A simple Sudoku puzzle solver written in PHP.

🚨 NOTE: This project is still under construction. 🚨

Run the following in the root of the projec to install sudoku-solver.

composer install

Square      One of the m x n values on the puzzle board
Row         Horizontal M squares
Column      Vertical N squares
Region      An m x n subset of squares
Group       M/N number of squares in a row (horizontally or vertically) 

The strategies used by this solver are based on the strategies as described by Sudoku Dragon.

Strategies should be repeatedly applied until the puzzle is solved.

1. Only Choice Rule

   • Scan each row, column and region for a single choice in the group.

2. Single Possibility Rule

   • Scan each intersecting row and column for a single choice in the group.

3. Only Square Rule

   • Scan each row/column with two empty cells.
   • Identify the two missing numbers, A and B.
   • Scan each intersecting row, column or region and try to eliminate option A.
   • If option A can be eliminated for a particular cell, fill in option B in that cell.
   • Fill in option A in the other empty cell.

4. Two out of Three Rule

   • From the top down, scan three rows or columns at a time for a particular number starting at 1.

5. Sub-group exclusion rule

   • TODO

6. Brute Force

   • Try all options until a solution is found :)

• https://www.educative.io/edpresso/how-to-check-if-a-sudoku-board-is-valid
• https://puzzling.stackexchange.com/questions/158/how-many-minimal-clue-sudoku-puzzles-are-there
• https://www.technologyreview.com/s/426554/mathematicians-solve-minimum-sudoku-problem/