Author: leo
Date: Fri Dec 30 08:31:54 2005
New Revision: 10793
Modified:
trunk/examples/pir/sudoku.pir
Log:
add (again) a TODO and an example to sudoku.pir
Modified: trunk/examples/pir/sudoku.pir
==============================================================================
--- trunk/examples/pir/sudoku.pir (original)
+++ trunk/examples/pir/sudoku.pir Fri Dec 30 08:31:54 2005
@@ -1902,6 +1902,44 @@ that in square 2 we have a unique '7' at
Now the tests in "create_inv_n" invalidate illegal positions
due to multiple-blocking and other tests are likely to proceed.
+=head2 TODO
+
+ # daily sudoku wed 28-dec-2005, very hard
+ # backtracking
+ ...52.63.
+ .5.....7.
+ 9....8..2
+ .17..4...
+ .9.....6.
+ ...8..31.
+ 1..6....5
+ .4.....9.
+ .86.95...
+
+This one starts backtracking early. The '6' is an 'X-Wing' like
+configuration (col 1 and row 2 with a common corner have just 2
+possible positions, just one is valid, when you try both).
+The same happens a bit later with '9'.
+
+ +---------+---------+---------+
+ | . 7 . | 5 2 . | 6 3 . | 666 666 666
+ | . 5 . | . . . | . 7 . | .66 6.. 666
+ | 9 . . | . . 8 | . . 2 | 6.6 6.6 666
+ +---------+---------+---------+
+ | . 1 7 | . . 4 | . . . | .66 6.6 666
+ | . 9 . | . . . | . 6 . | 666 666 666
+ | . . . | 8 . . | 3 1 . | ..6 6.. 666
+ +---------+---------+---------+
+ | 1 . 9 | 6 . . | . . 5 | 666 666 666
+ | . 4 . | . . . | . 9 6 | 666 666 666
+ | . 8 6 | . 9 5 | . . 3 | 666 666 666
+ +---------+---------+---------+
+
+Here is starts backtracking. A possible improvement would be:
+
+ - detect such digit configuration
+ - only backtrack try this digit ('6') above
+
=cut
# vim: ft=imc sw=4:
