... is to make a Sudoku solver. I'll probably never finish this being full time employed and active in many outside interests. But I love playing the game and I love using Go. Hard to resist at least thinking about the problem!

The challenge page is here: http://golang-challenge.com/go-challenge8/

The page lists the rules and provides a sample input and output. Main constraints are:

• Read a puzzle on stdin using format in test1.txt
• Output solution on stdout using format in test1_expected.txt

Since the approach below has never worked satisfactory, I have begun thinking of a different approach. The main differences will be:

• create a type to represent the cell
• switch to a linear 81 cell slice to represent the puzzle.

This design will complicate the square, row, and column validator logic, but I think it will make easier to introduce some parallelization, which was a secondary goal all along.

To save space, I am switching to uint8. Space is more important because of the go routines I hope to introduce. The puzzle must become immutable to make the parallelization work.

So this iteration will introduce the cell type and it will be used by the other types to build on.

the puzzle content Thinking of a 3D array of integers. The 2D slice would represent the input puzzle; with zeros for the blanks. The 3D slice would represent the pencil marks. Assuming a max number of pencil marks of 8, then the array would be 9 rows, by 9 columns, by 9 high: 9x9x9 = 729 slots. If the max number of pencil marks is 9, then 810 (9x9x10).

the 3x3 square Each square could be represented with a struct containing its data from the puzzle, the methods to interrogate the intersecting rows and columns, etc.

If each 3x3 square is handled separately, perhaps by its own go routine, then each square can figure out its own pencil marks without interfering with the others.

Each 3x3 square:

• construct an array 0 to 8 (9 integers) that represent its non-zero content; for example, a 5 would go in the 4th slot
• For each blank, for each missing number (not in the array above), test the number for presence in the row and column of the blank. If not there insert into the slot in the pencil marks.

For example, based on the test puzzle below, in the first square (upper left), the pencil marks should be: (0,1) --> 2 (1,0) --> 4 (1,2) --> 4,6 (2,1) --> 2,8

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