path: root/sample/trick2015/eregon/remarks.markdown
diff options
Diffstat (limited to 'sample/trick2015/eregon/remarks.markdown')
1 files changed, 70 insertions, 0 deletions
diff --git a/sample/trick2015/eregon/remarks.markdown b/sample/trick2015/eregon/remarks.markdown
new file mode 100644
index 0000000000..a56f24da71
--- /dev/null
+++ b/sample/trick2015/eregon/remarks.markdown
@@ -0,0 +1,70 @@
+### Remarks
+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]
+### Description
+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.
+### Internals
+* 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".
+### Limitation
+* The program does not want any *argument* with you and will quit quietly if you try some.