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Strategies for Popular Number Puzzles

Sudoku X Solver 
Please report any bugs and feedback welcome

Candidates can be Edited or Highlighted / Shown  
Enter clues or solutions 


Quick help: Using this Solver
Use the "Import a Sudoku" button or type in a Sudoku puzzle in the small board. You can also pick examples from the list above. Click on Take Step to step through the solution. Unknown squares are filled with 'candidates'  possible solutions. Any cells that are reduced to one possible candidate are solved. You can now use the << button to step back one go. Toggling between Take Step and << helps you see the changes. Pressing "Enter" on the keyboard after clicking on Take Step is a quick way to step activate "Take Step". Details of any solutions will be written out in the tex tbox below the big board. Strategies are ordered by complexity. Any strategy that is successful returns the stepthrough to the start. Click on the board to highlight sets of numbers. You can edit the sudoku at any time  entering solutions in the small board or editing candidates. (Toggle between highlighting and editing using the radio buttons at the top.) 



Sudoku X is a great variant of normal Sudoku and this solver is an extension of my Sudoku Solver to help you discover the logical solutions for this puzzle. The difference is that in Sudoku X the two diagonals are known to contain the numbers 1 ro 9 uniquely. These extra constraints allow you the puzzle solver to dervice new conclusions about candidates to eliminate and find solutions to cells. You can look along the diagonals (marked with a darked X on th board) and make deductions. However, the extra constraints mean that the puzzle creator can leave less clues than normal sudoku.
For the easier Sudoku X puzzles you won't really find a necessary example of a deduction based on the diagonals although you will want to scan them in case you see an easy 'single'. For tough puzzles and above the diagonals must be checked. In this solver they are checked before rows, columns and boxes. All the normal rules and logical posibilities apply to Sudoku X with some exceptions. There are pitfalls, for example, with Unique Rectangles, which rely on a certain formations. I have documented these here. Please check this stratgy guide if you want to use the advanced strategies.
I am now working independently on puzzle creation.
All feedback, comments, arguments, bug reports and strategy ideas are welcome. There is a new FEEDBACK form with a column displaying comments and questions. Many thanks to all the people who have done so and helped improve this solver.
New in version 2.01 (August 25th 2014)The examples in the list above illustrate some of the many strategies available. They are all 'necessary' examples in the sense that no easier strategy will bypass the requirement for their use  unless perhaps one reorders the strategies. The diabolical strategies could all be swapped around to no detriment but I have ordered them in what I subjectively believe to be an order of complexity. Some examples start at the beginning of the puzzles, some half way through. While one strategy has been picked out as the example many of the others will be required to complete.
12 clue tough is from "Taking Sudoku Seriously" by Jason Rosenhouse & Laura Taalman published by Oxford University Press, Inc.
Michael from Denmark has sent me the 'Unsolvable'  a great puzzle from a Sudoku magazine which I can't logically solve yet
Many people have written to me to comment about multiple solutions for a given Sudoku. There are no logical tricks the solver can use to detect this other than not complete correctly. The only way to check this is to perform a brute force analysis which tests every possible legal placement of a number. Computers are good at this and we now have a new yellow button called "Solution Count". Try this on any Sudoku to check if it has a unique solution.
Use New to empty the board before entering your own puzzle. Save will remember the current state of the board so you can Reload it again (even if you close your browser  you must allow cookie for this to work). Restart applies only to the example puzzles in the list. The current list contains an example puzzle that tests each strategy.
Take Step first displays the possibles or candidates for each unknown cell. These are the numbers that do not contradict any known or solved cells. Once these are displayed Take Step will step through other tests and then loop until it can go no further. The first few tests are the most productive and the solver will often loop between them. If any are successful and the board is changed in any way it will go back to the start and "Check for Solved Cells". The reason for this step is to make it easier to spot what's changed. Many of the strategies have knockon effects which means that they can't be run backtoback  it's essential that we return to the basic steps. We go back because we want the least hard solve route.
The first seven tests are the simplest and are required for any sudoku. After that you are allowed to choose which strategies the solver will use. Tick and untick the check boxes. For example, you may not want to use any strategies that rely on a unique solution. Uncheck test 15.
The order of these advanced strategies  and my inclusion of them in categories 'tough', 'diabolical' and 'evil' are my personal choice after close study and are roughly in order of complexity. While the logic is different for each, you should be aware that there is considerable overlap in their power to solve in certain situations. For example, 'Guardians' will never solve anything while 'Multicolouring' is switched on since they both attack similar configurations.
All strategies in the list have links to documentation, but it's worth describing what the first tests do: