I moved the site over to a new server over the last week. It took me some time to get everything up and running again so the site was down during the whole process. Everything seems to be running OK now, although some people may occasionally be directed to the old servers over the next couple of weeks. If you find any bugs, please help me and report to webmasterATgiroffelDOTcom.

I finally found some time to implement pencil marks. They can be switched on or off with the check box on the left side of the puzzle. They’re switched off by default because it gives a rather chaotic look to the puzzle. But they can sure come in handy! You can switch them on and off while solving a puzzle, they’ll keep their values even when switched off.

The grid interface in the play section received some minor upgrades:

• border thickness is corrected in Internet Explorer
• font changed to Times instead of Sans Serif, looks better IMO
• numbers should be correctly aligned (vertically) in all main browsers now
• corrected an error in the mouseOver color switching

Meanwhile, there are about 150.000 sudoku puzzles in the database, but I have yet to start rating them before they will show up on the site.

My first implementation of the b2evolution php blog was a bit rude: it just displayed the posts, but all functions (comments etc.) led you right outside the site I cleaned it all up, hoping everything works now. Why don’t you try leaving a comment?

OK, there’s something I have to confess. Up until now there have been 10,000 puzzles in the database. But of those 10,000 only 76 were classified as “hard” … As I wasn’t quite satisfied with this, I started work on an algorithm to make harder puzzles. The algorithm worked, but took way to long to make a new puzzle. Our basic algorithm for solving puzzles works fine for valid sudoku puzzles (i.e. puzzles that only have 1 unique solution), but takes a lot longer to exhaustively search the entire solution space of any grid. After some research I found a java-implementation of Knuth’s Dancing Links algorithm (check wikipedia) by Stan Chesnutt (check his page here). I translated it to PHP (all my pages & algorithms are written in php so far), but after 3 evenings of debugging my code, I found that my php-translation of the Dancing Links algorithm was even slower than my original implementation. Stan’s original (java) code however, was very fast. So I decided to rewrite my php code to java, and use Stan’s classes for solving puzzles in the process.
I finally got the new classes to work last night, and I’m very satisfied with the results. My php program constructed about 100 puzzles in 1 hour on an Intel Core 2 Duo 1,8 GHz, way too slow to my liking. But the new java implementation takes only 3 to 4 minutes to make 100 puzzles on my old iBook G4 1 GHz. I haven’t even bothered installing the Java Virtual Machine on my Core 2 Duo PC yet, but I suspect it will construct the same 100 puzzles in less than 2 minutes.
All I have to do now is find a way to feed the java output to the MySQL database and I’ll be upping the 10,000 puzzles to 100,000 very soon. Then there should be about 700-800 “hard” puzzles.