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World's Hardest Sudoku 179

Posted by Soulskill
from the addition-is-tricky dept.
jones_supa writes "A Finnish PhD in mathematics, Arto Inkala, has allegedly created the world's toughest sudoku puzzle. 'There's no straightforward way to define the difficulty level of a sudoku. I myself doubt if this is the hardest in the world, but definitely harder than my previous ones,' Inkala sets off humbly. The news agencies around Europe are nonetheless excited (Google translation of Finnish original). The particular difficulty in this version lies in the number of deductions you have to make in order to fill in a single number on the grid. 'It is a common misconception that the less initial numbers, the harder the puzzle. The most challenging ones have 21-25', the creator adds."
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World's Hardest Sudoku

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  • by Wild Wizard (309461) on Wednesday July 04, 2012 @04:59AM (#40539527) Journal

    I'm printing it out now, nothing like someone claiming the impossible to make you want to try and prove them wrong.

    • see you in 5 minutes after you realize it actually is kinda hard
      • by Chrisq (894406) on Wednesday July 04, 2012 @05:28AM (#40539661)

        see you in 5 minutes after you realize it actually is kinda hard

        If he's anything like my son he'll not come back until he's solved it - or has to be dragged away from it!

  • by BlackPignouf (1017012) on Wednesday July 04, 2012 @05:12AM (#40539585)

    Who cares about the Higgs boson?
    Sudoku is real science!

  • It wasn't hard at all - My Galaxy S3 with Google Goggles solved it in under 3 seconds.

    Of course, I personally, don't even know the rules of Sudoku. :-P
  • Would love to give it a shot!
  • Not so hard (Score:5, Insightful)

    by Laxator2 (973549) on Wednesday July 04, 2012 @05:18AM (#40539613)

    The following crappy solver I cobbled together solved it in 33 seconds under Cygwin:
    https://github.com/fhstoica/NumbersAndLettersSudokuSolver [github.com]

    Check out Peter Norvig's web site for a very elegant solver and look for the "impossible puzzle" if you really want a difficult one:
    http://norvig.com/sudoku.html [norvig.com]

    • Re:Not so hard (Score:5, Interesting)

      by BlackPignouf (1017012) on Wednesday July 04, 2012 @06:01AM (#40539773)

      Cool link, thanks!
      I love this quote :
      Ben Laurie has stated : Sudoku is "a denial of service attack on human intellect".

    • by Anonymous Coward

      I wrote my own solver a few years ago and got this;

      812 753 649
      943 682 175
      675 491 283

      154 237 896
      369 845 721
      287 169 534

      521 974 368
      438 526 917
      796 318 452

      I do think you need to work a little on your runtime;
      $ time ./CSPsudoku hardest.txt hardest_solve.txt
      real 0m0.041s
      user 0m0.027s
      sys 0m0.008s

    • 33 seconds? No offense, but methinks you need to review your algorithm, because either you

      a) have a bug, or
      b) have totally over-engineered the implementation of the algorithm.

      My solver in 106 lines of C++ solves it in by *brute* force in less then 1 second. I wrote this back on 6/6/6.

      There is also a 141 _byte_ sudoku solver in Javascript that someone else wrote. Check
      http://www.reddit.com/r/javascript/comments/ku0mb/tweet_sized_sudoku_solver_in_javascript/ [reddit.com]

      My input file is:
      8-- --- --- // stupid lameness f

  • Some excellent news about the Higgs particle is announced more than 0.5 hr ago, I come hear to Slashdot to read what everyone has to say.....and the font page headline here is about motherfucking Sodoku. Really? Are the editors asleep, apathetic or all of the above? Fuck.
    • by Anonymous Coward

      http://www.theonion.com/articles/caltech-physicists-successfully-split-the-bill,2037/

    • by Anonymous Coward

      Welcome to Slashdot, you must be new here. Slashdot is more newspaper-like than most other web resources, as it provides us with yesterday's news today!

  • by unitron (5733) on Wednesday July 04, 2012 @05:21AM (#40539627) Homepage Journal

    "'It is a common misconception that the less initial numbers..."

    When you have discrete, countable units, such as the symbols, in this case numbers, already present on the Sudoku grid, you have more or you have fewer .

    When it's something you can't count, you have more or you have less.

    I have more 16x16 grid sheets printed up for hexadecimal Sudoku, because those are the ones I copy from 'the net'.

    I have fewer (currently none, actually) of the 9x9 (4 to a page) printed because I quit doing the 1-9 version sometime back.

    I'm going to try this one out, but suspect it will turn out to be the type that lets you get just so far with logic and then leaves you no alternative but trial and error, just like the Saturday ones in a certain Raleigh newspaper.

    • by julesh (229690)

      I had a quick look at it a couple of days ago, and it seems to require you to resort to trial and error from the very first step. I figured I had better things to do with my life.

    • by alexhs (877055)

      I'm going to try this one out, but suspect it will turn out to be the type that lets you get just so far with logic and then leaves you no alternative but trial and error, just like the Saturday ones in a certain Raleigh newspaper.

      I've tried my own sudoku solver on it which puposefuly doesn't do the guessing/backtracking stuff. It didn't solve one single number. So, you might not want to waste time on trying by hand.

    • by wvmarle (1070040)

      The description of the puzzle by The Telegraph already says you have to resort to trial-and-error (unless you can think ahead ten moves in your head):

      Instead of being able to spot where a number goes based solely on the boxes that have already been filled in, most moves will face you with two or more spaces where a number could fit.

      Only one of these is correct, but to find it you must examine all possible options for your next move and perhaps the move after that, continuing in the same vein until all but one potential route results in a dead end.

    • Reposting what an AC said, because these things matter ;)

      Wrong. Fewer can never apply to [non]-count nouns, but less can apply to either.
      http://en.wikipedia.org/wiki/Fewer_vs._less [wikipedia.org]

      I could care fewer about people mixing these two up.

      • by unitron (5733)

        Thank you for your re-post, these things do matter.

        Wikipedia is bound to get something wrong somewhere along the line, as are the sources they cite.

        This seems to be a case of both.

        For instance, in one of the examples they give "However, in the case of singular but discretely quantifiable nouns as in "one less cup of flour in this canister", the adjective less should be used, the prescriptivists argue.", the actual wording should be "one cup fewer of flour", or "one cup of flour fewer".

        As for their example o

        • they make the mistake of thinking that the number of results has changed, when it is actually their individual and collective promise which is less.

          I'm not sure I see a mistake - isn't it exactly that ambiguity which they're trying to demonstrate?

  • The site that I used was http://www.sudokuwiki.org/sudoku.htm [sudokuwiki.org] . One of the feature of this site is to tell you the possible techniques that you can use on solving a given sudoku. Unfortunately, when it analyzed the sudoku found in the fine article it could not tell what technique to use. I used a sudoku app on my java phone to record the sudoku but it wont accept puzzles with less than 22 givens, Yeah the site brute forced the puzzle and solved it but it cannot solve it by normal means.

  • slashdot is pretty much done at this point, right? it always was a site made by its contributors, and the articles always were misleading and sensational.

    but at least they were topical.

    today's top story: worlds hardest sudoku

    summary: not actually the world's hardest sudoku.

    more at 11.

  • I am putting you all on sudoku watch.
  • Not the hardest one (Score:4, Informative)

    by eulernet (1132389) on Wednesday July 04, 2012 @06:17AM (#40539843)

    Definitely not one of the hardest sudokus.

    There is a tool to compute the difficulty of a puzzle, and you can also download a massive database of hard sudokus (5 millions+):
    http://code.google.com/p/skfr-sudoku-fast-rating/ [google.com]

    For reference, this one is rated 10.7:
    http://forum.enjoysudoku.com/the-hardest-sudokus-new-thread-t6539-420.html [enjoysudoku.com]

    BTW, there is a database of 31804 puzzles of difficulty 11 and above:
    http://gpenet.pagesperso-orange.fr/downloads/hard11.zip [pagesperso-orange.fr]
    Exactly 7 have a rank of 11.9.

    • Definitely, eh? Did you even read the first sentence of the summary?

      'There's no straightforward way to define the difficulty level of a sudoku. I myself doubt if this is the hardest in the world, but definitely harder than my previous ones,' Inkala sets off humbly

      [Yes, this completely contradicts the first link and the article title which both say this is the hardest Sudoku in the world. Terrible editing. Again.]

      • Actually that's an interesting problem in itself. "What constitutes hard" in a sudoku puzzle? Maybe he meant that there are differences of opinion on whether one type of concept is numerically more difficult than another combined with the depth of the process, aka a harder extension of an easier concept vs an easier extension of a harder concept.

        • by jfengel (409917)

          It's very literally a matter of opinion. Sudokus are trivial for computers. "Hard" for a sudoku means "not amenable to the kinds of tricks a human brain throws at the problem", and since brains vary, hardness is a matter of opinion.

          Many human strategies are known and categorized; a puzzle that is amenable to none of them is "harder" than one that isn't. Within the range, though, it will vary from person to person.

          It's kinda like the inverse of a Turing test. Too bad we can't use it for a reverse CAPTCHA: Ca

  • Wiped out my Android, started augemented reality solver, pointed camera at monitor at it took 45ms to show me the complete field.

    Makes Sudoku feel rather pointless.

    • by gl4ss (559668)

      sudoko IS pointless.

      much more pointless than folding paper cranes. has to be pretty pointless to top that off.

  • If you read the "official" difficulty level, it's 11.

    Yes, it goes to 11. You know it's harder when it's one more.

  • mainly because it's not deterministic, it's under-defined (or whatever it's called). If you can solve it with (at least) two different solutions, this PhD ought to give back his degree and go back to school for making such a claim without actually checking into the facts ... otherwise, here's my "hardest sudoku": "1" at E5. Send back the correct solution I have here.

    Btw, any Sudoku solver that goes by deterministic rules without using guesswork/brute force should stop after eliminating some numbers from emp

  • I can make a tougher puzzle than that. It's just a regular Sudoku, but if you make a mistake, I'll kick your shin.

  • One reason that you cannot solve this puzzle without making assumptions is that it has more than one solution!

    One of the comments in the FA provides a solution to the puzzle, which is different from the solution I found using a sudoku solver that I wrote back when I realized that I was spending too much time on these puzzles.

    When stuck, my solver starts selecting random values among the valid possibilities, backtracking if the guess does not lead to a solution. This makes it possible for the solver to solve

  • My program for converting a Sudoku to an exact cover took 0,015 second. My program for solving an exact cover took 0,031 seconds. This time is including producing 23K output file containing log and result. This mainly due to the fact that the exact cover can be solved with only applying logic reductions, taking two columns and see if there an implication. If this is the case, all rows that do not contain a 1 value in both columns can be removed. This results in 60 rows giving the solution to the sudoku, one
  • /Users/morten/Project/Sudoku> time sudoku worlds_hardest.sudoku
    |8 1 2|7 5 3|6 4 9|
    |9 4 3|6 8 2|1 7 5|
    |6 7 5|4 9 1|2 8 3|
    |1 5 4|2 3 7|8 9 6|
    |3 6 9|8 4 5|7 2 1|
    |2 8 7|1 6 9|5 3 4|
    |5 2 1|9 7 4|3 6 8|
    |4 3 8|5 2 6|9 1 7|
    |7 9 6|3 1 8|4 5 2|

    real 0m0.083s
    user 0m0.073s
    sys 0m0.003s

  • The challenge to me of this type of puzzle is to use the tools and skills available to me and to write my own solver program for it on computers with compilers that I have available. Like I've done for Maverick Solitaire in the past. And to have an efficient solution that doesn't need the Amazon Cloud to solve every possible combination through poor programming. The satisfaction of having my own program reach the answer well exceeds using anyone else's solver program.

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