WHAT IS?

This is a sudoku solver.  I implemented it in an attempt to break my
sudoku addiction, by proving to myself that human brains are far less
efficient at solving such things than computers are.

It worked... for a while.


HOW CAN?

Try typing "make".  If it produces a program called "sudoku-solver",
you're good.  If it gives you an error message, fix it.

If/when you get it compiled, feed it a sudoku game on the command line.
The game should be in simple text file form, see easy1.txt for an example.
It will try to solve it, making some comments about what it is doing along
the way.  Once it finds a solution, or gives up, it will show you what it
came up with.

The example text files in this repository were generated by the genina
"sudoku free" android app.  The file names correspond to the difficulty
setting with which they were generated.


HOW DOES?

The program keeps track of which cells it knows for sure, and which
possibilities there are for the cells it doesn't know for sure.  It
knows a few strategies for how to solve boards, and basically just keeps
repeating them until it finds an answer, or gives up.

Strategies:

* Direct elimination.

"mark_pending: handled 4 pending completions."

Every time it figures out an answer, it looks for other cells which had
that possibility, in the same row, column, and block, and eliminates all
of the others.  If only one possibility remains for an cell, then we have
discovered the answer for that cell, too, and the marking process repeats.

* Last one standing

"Only one 7 in column 8."
"Only one 9 in row 5."
"Only one 5 in box at (0,0)."

For a given value, if only one cell could possibly contain that value,
then that cell must contain that value.  This is true for a row of cells,
a column of cells, or a block of cells.

* Box items form a line

"The 6's in box at (3,3) form a horizontal line, which rules out (0,3)."
"The 3's in box at (6,3) form a vertical line, which rules out (7,1)."

For a given value, if all of the possibilities in a box are within a
single row or a single column, then that box must contain the value for
that row or column.  This means we can eliminate the possibilities for
that value in other boxes, within the row or column.

* Line items are in a box

"The 3's in column (8,*) are in a box, which rules out (7,8)."
"The 6's in line (*,0) are in a box, which rules out (7,1)."

For a given value, if all of the possibilities in a row or column
are within a single box, then the value must be in that row or column
of the box.  This means we can eliminate the possibilities for that value
elsewhere in the box.

* Inductive exclusion

"Set [6,6], [7,6], [6,7] of size 3 with 3 possibilities excludes [8,7]."
"Set [8,6], [8,7], [6,8], [7,8] of size 4 with 4 possibilities excludes [6,6]."

If two cells in the same row, column, or box have the same set of
possibilities, and there are only two values in that set of possibilities,
then those two cells must contain both of the values.  That means cells
in the same row, column or box as those two cannot contain either of the
values in that set.

More generally, a set of N cells with only N possibilities among them
must contain all N of those possibilities, so none of those
possibilities can exist elsewhere in the row, column, or box that
contains them.