I originally wrote the Python version of this on a long plane flight, after getting stuck on one of the sudoku puzzles in the back of the book. Then, on a similarly long flight, I adapted it into a web application. The web version is in the separate web branch.
You can check out the source code on the web branch of this repository for more information about its functionality or pictures of it in action, or just check out a live version at https://alexbeals.com/projects/sudoku.
It has an interface so that you can quickly enter in a sudoku puzzle, and either complete to see the solution, or step through move by move. It allows you to view the grid at any time, in both a small view
9 4 8 | _ 2 _ | 7 _ 5
_ _ 1 | _ _ 4 | 8 _ 9
_ _ 6 | _ _ _ | 1 2 4
------|-------|-------
_ _ _ | _ _ 3 | _ 1 2
1 _ _ | _ _ _ | _ 7 8
8 7 2 | 5 _ _ | _ 4 3
------|-------|-------
_ 8 7 | _ _ _ | 2 9 6
6 _ _ | 9 _ _ | 3 8 1
_ 1 9 | _ 8 _ | 4 5 7
and a (considerably) more zoomed in view that displays possible values in the corners.
| | # 1 3| |1 6 # |3 6|
9 | 4 | 8 # | 2 | # 7 | | 5
| | # 6 | | # | |
------------------#-------------------#------------------
2 3|2 3| # 3 6|3 5| # |3 6|
| | 1 # | | 4 # 8 | | 9
5 7|5 | # 7 |6 7| # | |
------------------#-------------------#------------------
3 5|3 5| # 3 7|3 5|5 7 # | |
| | 6 # | | # 1 | 2 | 4
7 | | # 8 |7 9|8 9 # | |
##########################################################
4 5|5 6|4 5 # 4 6|4 6| # 5 6| |
| | # | | 3 # | 1 | 2
|9 | # 7 8|7 9| # 9 | |
------------------#-------------------#------------------
|3 5|3 4 # 2 4|4 6|2 6 # 5 6| |
1 | | # | | # | 7 | 8
|6 9|5 # 6 |9 |9 # 9 | |
------------------#-------------------#------------------
| | # |1 6|1 6 # 6 9| |
8 | 7 | 2 # 5 | | # | 4 | 3
| | # |9 |9 # | |
##########################################################
3 4| | # 1 3|1 3|1 5 # | |
| 8 | 7 # | | # 2 | 9 | 6
5 | | # 4 |4 5| # | |
------------------#-------------------#------------------
|2 5|4 5 # |4 5|2 5 # | |
6 | | # 9 | | # 3 | 8 | 1
| | # |7 |7 # | |
------------------#-------------------#------------------
2 3| | # 2 3| |2 6 # | |
| 1 | 9 # | 8 | # 4 | 5 | 7
| | # 6 | | # | |
It also allows you to input row-column pairs to see the possibilities for that spot. In the above example, (9,1) would output [2,3], the available options for that square.
On the hardest Sudoku in the world it takes ~25 seconds to solve.
800000000003600000070090200050007000000045700000100030001000068008500010090000400
On a more average Sudoku, yet still hard it takes under 1/10 of a second.
940020700001004009006000120000003010100000008070500000087000200600900300009080057