One of my hardest grids

patrick84 · Posted: Tue Feb 08, 2011 10:45 am Post subject: One of my hardest grids

Hi all,

Here's a grid which I suppose to be VERY VERY hard, but solvable with known technics (no hypothesis) :
000005070001070200870000006096300820000090000047100950380000005005030700000004030

What do you think about it ? and.. how much time your solvers take to find the solution ? (i'm not sure it's an english sentence..)

JasonLion · Site Admin

SudokuExplainer rates that puzzle at 7.2, with 10 forcing chains. That is reasonably difficult, but not hard by the standards around here (which are extreme).

My brute force solver can solve it in under 1/1000th of a second, while my logical solver takes 6/1000ths of a second.

daj95376 · Posted: Tue Feb 08, 2011 5:34 pm Post subject:

Fun puzzle! Solution sans basics.

The (single/sole) generic chain is as close as it comes to difficult.

patrick84 · Posted: Mon Feb 14, 2011 10:19 am Post subject:

wow

thanks for your replies !

JasonLion how is made your logical solver ? mine takes 1/2 second to solve it (and describe each step) !!

JasonLion · Site Admin

My logical solver has been heavily optimized for speed, using bit vectors stored as ints, and other similar techniques, to optimize each of the logical solving techniques. My code needs to work well on smartphones with slow processors, so I had to put a fair bit of effort into making the code fast.

For examples of how I optimize things, take a look at JSolve, my open source brute force solver.

Once you get into the right mind set, it is simply a matter of thinking about how to best apply fairly straightforward optimizations to each of the solving techniques. Applying optimizations to may different solving techniques can be tedious, but it isn't magical or even especially difficult. It is simply a matter of finding a way to look at things that minimizes the search space and maps efficiently into machine instructions.

patrick84 · Posted: Wed Feb 16, 2011 12:47 pm Post subject:

Yep i've to make an effort with optimizations !

But making a good solver for a smartphone it's just amazing because there's no much memory and a very slow processor. Wow..