Build-you-own sudoku

Has anyone succeeded with this puzzle yet? Cause I’ve been trying to do it manually, but the clues only led so far and when I approached it with programming, there are too many possible configurations to check to find the right one. Does anyone have and would like to share any ideas how to best solve it?

Not a freakin clue…so frustrated!!

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It was really hard, but fun too. I wonder if you can solve it only by hand

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I solved it by hand but I love sudokus and solving the harder ones with logic puzzles (like from cracking the cryptic)
I had a lot of notes and scribbles but got there eventually just good to colour code and keep checking the rules :blush:

Phew. Solved it. I used an Excel spreadsheet for the Sudoku squares and highlighted the Givens with lots of manual manipulation. The key was to get the Givens matched to the rules and interpret the rules correctly. Keep re-reviewing the rules and ensure you are interpreting them correctly. The Givens do follow rules provided AND the Sudoku rules. Once you fit the givens/provided rules the remainder is just standard Sudoku with only 1 possible outcome.

I solved it by code to figure out the potentials that the givens could be and a sudoku solver. I could not get definite values for all of the givens from the rules. I ended up with a bunch of possibilities so i iterated over those and fed them into the sudoku solver until voila!

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Similar process here.
I did a first pass by hand to determine the possible values for each letters, given the rules.
Then wrote an ugly python program with many nested for loops on the letter values, starting with the most constrained or constraining letters (c,m,b,f,…), so that the program runs for a few minutes.
This resulted in 3 solutions for the letters, giving only one puzzle solution after solving the sudokus.

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FWIW, it is possible to solve this entirely manually, once you realise one significant logical inference which can be drawn from the given conditions. That is the golden key which unlocks the puzzle!
That said, I originally smashed the door down by brute forcing the diagonal with code and finding the answer without solving the sudoku.
It was much more satisfying when I finally found the key and went back and did it properly though!

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I did it manually too. The first time I felt like I’d solved it, I noticed the only thing that was off was that the column intended to be 25 was 24 in my case. I figured it’s close enough, solved the sudoku, got a nonsense answer but miraculously guessed the correct one.

Then I went back and re-solved it knowing the diagonal numbers which made the rest a breeze. Turned out i had 10/24 wrong givens in the first attempt :speak_no_evil:

So my advice is, if you’re “close enough” (valid sudoku), give it a go and you might get it.

I’ve just finished it, and yes, I too (finally) found the logic in those conditions. Took three days though😅

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Finally finished solving it by hand, but it took a while! I thought the givens were much harder to figure out than the sodoku itself. One thing I assumed was that the unique solution to the givens did not depend on traditional sodoku constraints applied to the non-givens. Basically, you could solve the givens without having to start solving the rest of the sodoku at the same time. Might seem obvious (and it did turn out to be true), but it wasn’t stated. I think this is one of the few puzzles that could have been made much harder to the point of being impossible to do without a computer. I have been enjoying this competition very much, but I think it would also be interesting to have a similar competition where the puzzles are all impossible to solve by hand and require writing code.


Now that this puzzle is expired, anyone cares to share their code on how they solved it?

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Didn’t find a mention in T&C whether solution/code sharing is allowed so hopefully it is. Here you go, in ugly C:

puzzle30.c (
pw to open: hackthegalaxy

Didn’t have the energy to re-implement the sudoku solver, so do that by hand or use an online solver.

also, as mentioned in the other thread (seems to be locked now), the adjustments of the main diagonal by themselves are sufficient to find the solution with no effort at all:

shell 1-liner

$ zcat words_alpha.txt.gz | grep -i -E -e '^[c-k][a-i][e-m][g-o][h-p][d-l][c-k][a-i][p-x]$'


Nice one!
I only thought of that after having spent several hours to solve it with a brute force program.

yes, solved it quite quickly (this one surely the fastest one in the last dozens). sorry, don’t wanna show off. just a little. :slight_smile:

Does anyone mind sharing the key clue that unlocked this puzzle? Since the puzzle is over now.

Sure. If you look at rules 6 & 11, you’ll see the following key points:

  1. There are equal numbers of 1, 2, 9 and also of 3, 4, 5, 8. There are more 7s than 6s.

All values 1-9 are contained within this statement, meaning that all givens are contained within four groups.

  1. At least one value appears once, one twice, one three times, one four times.

Again four groups, so the four groups from rule 6 must correspond with those from group 11.

You know that there are 24 givens in total. So, you can figure out how the values are distributed into these groups.

e.g. [1, 2, 9] x3 + [3, 4, 5, 8] x 4 + [7] x2 + [6] x1 = 28 … no good.

Keep playing with different combinations until you can hit exactly 24 givens (of which there may be more than one possibility). Then you can incorporate other rules until you can determine exactly which grouping must be correct. The rest of the puzzle should then fall into place.

The puzzle is over, but i cant figure out where the mistake is in my thought process, if someone could help its very much appreciated, can be a private message too.

I wrote out every combination for combining rule6 with rule11. This has 24 combinations.

Only 4 of those add up to 24. And only one of those four follows rule#6 which says there are more 7s than 6s.

With that in mind, looking at the first part of rule#8, F can only be 3 possibilities.

Looking at the 3 possibilities for F and rule#5 and the second part of rule#8, we can now know F.

Now we can combine the first part of rule#8, the first part of rule#10, and knowing F, to determine the two numbers that make up D and E, although unsure which is which.

And now theres a conflict: knowing from earlier how many times there should be a 4, there is currently no possibilities anymore that can satisfy rule#2 without either breaking the sudoku rules or breaking rule#5 or breaking rule#9.

Glory to the person who can spot the mistake in this logic!