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multiple solutions to 1-Oct-14, 4x4 Difficult?
http://www.calcudoku.org/forum/viewtopic.php?f=18&t=625
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Author:  Rob  [ Wed Oct 01, 2014 2:31 am ]
Post subject:  multiple solutions to 1-Oct-14, 4x4 Difficult?

It appears to me that there are multiple solutions to the 4x4 difficult puzzle today.

Here is what I have so far:

Image

It appears to me that the first row can be either 3412 or 4312 equally correctly.

Author:  apolaris  [ Wed Oct 01, 2014 2:56 am ]
Post subject:  Re: multiple solutions to 1-Oct-14, 4x4 Difficult?

I found a similar problem:


4 3 1 2
3 4 2 1
2 1 4 3
1 2 3 4

and

4 3 1 2
3 4 2 1
1 2 4 3
2 1 3 4

are both equally valid solutions, yet both are being rejected as "incorrect" when submitted.

3 4 1 2
4 2 3 1
2 1 4 3
1 3 2 4

is being accepted as the solution, but it's nowhere near being the only one.

Author:  bram  [ Wed Oct 01, 2014 4:12 am ]
Post subject:  Re: multiple solutions to 1-Oct-14, 4x4 Difficult?

Apart from four correct solutions with this arrangement of numbers in the right half of the puzzle:

x x 1 2
x x 2 1
x x 4 3
x x 3 4

there is also this (fifth) correct solution:

3 4 1 2
4 2 3 1
2 1 4 3
1 3 2 4

On my system (latest Firefox version on Windows Vista SP2) all five solutions are accepted as correct when submitted.

Author:  calcpnm  [ Wed Oct 01, 2014 9:16 am ]
Post subject:  Re: multiple solutions to 1-Oct-14, 4x4 Difficult?

Thanks for letting me know. I'm as far as having figured out that this is a bug
introduced in a more recent version of my solver [mad]

I ran an old version of the solver (from July 2010 (!)) on this puzzle, and it found
all five solutions.

Edit: found the bug, a fix for a problem which caused it to not be able to solve puzzles
larger than 15x15 (which I needed for the new book, which has one 17x17 puzzle) introduced
a new bug for puzzles with cages larger than 5 cells [crying]

Challenge to you all: how to modify the current puzzle so only this solution:
Code:
3  4  1  2
4  2  3  1
2  1  4  3
1  3  2  4

is valid?? [biggrin]

Edit #2: I changed the puzzle a bit so the above is a single valid solution.

Patrick

Author:  clm  [ Wed Oct 01, 2014 10:30 am ]
Post subject:  Re: multiple solutions to 1-Oct-14, 4x4 Difficult?

calcpnm wrote:
... I'm as far as having figured out that this is a bug
introduced in a more recent version of my solver [mad]

I ran an old version of the solver (from July 2010 (!)) on this puzzle, and it found
all five solutions.
...

Patrick


It was one possibility among 5, but I was so unlucky that only when I entered the fifth I got the 2 points :-) :-).

I was really surprised since it was the first time (IIRC) that I was seeing a violation of the "dogma" of the unicity, and thinking what would have happened in case of a 9x9, ... . I supposed most players were going to feel the same and find the solution given as correct by the program in a couple of minutes (the other four times the full grid was green but the program insisted in an "incorrect solution") and I was waiting until 24.00 CET today to talk about this bug (this time only 2 points, anyway [confused] ).

Author:  pnm  [ Wed Oct 01, 2014 10:44 am ]
Post subject:  Re: multiple solutions to 1-Oct-14, 4x4 Difficult?

clm wrote:
I supposed most players were going to feel the same and find the solution given as correct by the program in a couple of minutes

Yes, and every time this happens (a validated solution is submitted but rejected by the server) I get
a warning message, to alert me of a potential bug.

So this morning I had 400+ emails in my inbox [scared]

Edit: and after exhaustively testing all October puzzles, I found that I have to replace five more 4x4 difficult puzzles
(which had 3 or 5 solutions)

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