Easy Peasy Sudoku Advent (23) - Parity Poopers
(Published on 23. December 2025, 06:00 by Richard)
This sudoku advent calendar contains 24 variants that were included in my Sudoku Variants Series (SVS) but turned out to be too hard to be solved by a wide range of puzzlers. I aim for difficulties of 1-2 stars and have written some general and/or specific hints as well. I hope some players will try the original SVS puzzle #415 after solving the advent version.
Parity Poopers
General hint for this type:
Mark every cell with a color (or a circle/square if you solve on paper; like me) to distinguish between odd and even. This is very helpful! (That’s why I used crosses this time, instead of grey cells in the original puzzle.)
Apply classic sudoku rules.
Digits cells with a cross are parity poopers: digit N in a cell with a cross has exactly N-1 digits of opposite parity (odd/even) in the horizontal, vertical and diagonal adjacent cells. (An 8 in a such a cell has exactly 7 odd digits around it; a 5 exactly 4 even digits etcetera.)
All possible cells with crosses are marked!
If the central cell of a 3x3-block contains a cross, it’s digit must be a 5 (four evens) or a 6 (five odds).
Cells with crosses can contain a maximum digit of 4 in case they are in a corner or 6 in case they are elsewhere along edges.
Take the negative constraint into account!
Sudokupad (no answer check)
Sudokupad (with answer check)
or
Penpa+.
Solution code: Row 7
Comments
yesterday, 00:35 by packt
a nice puzzle little puzzle!
on 23. December 2025, 21:57 by wuc
Great coloring puzzle. Id remove the negative constraint. Fun 2* without. Otherwise you spend time looking for complex implications that are not needed. Anyway round, cool puzzle thx.
---
Hi wuc, thanks for your useful comment. I just simply used the same ruleset as in the original SVS-puzzle, and there the negative constraint is definitely needed. Quite often, in my harder puzzles, they are even build on the negative constraint (if applicable). So: when I start writing and while test solving, that negative constraint is always around, but I didn't check if it was indeed needed at all.
And I agree with you that it isn't helpful to have a redundant addition in the rules.
So that is a good thing to keep in mind if I repeat a project like this next year.
The only reason to keep it in, is to avoid that the original puzzle and the easier puzzle have different rulesets, and if players want to try the harder one, they can't rely on what they learnt in the easier puzzle.
All in all: food for thought! :)
on 23. December 2025, 19:32 by dzamie
Got a bit tripped up a couple times early on with an assumption that I really shouldn't have made. Very interesting rule! I wound up using the negative constraint towards the end, but it's neat to see that others solved it without.
on 23. December 2025, 15:50 by vidarino
Very nice! I didn't notice, nor need to use, the negative constraint, though. :)
on 23. December 2025, 13:03 by Franjo
Perfect appetizer again! I already had solved the original, but enjoyed to solve this one, too. Thank you very much for creating and sharing.
on 23. December 2025, 12:48 by akodi
Thanks Richard, for all the great puzzles and all the work you put into this!
Its great fun! With or without answer check ????
on 23. December 2025, 09:59 by Richard
Fixed minor error in solve links with solution checks.
on 23. December 2025, 09:04 by Piatato
Lovely!
on 23. December 2025, 08:29 by Klausku
Very nice. Interesting but mind-twisting ruleset. Thanks.
on 23. December 2025, 06:09 by Richard
fixed small typo
