I too did one back in April - and have modfied it to
read (and write) the current intput format.

Here's the puzzle that takes a *long* time - I don't
remember if it's even solvable, but my solver has
been running for over an hour so far ...
+-------+-------+-------+
| _ _ _ | _ _ _ | _ _ _ |
| 7 _ _ | _ 8 _ | 3 _ _ |
| _ _ _ | 7 _ _ | _ _ _ |
+-------+-------+-------+
| _ _ _ | _ _ _ | _ _ _ |
| _ _ 9 | _ 1 _ | _ _ _ |
| _ _ _ | _ _ 7 | _ _ _ |
+-------+-------+-------+
| _ _ 1 | _ _ 8 | _ 2 _ |
| _ _ _ | _ 2 6 | _ _ 1 |
| _ _ _ | 3 _ 5 | _ _ _ |
+-------+-------+-------+

The 4 problems we've been given (the orignal with the
quiz and the 3 new ones seemed to yield fairly
quickly.  I'm looking forward to see other
solutions and try them on this one.

Here's the results of running the 1st 4.

Vance

heron-linux:for i in rqi* ; do time sd2.rb $i ; done
Input
+-------+-------+-------+
| _ 6 _ | 1 _ 4 | _ 5 _ |
| _ _ 8 | 3 _ 5 | 6 _ _ |
| 2 _ _ | _ _ _ | _ _ 1 |
+-------+-------+-------+
| 8 _ _ | 4 _ 7 | _ _ 6 |
| _ _ 6 | _ _ _ | 3 _ _ |
| 7 _ _ | 9 _ 1 | _ _ 4 |
+-------+-------+-------+
| 5 _ _ | _ _ _ | _ _ 2 |
| _ _ 7 | 2 _ 6 | 9 _ _ |
| _ 4 _ | 5 _ 8 | _ 7 _ |
+-------+-------+-------+
Solution
+-------+-------+-------+
| 9 6 3 | 1 7 4 | 2 5 8 |
| 1 7 8 | 3 2 5 | 6 4 9 |
| 2 5 4 | 6 8 9 | 7 3 1 |
+-------+-------+-------+
| 8 2 1 | 4 3 7 | 5 9 6 |
| 4 9 6 | 8 5 2 | 3 1 7 |
| 7 3 5 | 9 6 1 | 8 2 4 |
+-------+-------+-------+
| 5 8 9 | 7 1 3 | 4 6 2 |
| 3 1 7 | 2 4 6 | 9 8 5 |
| 6 4 2 | 5 9 8 | 1 7 3 |
+-------+-------+-------+

real    0m0.060s
user    0m0.050s
sys     0m0.000s
Input
+-------+-------+-------+
| _ _ 2 | _ _ 5 | _ 7 9 |
| 1 _ 5 | _ _ 3 | _ _ _ |
| _ _ _ | _ _ _ | 6 _ _ |
+-------+-------+-------+
| _ 1 _ | 4 _ _ | 9 _ _ |
| _ 9 _ | _ _ _ | _ 8 _ |
| _ _ 4 | _ _ 9 | _ 1 _ |
+-------+-------+-------+
| _ _ 9 | _ _ _ | _ _ _ |
| _ _ _ | 1 _ _ | 3 _ 6 |
| 6 8 _ | 3 _ _ | 4 _ _ |
+-------+-------+-------+
Solution
+-------+-------+-------+
| 3 6 2 | 8 4 5 | 1 7 9 |
| 1 7 5 | 9 6 3 | 2 4 8 |
| 9 4 8 | 2 1 7 | 6 3 5 |
+-------+-------+-------+
| 7 1 3 | 4 5 8 | 9 6 2 |
| 2 9 6 | 7 3 1 | 5 8 4 |
| 8 5 4 | 6 2 9 | 7 1 3 |
+-------+-------+-------+
| 4 3 9 | 5 7 6 | 8 2 1 |
| 5 2 7 | 1 8 4 | 3 9 6 |
| 6 8 1 | 3 9 2 | 4 5 7 |
+-------+-------+-------+

real    0m1.626s
user    0m0.830s
sys     0m0.010s
Input
+-------+-------+-------+
| _ _ _ | _ 7 _ | 9 4 _ |
| _ 7 _ | _ 9 _ | _ _ 5 |
| 3 _ _ | _ _ 5 | _ 7 _ |
+-------+-------+-------+
| _ 8 7 | 4 _ _ | 1 _ _ |
| 4 6 3 | _ 8 _ | _ _ _ |
| _ _ _ | _ _ 7 | _ 8 _ |
+-------+-------+-------+
| 8 _ _ | 7 _ _ | _ _ _ |
| 7 _ _ | _ _ _ | _ 2 8 |
| _ 5 _ | 2 6 8 | _ _ _ |
+-------+-------+-------+
Solution
+-------+-------+-------+
| 2 1 5 | 8 7 6 | 9 4 3 |
| 6 7 8 | 3 9 4 | 2 1 5 |
| 3 4 9 | 1 2 5 | 8 7 6 |
+-------+-------+-------+
| 5 8 7 | 4 3 2 | 1 6 9 |
| 4 6 3 | 9 8 1 | 7 5 2 |
| 1 9 2 | 6 5 7 | 3 8 4 |
+-------+-------+-------+
| 8 2 6 | 7 4 3 | 5 9 1 |
| 7 3 4 | 5 1 9 | 6 2 8 |
| 9 5 1 | 2 6 8 | 4 3 7 |
+-------+-------+-------+

real    0m0.960s
user    0m0.450s
sys     0m0.000s
Input
+-------+-------+-------+
| _ _ 1 | _ 2 _ | 8 _ _ |
| _ 7 _ | 3 1 _ | _ 9 _ |
| 3 _ _ | _ 4 5 | _ _ 7 |
+-------+-------+-------+
| _ 9 _ | 7 _ _ | 5 _ _ |
| _ 4 2 | _ 5 _ | 1 3 _ |
| _ _ 3 | _ _ 9 | _ 4 _ |
+-------+-------+-------+
| 2 _ _ | 5 7 _ | _ _ 4 |
| _ 3 _ | _ 9 1 | _ 6 _ |
| _ _ 4 | _ _ _ | 3 _ _ |
+-------+-------+-------+
Unsolvable
+-------+-------+-------+
| 4 6 1 | 9 2 7 | 8 5 3 |
| 8 7 5 | 3 1 6 | _ 9 2 |
| 3 2 9 | 8 4 5 | 6 1 7 |
+-------+-------+-------+
| _ 9 _ | 7 _ _ | 5 _ _ |
| _ 4 2 | 6 5 8 | 1 3 9 |
| _ _ 3 | _ _ 9 | _ 4 _ |
+-------+-------+-------+
| 2 _ _ | 5 7 _ | 9 _ 4 |
| _ 3 _ | _ 9 1 | _ 6 _ |
| _ _ 4 | _ _ _ | 3 _ _ |
+-------+-------+-------+

real    0m0.046s
user    0m0.030s
sys     0m0.010s

On Sat, 2005-08-20 at 23:41 +0900, Karl von Laudermann wrote:
> In article <15hhca6w53u5l.dlg / example.net>,
>  Chris Game <chrisgame / example.net> wrote:
> 
> > Karl von Laudermann wrote:
> > 
> > > Heh. I already wrote a Sudoku solver back in May; all I have to do
> > > is change the input/output format a bit to match the quiz
> > > example. In the meantime, here's a harder puzzle to test your
> > > programs with:
> > 
> > Why is it harder? If an algorithm works, all solvable puzzles are
> > the same?
> 
> Well, it's harder for a human to solve; I forget where I snagged it 
> from, but it was labelled as "really hard". And it took my solver 
> program a whole 7 seconds to solve it (on my old computer), while other 
> examples I tested it with took less than a second to solve. So it's 
> probably useful for profiling your program's performance.
> 
> Ok, here's one that's *not* solvable, useful for making sure that your 
> program can handle such a case gracefully:
> 
> +-------+-------+-------+
> | _ _ 1 | _ 2 _ | 8 _ _ |
> | _ 7 _ | 3 1 _ | _ 9 _ |
> | 3 _ _ | _ 4 5 | _ _ 7 |
> +-------+-------+-------+
> | _ 9 _ | 7 _ _ | 5 _ _ |
> | _ 4 2 | _ 5 _ | 1 3 _ |
> | _ _ 3 | _ _ 9 | _ 4 _ |
> +-------+-------+-------+
> | 2 _ _ | 5 7 _ | _ _ 4 |
> | _ 3 _ | _ 9 1 | _ 6 _ |
> | _ _ 4 | _ _ _ | 3 _ _ |
> +-------+-------+-------+
>