This is the archive of the old posts from Djape .Net, more or less as they used to be. Please go to to see the new website.

Why do I make puzzles? A brief history of

A short answer to that question would be: because I enjoy doing it! … but here is a long version, too. Back in the summer of 2005 I realized that it’s about time I had my own website. At that point I had been computer programming for years and it kinda made sense that I should have a piece of the www for myself. So I bought the domain Why that one? Well, because .com was already taken (and still is and it’s still “under construction”!!!). Djape is popular among people with my first name. 🙂 Oh, and why Djape? Well, because that’s my nickname. Actually, my nickname is Pedja (just like recently retired All Star Pedja Stojakovic of the Dallas Mavericks), but Djape is a common variation (it’s an anagram, the two syllables are reversed). I also thought DJ APE, no matter how stupid it sounds, would be something easy to remember for English speaking folks. Here you can hear me pronounce my nickname. Yes, that was my voice, recorded a few minutes ago. Anyway… now that I had a domain, what should I put on it? Earlier in 2005 I learned about Sudoku and actually I had already started creating my own software for solving and generating Sudoku puzzles. I thought, this could be interesting to other people, so I decided to put it on my website. Now, I’m a software developer, not a web-designer, so I installed a WordPress blog and started “blogging” about Sudoku. At that time a number of Sudoku websites already existed and I realized that I had to offer something different. I added Diagonal Sudoku and Sudoku of different sizes (4×4 or 16×16 etc) first. However, in early September 2005, while surfing around, I stumbled upon The Times of London website and learned about Killer Sudoku. It immediately impressed me as a cool new variant of Sudoku and I immediately started working on my algorithms, first for solving Killer Sudoku and later for creating them. It took about 10 days to implement a Killer Sudoku generator. On September 26, 2005, I started posting Daily Killer Sudoku puzzles. On September 27, the Killer puzzle I posted caused some controversy. Why? Well… at that time my Killer Sudoku generator algorithm was still far from perfect. It used trial and error to create&solve puzzles. It even had two singleton cages, something that I would NEVER publish now! Frankly, I wasn’t paying much attention to which puzzles I was posting on a daily basis, I spent most time working on the algorithm. And that one puzzle changed the course of this website. People showed interest, The Times’ puzzles were too easy, everybody wanted something more difficult. The September 27 puzzle was valid in the sense that it had a solution and it had only one solution, just like all other puzzles I ever posted. If you click on the above link to Sep 27 puzzle, you can still access it, but don’t try to solve it. It’s almost impossible. It was the first INSANE Killer Soduko puzzle. Some visitors had made a lot of progress on it, and a guy called udosuk provided a solution to the September 27 Killer Sudoku puzzle with only minor trial and error. By the way, if udosuk is still visiting this website or if anyone has his contact, I’d like to get back in touch. In the meanwhile, I perfected the Killer Sudoku generator and starting posting daily puzzles only of top quality. There was no more trial and error (although many visitors kept asking for INSANE Killers), every puzzle was human solvable, mostly by using variations of the Rule of 45 technique. Later I started adding a personal touch to the puzzles, by interactively designing them and only letting the computer do the hard work. Anyhow… the Sep 27 puzzle and the people’s reaction to it, gave me incentive to keep making puzzles. The number of visitors to the website increased, I was interacting with people from all over the world from the comfort of my home (there was no Facebook back then! :)) and it only made sense to continue doing it. Later that year I added overlapping Sudokus to my software (much easier implemented than Killer Sudoku) and among them, of course, Samurai Sudoku. Eventually, publishers started contacting me. First was Via Optima from Portugal, who published my first book of Killer Sudoku puzzles. Then some lady from the UK sent an e-mail and later called me on my phone, also showing interest in publishing Killer Sudoku, but then disappeared. Finally, through a guy from another Sudoku website, I got in touch with the editors of The Washington Express who wanted Samurai Sudokus in their paper. They started printing Samurais in December 2005. Shortly afterwards, The Washington Post followed suit and in January 2006 published my first Samurai Sudoko puzzle. Later a publisher from Finland asked me if I could made Hanjie puzzles for them. And I did. These are, of course, hand-made puzzles, only checked for validity by my software. Other publishers followed. I started self-publishing books on Amazon. This was an important step and I am really happy to read your reviews of my books. Please, if you buy my books, please review them on Amazon, it means so much to me. There’s more history, but even this is more than just a “brief” version, so I’ll leave the rest for some other time. I make puzzles and puzzle books because it’s my passion. I do it all by myself, even the book covers! That’s why they look so childish (except for a few which were done by a friend who is a designer). Most of all I enjoy programming new types of puzzles into my software and communicating about new puzzles and my books with people who buy them and solve them. This is why I engage so much in social networks. Your reaction is important to me and it does influence my decision. Add me on Facebook, Twitter or Google+. And share with your friends! 🙂 When I publish the puzzles, I either interactively create them using my software, or generate them automatically and then hand pick them, verify the solving paths and check the difficulty. Most puzzles by other authors are too easy. I know, that because I create difficult puzzles the target audience is smaller; but I believe that the joy of having solved a difficult puzzle is much much bigger than solving a trivial one. Hence, my joy as the author also grows! I prefer to create puzzles that will get a review like this one. This must be the longest post ever on this website. It was intended as a “brief history” but it turned out much longer than I anticipated. Oh well… 🙂 I felt like writing it. Hope it wasn’t too boring!
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  1. Roland
    Posted January 15, 2012 at 7:19 am | Permalink

    Interesting story, Pedja. I’m definitely curious to hear the rest of it. Especially why you’re not available on Lulu anymore with your latest books! I live in South Africa and the electronic format is the only option (well, practically spoken, I don’t have a Kindle). Any chance that your puzzles will be electronically available again in PDF format in the near future?
    Like most people I started with ordinary Sudokus, but I got sort of bored with them. Not because of the lack of difficulty, but I just lost interest in the format. Presently my diet is mainly Killer Sudokus, Jigsaws, Calcudokus and all Samurai/Flower/Butterfy/Diagonal/Zero/Twodoku/Tridoku versions.
    I enjoyed in particular the Hidokus/Hidatos and Slitherlinks, but you don’t seem to publish those anymore? Sudoku Straights, Greater/Less Sudokus complete my daily intake.
    I would also like to know how you determine the degree of difficulty. To me IQ sometimes seems relatively Easy and vice versa.
    That brings me to your notorious Insane killer sudoku of 27 september, which I just solved relatively easily with indeed minor trial and error. Personally, I would rate the difficulty as IQ. I still have an unsolved Insane which you published a few months ago. That one is really Insane to my opinion.
    Anyway, thanks for all your great efforts!

    • Posted January 16, 2012 at 1:40 am | Permalink

      Thank you for your comment Roland. I do intend to write a little bit more on this topic, at some point.

      New PDF books are not available for suspected copyright infringement issues. I do travel a lot, but I can’t visit every country and buy every puzzle magazine out there. It is unfortunate that people who enjoy my puzzles worldwide are deprived from them because of this.

      It is a tough job determining the degree of difficulty of a classic Sudoku puzzle, let alone a Killer Sudoku. At the end of the day, does it matter? If you enjoyed solving an Easy puzzle because it was more difficult than “Easy”, what’s more important? 🙂 I really think the difficult of a Killer Sudoku depends on who’s solving it, because of the complexity of solving methods involved in the process. Perhaps some technique is simple to you, but a challenge to others. Further, and perhaps more importantly, there are numerous solving paths: if you start at one end, the puzzle is difficult; had you noticed another way to crack the code, it would’ve been much simpler.

      Anyhow… to answer your question: the rating of Killer Sudokus I publish is a complicated process: it depends on the solving techniques my algorithm uses in order to solve them. “Cage splitting” techniques are ranked most difficult. It also depends on the number of times a certain technique is used and the number of initial innies/outies (the more innies/outies, the easier the puzzle is). I haven’t changed this process in years, perhaps I should? 🙂

  2. Posted January 17, 2012 at 6:53 am | Permalink

    Thanks for your story; as a programmer myself I found it interesting. Like Roland, my boredom with regular Sudokus wasn’t that they were too easy, but that they’re all the same after a while. Every one requires the same basic solving process, and either you solve it or you get to a point where you don’t know the advanced method needed and you’re stuck. (I don’t see the point of trial-and-error; I’ll move on to another puzzle instead.)

    Your jigsaws, flowers, and other varieties solve that problem for me. Maybe each variety could be boiled down to its own repeated solving process like ordinary Sudokus can, but for me at least, each puzzle seems different. One jigsaw will have some long pieces that send me thinking in one direction, while another requires tracking down a bunch of complex innies and outies. Throw in killers, the outside puzzles, the ones with some squares blacked out, diagonals, and all the combinations thereof, and I never get bored. (The only ones I don’t like are the tridokus. I don’t know why; they just leave me cold. Guess I’m just a square at heart.)

    Your free daily killer even inspired me to finally learn to do killers. I’d seen them in puzzle magazines, but never understood how to do them. Yours got me over the hump somehow, and I looked up some more advanced solving techniques, and now killers are my favorite kind — especially when combined with other variations like jigsaw or samurai.

    I know I’m going on and on here, but one thing I like about your puzzles, especially the killers (and zero killers), is that I can work on a puzzle for a while, get stumped and set it aside, and then come back the next day and see the next thing that I couldn’t see the last time. I don’t know why that is, but they really flow at a good steady pace for me, even when that pace is very slow, like one cell solved every 15 minutes. A puzzle can stay difficult for me — but not too difficult — for a surprisingly long time, sometimes even when I’ve gotten a good portion of the cells filled in.

    Lastly, have you ever considered releasing or selling your software? It sounds like the software doesn’t do the whole job; you mentioned that you do some manual work on the puzzles yourself. Still, I’d love to see how they’re created, and the algorithms that go into them. They have to be beyond anything I’ve done.

  3. Allison M.
    Posted January 17, 2012 at 12:24 pm | Permalink

    This is a very interesting read. Thanks for sharing, Djape.

    I encountered my first sudoku (it was one of yours) published in a newspaper left on a train headed into Washington D.C. I had about 30 minutes to spare, so I picked up the newspaper and went to the puzzle section. I had never heard of sudoku, but I love math and logic, so I read the directions and started. Luckily it was an easy puzzle, just right for a first try. The rythem and logic of it was very soothing and fun. It gave my brain a much needed workout! Now I buy the toughest books you publish, and love working on them. I have tried other sudoku publishers, but always come back to your website and books because the are the best in my opionion. Thanks for making them!!

  4. Marty Youngs
    Posted January 19, 2012 at 6:53 pm | Permalink

    It was a pleasure to actually “hear” from you and read your story about how this site all came about.

    I’m totally hooked on the killers. As said above, regular sudoku isn’t very fun after a while. Even a difficult one is more an ordeal than anything else. Jigsaws I just don’t get.

    I really can’t thank you enough for all the hours of pleasure you’ve given me. Especially from the Sunday ones. The first time I looked at one of those I thought “you have to be kidding!” But I finally tried one and – probably about six hours later – solved it. Now I look forward to them every week.

    I’m not sure what my sister’s feelings are for you, though. She was doing regular sudokus and I turned her on to killers. I showed her your site. I got a very snippy email from her at 5AM one morning that she had finally solved one of the Sunday ones and was going to bed. She wasn’t happy with me – or you. She’s probably gotten over it by now.

    I just solved Jan 8th’s last night. Like Aaron said, if you’re at the point you’re just going around in circles and finding nothing, put it aside. It’s amazing how you can pick it up, take a quick glance and see the glaringly obvious clue staring you in the face.

    I’ve always assumed that a given puzzle is more or less difficult depending on the number of possible routes to the solution. Is that right? An easy puzzle you can just pick anyplace and start, a really tough one you can take quite a while coming up with your first number. What I think’s unique about your puzzles is that even though you have most of the numbers filled in, it’s not just a cakewalk to the finish, you still have to work at it.

    Again, thank you for all you do. And keep them tough. Your readers are tough 😉

  5. Bob Park
    Posted April 23, 2012 at 10:18 am | Permalink

    Enjoyed the story of how you make your puzzles! I have done the Friday and Sunday killers for several years and bought the Zero Killer 101 maximum challenge to use on vacation. Some of these IQ puzzles are much more difficult than others.. A good share of the book IQ section went very well but a few gave me lots of trouble. Toughest was number 60 which I had to start several times. I also enjoyed the ones where the number boxes were on a diagonal. Hope to find more of those to do in the future.

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