-way-possibilities in the cells (very efficient for the right starting points). But this does need a lot of erasing, if or if not it was successful.

So i am curious. And i wonder, if such marking would not be interesting for programs. I am all but glad about what the ones i know offer. Apart from highlighting they just tell me, how they could/would do it. This way they spoil the fun of finding it myself.

btw i find, you should change the title of this thread. It only started with xy-loops.
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Re: What's wrong with XY-loops?

Postby SpAce » Mon Oct 09, 2017 6:10 am

StrmCkr, thanks for explaining and the link to your program! I'll be sure to study it at some point, but only after I've coded my own solver :) I think coding is a great way to improve one's manual solving skills because it requires one to really understand each solving method. Last spring I coded a basic solver, and that little project pushed me into improving my manual system as well. Seeing my solver easily find patterns I hadn't manually spotted made me understand that neither I or my system were quite as good at even basic solving as I'd thought.

So my first upgrade was to make my manual system reveal basic patterns almost as well as the solver, and now it does. All naked and hidden singles and doubles, pointing pairs/triples, and BLR are guaranteed to be found. The only basic patterns it doesn't reveal on its own are quads and those triples that aren't on a boxline, but I can live with that. The same upgrades made also some intermediate patterns, such as simpler UR types, BUGs, and X-Wings easy to spot, and also helped with chaining (although I've since made additional upgrades for that).

All that happened because I spent a couple of evenings coding a basic solver. That makes me believe that if I now coded what I think I know about advanced techniques, I'd probably learn much more about them as well. Did that happen to you?
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Re: What's wrong with XY-loops?

Postby SpAce » Mon Oct 09, 2017 7:32 am

eleven wrote:Oh yes, most of my mistakes i make, when the hard part of a puzzle is already solved. That's natural, because i always double-check an elimination/placement from more complex steps.


Same here. I don't think I've ever made a mistake in a really hard step, as I double-check it so many times and try to find different ways to confirm it, too.

Of course i am also using solvers, where i can highlight the single digits, which is rather tiring without (i can spot x-chains, but hardly a finned swordfish).


Yeah, the single digit highlighting is probably one of the most helpful features of software solvers. I don't have that, but that's why I have a separate helper grid with each box representing a mini grid dedicated to a single digit (sounds awfully packed but works really well, actually). I mostly use it to make X-Chains easy, but obviously it also helps with fishing. I haven't done much fishing, though, but recently I concluded that I should. I've tried to figure out methods that would make it easier to spot finned and sashimi variants.

For harder grids i also used a kind of Medusa, where i just filled in 2-way-possibilities in the cells (very efficient for the right starting points). But this does need a lot of erasing, if or if not it was successful.


I use GEM (with my own markers) for that, as it's like Medusa on steroids and about as easy to use (or even easier, because it shows the resulting inferences explicitly). I used to do it on my main grid but it was restricted to one seed point. That's why I recently moved it to a helper grid as well. I originally dreaded that because I thought I'd have to copy the whole grid to it reliably, but it turned out to be unnecessary. I just add the GEM markers into an empty grid and there's no extra work at all.

So i am curious. And i wonder, if such marking would not be interesting for programs.


Me too. My system is probably a bit too detailed and tedious for most people want to adopt into their manual style (fully at least), but it might be helpful on a software solver. I'm actually planning to code one at some point but I'm not that great with graphical coding. It would be easiest to integrate into an existing solver like Hodoku, but is that open source? I noticed it has this ColorKu mode which superficially resembles my system a little bit. I also use circles to denote candidates but I have the various link markers inside them. It should be really easy to add those markers into that ColorKu mode.

I am all but glad about what the ones i know offer. Apart from highlighting they just tell me, how they could/would do it. This way they spoil the fun of finding it myself.


That's why I chose to develop my system instead of using existing software tools. I don't find their (the few I've seen) chaining support very good, especially for more complex chains.

btw i find, you should change the title of this thread. It only started with xy-loops.


Hmm. Can I do that? What should the title be anyway?
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Re: What's wrong with XY-loops?

Postby SpAce » Mon Oct 09, 2017 12:15 pm

eleven wrote:Nice to read an emotional debate from native speakers - it improves my English.


Well, I'm not a native speaker, but thanks I guess :) The last thing I would have wanted in a logic-oriented forum was an emotional debate.

SpAce, please forgive David, his skin is getting thinner and thinner (for the moderator: i say this with sympathy for David).


I have no problem with forgiving. I'd like to put this crap behind us and concentrate on discussing sudoku, preferably with David as well, but I don't think he shares that sentiment. It seems to me he thinks only one of us has anything to apologize for. With that I have a slight problem.
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Re: What's wrong with XY-loops?

Postby eleven » Mon Oct 09, 2017 4:43 pm

You can change the title by editing the first post. But it is hard to find one for this mixture of topics.
Maybe the moderators can do the work and collect the posts relevant for manual solving markups into a new thread.
For now i suggest: "XY-loops, Arto's fall and manual solving markups"
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Re: What's wrong with XY-loops?

Postby eleven » Mon Oct 09, 2017 5:27 pm

SpAce wrote:It would be easiest to integrate into an existing solver like Hodoku, but is that open source?

Yes, i saw that the source is available at sourceforge (https://svn.code.sf.net/p/hodoku/code/HoDoKu/trunk/), it is published under the GPLv3 licence.
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Re: What's wrong with XY-loops?

Postby SpAce » Tue Oct 10, 2017 2:27 pm

David, you challenged me to justify my point of view, and I did. You, however, haven't answered my questions, one of which I've now asked twice. Like I said, I'm willing to apologize for my admittedly harsh words, but it's not a one-way street. For that to happen I'm expecting you to either accept that I had a bit of reason to feel insulted or successfully argue that I completely misunderstood everything you said. So far you've picked neither of those options. Yes, I probably misinterpreted some parts but apparently not the most significant bits that released my venom. I'm not proud of what I wrote but I can't fully take it back either unless you give me a reason to do so.

In the mean time, let me explain my point of view a little better. I already explained that I wasn't very happy about your insinuations (or what I perceived as such) about my integrity. That was a red line for me. More generally but less seriously I got a little impatient with your constant assumptions during the discussion. A few examples:

David wrote:I appreciate that you are just gathering ideas at the moment

However both approaches make it straightforward to keep the mark-ups up to date - the problem you are trying to overcome.

Surely to say your system is "extreme-capable" is premature


The first two assumptions were obviously benign, but still a bit annoying because they weren't exactly accurate. In general, assuming a lot about your discussion partner is annoying. The third one was the most puzzling, however, and this is now the third time I'm asking you to explain it.

The trending assumptions were one reason why I called your style patronizing. Ironically, in your first, more conciliatory response, you apparently assumed (again) that I meant something else:

I'm sorry that you find my writing style pedantic, that was never my intention.


I never used the word "pedantic" which means quite a different thing. Your style may have hints of that too but it doesn't annoy me nearly as much, nor did I say it. I may be pedantic too. I meant what I said: patronizing (="treat with an apparent kindness that betrays a feeling of superiority"). Don't get me wrong -- you're fully justified to feel superior in this context. You're obviously a far superior sudoku solver than I may probably ever hope to be, and so is everyone else who's kindly answered my questions. I'm really grateful for all the advice I've gotten from all of you, and I've tried to express it too.

However, that's exactly why I don't really understand why you'd feel the need to -- ever so subtly -- put me in my place, so to speak. I think I have a pretty realistic idea of my "ranking" here, and I don't think I've given reasons to believe otherwise. That doesn't mean you'd be wise to automatically dismiss my one claim as hubris. That's just arrogant, and arrogance is the opposite of an open mind. A true master knows that he doesn't know everything, and that even complete beginners may sometimes have decent ideas he hasn't thought himself. Any other attitude discourages creativity and the free flow of ideas.

I am not interested in getting involved in heated arguments.


Me neither, but here we are. If such an unpleasant thing does happen, like it unfortunately did, I would prefer both sides to actually argue their sides so something could be learned and similar episodes avoided in the future. On my part, I've tried to clarify the reasons for my reactions as well as I can. I'd be grateful if you did the same.

I'm now too old to care much.


Apparently not. It's kind of funny that you said you didn't care but still expected me to understand that I had a 36-hour deadline to take back what I said. I usually take people's words at face-value, unless there's a historical or other reason to suspect hidden meanings. That's why I originally chose not to throw any more gas into the fire. I thought that's what you wanted, but obviously I totally misread you.
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Re: What's wrong with XY-loops?

Postby SpAce » Tue Oct 10, 2017 9:09 pm

David, I'd like to cover one more thing I kind of skipped earlier. After that it's up to you to decide whether you want to argue your side or not.

You wrote:To pick on the point that triggered your venomous tirade:
I wrote: I also find it interesting that you've now made ad hominem attacks on three different people in one thread (first John Welch, then Arto Inkala, and now me).

I challenge you to justify that. If any-one is leaping to conclusions it is you.


I didn't fully cover that part in my first response because I had to reevaluate whether you were at least partially right to question that. Let's see. What should be uncontested is that you made negative insinuations about all three of us, which is what I really meant, but that doesn't necessarily mean they were ad hominem attacks (as it requires a related argument to be present). I'm using the Wikipedia definition as a reference: "Ad hominem is where an argument is rebutted by attacking the character, motive, or other attribute of the person making the argument, or persons associated with the argument, rather than attacking the substance of the argument itself."

About John Welch (Sysudoku guy):

Okay, that's a hard call. You didn't say anything negative about him directly, but you did admit to sharing StrmCkr's negative experiences. Note that I don't doubt those experiences at all. I could have predicted them just by reading his blog. However, I consider that irrelevant as far as my original question was concerned -- whatever his faults, that doesn't necessarily mean he's wrong about something (assuming someone can decipher what he's saying). Problem is, you didn't really address my question (without prompting), so there was no argument present -- hence no ad hominem. Then again, it could also be interpreted as though you dismissed my question based on the source I presented and not the content of it. If so, that would sort of qualify for ad hominem, but I guess we'll never know. I must give you at least half a point on this one.

About Arto:

Based on what we know, your recommendation to search his name couldn't be interpreted as anything but a negative character reference. However, there was no explicit argument present, so I guess it can't be called an ad hominem attack, unless you wanted to imply that Arto's puzzles couldn't be trusted. Probably not. I give you a full point for this.

About me:

You appeared to express thinly veiled suspicions that I was on the same path as Arto and others who'd allegedly made exaggerated claims. That's guilt by association which is a type of ad hominem attack. I give myself a full point on this.

So, all in all, I think you win this one. I concede that I probably used the term ad hominem a bit too liberally. It doesn't disqualify my other points, however.

Now, this is what I call pedantic :D
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Re: What's wrong with XY-loops?

Postby David P Bird » Wed Oct 11, 2017 6:01 am

SpAce, What amazed me was that, after the welcome I had given you, you considered that I had turned against you and was belittling your work. Without pausing to reflect, you shot from the hip and falsely accused me of making personal attacks on not only yourself but also John Welch and Arto Inkala. You then continued with a very spiteful but quite revealing appraisal of what you thought of me in contravention of the forum's guidelines.

Although I was bristling, I posted that I was only trying to warn you to avoid using terms that could affect how you were received on the forum. However you chose to believe that this was a lie, so never made any retraction and let your original post stand. That was (and still is) intolerable to me. I don't know if your 'alternative facts' defence is a contrivance or a genuinely held belief, but that's not important, it doesn't wash. If it helps you, I will admit some of my wording was clumsy, but that's as far as it goes.

Although I reported your post, for whatever reason, no action has been taken. It seems that the standards of old are no more and I am now the odd man out.

In my dotage, the time I can spend on Sudoku is reduced, and I want it to be a source of pleasure rather than anguish. So, I'm sorry, but I will only engage with people who I judge have ethical values closer to mine than those of Donald Trump. As the Russian saying goes, 'I've stepped on that rake before'.
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Re: What's wrong with XY-loops?

Postby SpAce » Wed Oct 11, 2017 3:25 pm

David, I don't really know what to say anymore. I offered you an olive branch but you threw it away. You also effectively removed any good will I had left by playing the ridiculous Trump card. You're right that in my frustration I shot from the hip and took a huge risk of missing my mark, but it doesn't seem that I actually missed that much. My original concern was summarized in this question "Are you perfect, David?", and I think you have answered it. Glad we got that out of the way, because I've learned that it's really hard for us mere mortals to have mutually respectful conversations with perfect people.

You say you only wanted to warn me to avoid terms that could affect how I was received on the forum. I can buy that your intentions were friendly, but that doesn't really change much. Was it your duty to concern yourself with my communicating style that, until then, should not have affected you personally? Why not treat me as an adult who had the right to make his own mistakes? Anything else smells of patronizing and controlling, and I consider them disrespectful, even when done for benign reasons.

We all have different personalities and, among other things, they affect how we communicate. As long as I wasn't way out of line you might as well have let me be myself, and we could be discussing sudoku now. Are you sure you'd like it the other way around yourself? As far as I can tell, you don't take it so well if someone criticizes your way of communicating (which I only did as a return gift), so what makes you think others should be grateful to be on the receiving end? In my experience perfect people tend to have a bit of double-standards in the departments of giving and accepting criticism.

I leave it up to the moderator to decide what to do with my offending post and what judgment I deserve. Like I've said, I'm not particularly proud of what I wrote in the heat of the moment, but it's history that happened. Unlike you just suggested, I don't like to alter facts, even if they're not flattering myself. Also, there's still that one open question in that post that you've repeatedly refused to answer. I let it stand there as a reminder in case you want to reconsider at some point. My door remains open.
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Re: What's wrong with XY-loops?

Postby eleven » Wed Oct 11, 2017 9:53 pm

David P Bird wrote:... I want it [Sudoku] to be a source of pleasure rather than anguish. .

But instead you are still biting newcomers and i can't see any interest by you in that topic (which seems to be finished for you with GEM).

What's wrong with David P Bird ?
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Re: What's wrong with XY-loops?

Postby tarek » Wed Oct 11, 2017 11:26 pm

ArtoI's Etana(Escargot, Snail) was indeed the hardest known sudoku puzzles for a very brief period of time. http://forum.enjoysudoku.com/the-hardest-sudokus-t4212-510.html. The morphology followed strategies that we were using to generate hardest puzzles at the time. It was very quickly surpassed by harder puzzles. It is now no where near the hardest. The current puzzles posted in the patterns game would trump any snail :lol:

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Re: What's wrong with XY-loops?

Postby JasonLion » Sun Oct 15, 2017 7:09 pm

If you want to talk about "the hardest known puzzle", it is important to understand that there isn't any generally agreed upon system of measuring puzzle difficulty. Any experienced Sudoku solver has an intuitive understanding of what makes a puzzle difficult, but quantifying that intuition has turned out to be essentially impossible. Dr. ArtoI's puzzle ranking system turns out to have some dramatic deviations from the common intuition about what makes a puzzle difficult, as does every other system proposed to date.

As a practical matter, rating difficulty of puzzles is extremely useful, so on this forum we have generally agreed to use Sudoku Explainer's difficulty ratings when difficult ratings are needed, but SE ratings have significant deviations from intuitive understanding (as do all proposed systems to date) so that should not be taken to imply that SE ratings have any absolute meaning. The way SE rating work is well documented and well understood and each individual is expected to compensate for the difficulties with the SE rating system on their own when interpreting SE ratings.

So, anytime someone talks about "the hardest known puzzle" you need to ask what ranking system they are using for determining "Hardest" and realize that no matter which system they are using that it won't correspond to your intuition very well, if at all. This also means that using terms like lying or fraud is fraught with difficulty given that the domain is so ill-defined. Likewise, SpAce, you shouldn't use words like extreme in the context of Sudoku as if "extreme" has a well-defined meaning, it does not. There are many many systems of ranking puzzles and they all use/define the words like "easy", "hard", and "extreme" differently.
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Re: What's wrong with XY-loops?

Postby eleven » Mon Oct 16, 2017 10:01 pm

In fact, SE is not up-to-date, but as i see it, only for three more or less relevant reasons, regarding hard puzzles:
the lack of not using ALS's, JExocet's and SK-loops.
For hardest puzzles, personal manual preferences don't play much role at all (with a few exceptions).
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Re: What's wrong with XY-loops?

Postby SpAce » Tue Oct 17, 2017 11:24 pm

JasonLion, that was an excellent piece of philosophy! I couldn't agree more. Difficulty is a subjective and relative matter, just like morality. If "good" and "evil" have no meaning without a reference (and there's no absolute reference that everyone could agree upon), how could "easy" and "difficult" have absolute meanings either? I guess that means ArtoI can make any claims he wants as long as he makes it clear that he's using his own ranking system which is just one of many.

JasonLion wrote:Likewise, SpAce, you shouldn't use words like extreme in the context of Sudoku as if "extreme" has a well-defined meaning, it does not. There are many many systems of ranking puzzles and they all use/define the words like "easy", "hard", and "extreme" differently.


I understand. I learned a long time ago that if I go to my local book store and buy any "Extreme Sudoku" booklet it most likely has only basic puzzles. I just presumed that I'd made it clear in an earlier discussion what kind of minimum level I meant when I used the term "extreme". My reference for that term was taken from Andrew Stuart's SudokuWiki solver and its ratings. Being a newbie, I thought those levels (..., tough, diabolical, extreme) were at least somewhat standard in the community, but I guess I was wrong.

Of course even that definition of "extreme" is highly ambiguous because it actually covers everything up to the most difficult puzzles. I surely haven't implied that I or my system could solve the highest levels of that range at this time (or probably ever). The way I see it is kind of like in martial arts a "black belt" doesn't tell much without the actual degree number, but it does tell something about the minimum level, and even a first degree black belt is still a black belt.
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