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Just run it without arguments:

ruby entry.rb

I confirmed the following implementations and platforms:

  • Linux:
    • ruby 2.3.0dev (2015-10-30 trunk 52394) [x86_64-linux]
    • ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux]
    • ruby 2.0.0p647 (2015-08-18) [x86_64-linux]
  • Darwin:
    • ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin10.8.0]
    • jruby (2.2.2) 2015-10-21 633c9aa Java HotSpot(TM) 64-Bit Server VM 25.11-b03 on 1.8.0_11-b12 +jit [darwin-x86_64]
    • rubinius 2.2.6.n74 (2.1.0 94b3a9b4 2014-03-15 JI) [x86_64-darwin12.5.0]


This program shows all solutions of any sudoku puzzle.

The embedded sudoku puzzle can be changed at wish.

Giving an empty puzzle (all 0 or _), the program will print every possible completed sudoku puzzle. We do not however make any time guarantee on such behavior.

The program is rather small for the task: the solver is actually 302 characters long,
assuming the sudoku puzzle is in a variable s and encoded as an array of rows of numbers.


  • The program implements backtracking and keeps state in a very elegant way.
  • The whole program never goes deeper than 9 stack frames, but yet can backtrack up to 81 levels!
  • The main loop of a program is a dance between cells. On one end is the solutions, on the other the program ends.
  • The program only uses infinite loops and no break.
  • The program interleaves the creation of the solver and the puzzle.
  • The program is easy to deobfuscate but finding how it works will be more challenging.
  • The last line contains a smiley.

The author likes good numbers:

$ wc entry.rb
      15      42     600

The inspiration for this entry comes from:

  • A newspaper sudoku with multiple solutions
  • An inspiring paper: Revisiting Coroutines

Various tricks used for brevity:

  • The method defined is one of the fews which may contain neither parenthesis nor spaces.
  • The program uses the return value of Fiber.yield without arguments.
  • String#b is used as a very short self.

Design issues:

  • Since return-ing from a Fiber is not allowed, the programs must exit.
  • The program reveals that the cartesian product operator is still too long: a.product(a) while it could be a*a.


  • In the original code, the last cell was:{loop{yield s; C.yield}}, implementing some sort of "forwarding coroutine".


  • The program does not want any argument with you and will quit quietly if you try some.