Go Challenge - 8


Winners have been declared by Andrew Gerrand

Winner #1 - Jakub Rozanski - A readable dancing links implementation.

Winner #2 - Jan Mercl - I enjoyed reading this clean solution.

Special mentions

Dan McCandless - “Las Vegas” algorithm gave me pause, and then a good laugh.

Robert Ortega - “suGOku” web app is nicely done.

The November 2015 Go Challenge for developers (newbies included)

Andrew Gerrand: Author of the 8th Go Challenge

Andrew Gerrand The 8th Go Challenge author, Andrew Gerrand, is from Melbourne, Australia and now lives in Sydney working at Google Australia on the Go project. He has been writing code since he was a little kid, and has been working in the industry since he was a teenager. Before Google he worked for various Internet companies using most of the common languages in that sphere (Perl, PHP, Python, JS, etc). At the same time he spent most of his personal hacking time writing assembly for 8-bit systems from the 80s. He also has a Bachelors degree in Fine Arts (his major was photography).

Andrew has this to say about the challenge:

“I set this puzzle because it suits a range of skill and knowledge levels. It’s tractable for a novice to implement the brute force approach, while an expert can go all-out with a more creative approach. I look forward to seeing what the gophers can come up with!”

The Go Challenge 8

A Sudoku Solver

The game of Sudoku is simple to understand but it can be challenging to play. The same can be said for the “metagame” of writing a Sudoku solver: the algorithm can be easy to imagine, but when you set about implementing it things can get interesting. And it clearly is interesting: Donald Knuth used Sudoku to demonstrate his “dancing links” technique, the Prime Minister of Singapore released the source code for his Sudoku solver to demonstrate his IT credentials, and many years ago I explored the problem as a novel application of Go’s concurrency primitives.

It was back in 2010, when we were all still learning Go, that I wrote a Sudoku solver using goroutines and channels. I wrote it up on my blog (part 1, part 2), but it was only ever able to solve puzzles that require no backtracking (guessing). All challenging Sudoku puzzles do require some guesswork, so I couldn’t say my solver was ever finished. Indeed, I don’t believe it is even possible to finish my solver without abandoning its fundamental design.

The Goal of the challenge

The goal of this challenge is to implement a Sudoku solver.

Requirements of the challenge

Your program should read a puzzle of this form from standard input:

1 _ 3 _ _ 6 _ 8 _
_ 5 _ _ 8 _ 1 2 _
7 _ 9 1 _ 3 _ 5 6
_ 3 _ _ 6 7 _ 9 _
5 _ 7 8 _ _ _ 3 _
8 _ 1 _ 3 _ 5 _ 7
_ 4 _ _ 7 8 _ 1 _
6 _ 8 _ _ 2 _ 4 _
_ 1 2 _ 4 5 _ 7 8

And it should write the solution to standard output:

1 2 3 4 5 6 7 8 9
4 5 6 7 8 9 1 2 3
7 8 9 1 2 3 4 5 6
2 3 4 5 6 7 8 9 1
5 6 7 8 9 1 2 3 4
8 9 1 2 3 4 5 6 7
3 4 5 6 7 8 9 1 2
6 7 8 9 1 2 3 4 5
9 1 2 3 4 5 6 7 8

It should reject malformed or invalid inputs and recognize and report puzzles that cannot be solved.

(Incidentally, the puzzle above makes a nice test case, because the solution is easy to validate by sight.)

Bonus features:

  • Print a rating of the puzzle’s difficulty (easy, medium, hard). This rating should roughly coincide with the ratings of shown by sites like Web Sudoku.
  • Implement a puzzle generator that produces a puzzle of the given difficulty rating.
  • Maximize the efficiency of your program. (Write a benchmark.)
  • Write test cases, and use the cover tool to make sure your tests are thorough. (I found a bug in both my implementation and a test case when I checked the coverage.)
  • Use a non-obvious technique, like Knuth’s “Dancing Links” or something of your own invention.


  • For an elegant and efficient representation of the puzzle, try using an array (not a slice).
  • Recursion can dramatically simplify your implementation.

Challenge Rules and how to enter the Go Challenge?

By participating in this challenge, you agree to be bound by the Challenge Rules below:

  • The Challenge is open to individuals.
  • Evaluators cannot enter the challenge except under the “Just for Fun” category.
  • Each entrant shall indemnify, defend, and hold JoshSoftware Pvt. Ltd. (who has sponsored the domain and is the organizer of these challenges) harmless from any third party claims arising from or related to that entrant’s participation in the Challenge. In no event shall JoshSoftware Pvt. Ltd. be liable to an entrant for acts or omissions arising out of or related to the Challenge or that entrant’s participation in the Challenge.
  • Odds of winning depend on the number and quality of entries received.
  • All taxes, including income taxes, are the sole responsibility of the winners.
  • No prize substitution is permitted.
  • Create a zip of your Go source code and test cases and send the zip file to golangchallenge [at] gmail.com before 24th of November 2015 (6 am IST). Use this link to find the equivalent time in your city/country. No new solutions will be accepted after that. In the email mention your full name, country of residence, twitter or GitHub id (if any) and participating under which category - Just participating or Participating and exploring further or Just for Fun or Anonymous entry. We are accepting anonymous submissions and will evaluate them too but then these participants are not eligible for the prizes.
  • We shall be publishing on this blog, a list of participant names. If you don’t want your name to appear kindly mention the same in your email.
  • You are allowed to re-submit your code if you feel it is necessary.
  • After the challenge is over, all submissions will be made available online on GitHub under the BSD 3-Clause License or the GNU General Public License, version 3 - GPL-3.0 unless a participant has indicated that his/her solution should not be made public before the challenge ends.

How will the challenge be evaluated?

Entries will be evaluated by the challenge author and a team of evaluators.


If you have any questions about this challenge, please join the golang-challenge channel on slack and ask your questions. This is a room for people who are going to participate in the Go Challenge. You can also send us an email at golangchallenge [at] gmail.com


Nathan Youngman had set the guidelines for evaluation for the first Go Challenge. Subsequently Dominik Honnef modified the guidelines based on his experience as an evaluator for the first challenge. Austin Riendeau, Cory LaNou, Edd Robinson, Gautam Dey, Jyotiska NK, Kevin Gillette and Pravin Mishra have agreed to go through all the submitted solutions of a challenge. They will comment and rank these solutions.

Best Solution

The author of the Go Challenge will decide the best five solutions. The author shall have the sole authority and discretion to select the award recipients.


The winning entries will be announced here on this blog. The winners will be sent their prizes by email/postal mail.


The author of the challenge will select 2 winners and prizes will be awarded to them.

Here are some great prizes provided by our sponsors for the event.

Winner 1:

Winner 2:

Challenge Solutions

All the solutions submitted by the participants will be available here after the challenge ends.


This challenge’s sponsors are: Anand D N, Apcera, Cube Root Software, InfluxDB, JoshSoftware Pvt. Ltd., Manning Publications Co., Packt Publishing and Qwinix Technologies.


  • The Gopher character is based on the Go mascot designed by Renée French and copyrighted under the Creative Commons Attribution 3.0 license.
  • GitHub for the yearly sponsorship of a GitHub Bronze Organisation plan for the Go Challenge.
  • The Go Challenge is being organized by JoshSoftware Pvt. Ltd. with help from the Go community.
Written on November 1, 2015