Robot Lines
(Published on 18. November 2024, 12:41 by marty_sears)
Rules (for part 1): Normal sudoku 4x4 rules apply. Each line contains an inspection robot, which moves backwards and forwards along it. The starting position must be deduced. The robot inspects this starting digit, which tells it how many cells along its line it must move. It can move in either direction, as long as there is enough room for it to move the correct distance without going off an end of the line (it can't reverse direction mid-movement.)
The robot then inspects the digit it stops at, which again tells it how many cells to move, again in either direction. The robot must repeat this process, each time obeying the digit it stops at, until it has inspected EVERY cell along the line. (It can ignore the last digit it inspects, and stops there.)
The robot must never stop at the same cell more than once. Digits may repeat on lines if allowed by sudoku.
Please leave a comment if you enjoy :)
Solution code: Column 8 of the 9x9 grid, from top to bottom
Comments
on 19. October 2026, 12:01 by Angara
I’m solving it for a third time to refresh my memory on the ruleset for the Scojo puzzle. Just like the first two times, I got stuck for ages at the beginning—until I read an important rule - (it can't reverse direction mid-movement.) Masterpiece. Thank you
on 11. October 2026, 09:48 by davidz32z
Brutally hard, but really really interesting rule set! Wish I'd read some of the notation ideas here before doing it as that was really a struggle for me
on 1. October 2026, 03:44 by The Pi Guy
Brilliant trio of puzzles. Loved all 3. :)
on 29. September 2026, 06:55 by GrouseHound
Just barely under 3 hours and my second 5/5 ever!
on 25. September 2026, 19:39 by KelleyCook
Twas very hard, now on to the feeding frenzy one
on 23. September 2026, 23:16 by heliopolix
Bzzzzzrttt!
on 23. September 2026, 19:41 by blackjackfitz
Huzzah!
on 23. September 2026, 17:32 by LunchEntity
Feeding Friendsy sent me back here as well ... I remember solving the first two and getting stuck on the third, but I got through it this time. Whew!
on 23. September 2026, 14:52 by amirs
I see I'm not the only one solving this before attempting Scojo's feeding friendsy puzzle :) Very creative and interesting ruleset, I enjoyed it a lot.
on 23. September 2026, 03:00 by Thanos13
I had to solve this one before trying Scojo's Freeding Friendsy level because I found the ruleset quite daunting! After almost 2 hours I now think I'm ready to battle with it!
on 23. September 2026, 01:53 by TheJRMY
Hmm. This was my first 5* solve, and it took me 4:56. Interesting. I actually found this very simple. Obviously late to the game, but I guess I need to try the next level.
Edit: disregard. Realized that this post covers all 3, not just the 4x4. That makes more sense.
on 2. May 2025, 14:16 by bboom
Well that took a solid three hours. Once again a very original constraint used incredibly well. Would love to see a few more puzzles using this constraint now that I'm finally starting to understand it a bit...
on 27. January 2025, 10:54 by EDL
Amazing puzzles and a very interesting ruleset!
I wonder what notations people use for it. I ended up using colours that I arranged in a strict order (e.g. green always follows yellow, blue always follows green etc) to keep track of what I know about the robot's movement on a line, but I guess there is some better way to do it.
Marty: that's a cool idea. I used red for known endpoints, green for cells that can't be the endpoint, and then turned them yellow when they are being pointed at by a cell
on 21. December 2024, 10:53 by NordenShield
This was a very interesting ruleset. I struggled with finding a good notation in the app to note down some logical conclusions. I made some incorrect assumptions along the way, and had to redo parts several times after the solution checker told me I was suddenly way off. All in all an absolutely great experience.
on 20. December 2024, 13:42 by Brains
I finished the first puzzle but couldn't find where the second and third are?
Marty: The link to the second puzzle appears in the 'success!' message after completing the first puzzle
on 7. December 2024, 04:17 by QuiltyAsCharged
System overload... central processor overheated... beep, boop. My brain is fried but these were excellent! I always wanted to be a Turing machine
on 1. December 2024, 20:37 by Puzzlefan
Wow, these took me a while. The first two went very fast, but then came the third... Interesting idea, worked out really nice. Thanks!
on 25. November 2024, 07:08 by henrypijames
The intro puzzles are neat, but I don't like being forced to go through them - I generally prefer to skip them and jump right into the cold water. Had I done so, this puzzle would've been more challenging, but after the intros, it's only 3 difficulty.
on 20. November 2024, 17:19 by Isael
Really nice ! The warm ups come easily, then I was a bit stuck on the last one. When I got it, I really felt like I had learnt something
on 20. November 2024, 14:04 by peacherwu2
Very nice warmups for the final deal! Love it!
on 20. November 2024, 13:20 by GoodMorningCallum
A great invention! The last puzzle was really difficult, and I think when the lines get more complex, I ended up doing a lot of trial and error. I suspect there's a more logical way of solving it, so it would be great to know if anyone else found a more elegant way.
Marty: thanks for the comment and for playing :) Yes no trial and error is really needed, I think part of the puzzle is about finding a good system for keeping track of what you know about each line
on 19. November 2024, 19:54 by Franjo
Yeah, the last one was quite hard to solve, but so satisfying. I don‘t think I really have gotten my head around these interesting new lines. Need more exercise… Thank you so much for creating and sharing another wonderful novelty.
on 19. November 2024, 17:56 by SuperCoolGuy
What a brilliant puzzle! I'm completely in awe of this perfect trio of sudokus. I would love to see more robot lines in the future, it seems to be a ruleset with vast potential. This might just be my favorite sudoku of the year.
on 18. November 2024, 20:00 by jakestilesowen
The first two were 1-star difficulty while the last one had some more difficulty. All three parts were very fun. Great job on the new line invention!
on 18. November 2024, 18:18 by mir85
Really fun!
And liking these three-part puzzles. It's nice to be able to explore a new ruleset in small easy puzzles before attempting a more difficult one.
on 18. November 2024, 13:13 by ChinStrap
This was a work of brilliance. Even after going through everything, developing the logic for them etc, I still feel like these are somewhat magical in the best way possible. The interplay between the sudoku and the lines was brilliance, and I was very thankful for the first two training levels to gently get me started on the concept before being thrown into the gauntlet. Bravo Lake and Marty!
