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Sudoku solver project

Will we get to the end of this? Who knows. EDIT: We did it!

Compiling instructions

Tentative compilation instructions:

bash$ mkdir build
bash$ cd build
bash$ cmake .. -DCMAKE_INSTALL_PREFIX=. 
bash$ make

Run

Once the project has been built, the program sudoku_solver should be in the selected install path (the build directory). It can be launched with the commmand

bash$ sudoku_solver input.txt

where input.txt is a text file of the format

5**67*9**
*4*8*****
8**5**613
*624***7*
1****3*2*
3749*8***
*961*78*2
218**6*45
*5**8**9*

where * stands for an empty space. The ouput will be given in the same format.

Algorithm

The search tree is represented as a 9x9x9 cube of booleans. The "x" and "y" directions map to the two axes of the puzzle. The "z" direction is used to contain the possibilities: the element (i,j,k) of the array being true means that the digit k+1 is a candidate for position (i,j).

If in the column (i,j, _ ) only one value is true, then this information is used to set to false the right elements of the cube, according to the Sudoku rules. Similar considerations apply for x-rows (_,j,k) and y-rows (i,_,k).

Algorithm 1

This algorithm consists of propagating all the consequences of the "certain" information, i.e. when there is only one possiblity for a number in a certain position all the consequences of that piece of knowledge are applied and the "search space" is pruned accordingly. This is not enough to solve all puzzles (see, e.g., `example_tables/hard.txt').

Algorithm 2

This algorithm adds additional assumptions to what is already known in order to get to the point where a single digit is possible in any cell. Of course there are many different ways to do so. It will try first to add assumptions to the cases allowing the least amount of mutually exclusive assumptions. If it is possible to always find a case where the number of mutually esclusive assumptions is 1, this is equivalent to Algorithm 1. This generates a tree of assumptions, which is explored with multiple recursion in a depth-first fashion. Once a solution is found or deemed impossible, the recursion returns.