71 lines
2.5 KiB
Markdown
71 lines
2.5 KiB
Markdown
### Remarks
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Just run it without arguments:
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ruby entry.rb
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I confirmed the following implementations and platforms:
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* Linux:
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* ruby 2.3.0dev (2015-10-30 trunk 52394) [x86\_64-linux]
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* ruby 2.2.2p95 (2015-04-13 revision 50295) [x86\_64-linux]
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* ruby 2.0.0p647 (2015-08-18) [x86\_64-linux]
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* Darwin:
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* ruby 2.0.0p247 (2013-06-27 revision 41674) [x86\_64-darwin10.8.0]
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* jruby 9.0.3.0 (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]
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* rubinius 2.2.6.n74 (2.1.0 94b3a9b4 2014-03-15 JI) [x86\_64-darwin12.5.0]
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### Description
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This program shows all solutions of any sudoku puzzle.
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The embedded sudoku puzzle can be changed at wish.
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Giving an empty puzzle (all `0` or `_`), the program will print every possible completed sudoku puzzle.
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We do not however make any time guarantee on such behavior.
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The program is rather small for the task: the solver is actually 302 characters long,
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assuming the sudoku puzzle is in a variable `s` and encoded as an array of rows of numbers.
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### Internals
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* The program implements backtracking and keeps state in a very elegant way.
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* The whole program never goes deeper than 9 stack frames,
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but yet can backtrack up to 81 levels!
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* The main loop of a program is a dance between cells.
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On one end is the solutions, on the other the program ends.
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* The program only uses *infinite* loops and no `break`.
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* The program interleaves the creation of the solver and the puzzle.
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* The program is easy to deobfuscate but finding how it works will be more challenging.
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* The last line contains a smiley.
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The author likes good numbers:
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$ wc entry.rb
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15 42 600
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The inspiration for this entry comes from:
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* A newspaper sudoku with multiple solutions
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* An inspiring paper: `Revisiting Coroutines`
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Various tricks used for brevity:
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* The method defined is one of the fews which may contain neither parenthesis nor spaces.
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* The program uses the return value of Fiber.yield without arguments.
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* `String#b` is used as a very short `self`.
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Design issues:
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* Since `return`-ing from a Fiber is not allowed, the programs must `exit`.
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* The program reveals that the cartesian product operator is still too long: `a.product(a)` while it could be `a*a`.
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Note:
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* In the original code, the last cell was: `C.new{loop{yield s; C.yield}}`,
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implementing some sort of "forwarding coroutine".
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### Limitation
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* The program does not want any *argument* with you and will quit quietly if you try some.
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