Q. How do I play?
A. As per normal Sudoku, the aim is to completely fill the grid such that each column, row and 3x3 box contains all of the numbers 1 to 9.
Q. How do I start?
A. Either enter your own puzzle manually, perhaps from a paper or magazine, by using the numeric keypad, or just press 'Generate new' to let the app generate a new puzzle for you. Use the slide control to vary the level of difficulty of the generated puzzle.
Q. Do you actually generate puzzles, or do you just have a finite database?
A. All puzzles are randomly generated from scratch within the app, and each one is guaranteed to have a unique solution.
Q. OK, I've got my initial puzzle. What now?
A. The app aims to replicate the pencil-and-paper solving experience as closely as possible, but with some optional assistance if you require it. Enter numbers in the blank cells using the keypad. You can overtype existing entries with new numbers if you change your mind, or use the DEL key to clear the cell. You can add your own pencil marks to each cell too, just as with a paper puzzle, to aid the solving process. To save frustration, if you accidentally  enter a number that already exists in either the same row, column or 3x3 box, then the conflicting cells will be highlighted red to alert you. The user entry causing this will be separately highlighted in red and yellow diagonal stripes, with black text. The highlighting will disappear when the error is corrected.
Q. How do I use the pencil marks feature?
A. There are 3 modes of operation for the pencil marks, as follows:
Auto - in this mode, the app will automatically draw pencil marks in every blank cell, and update them as you continue to solve the grid. Note that it only displays the set of legal values remaining for each cell, based on the cell's row, column and 3x3 box. It doesn't make any further logical deductions.
Manual - in this mode, you have full control over the pencil marks. Double click on an empty cell to bring up the pencil mark keypad. Each number will be displayed on either a red or green background, where red denotes "don't draw" and green denotes "draw". Tap the numbers to toggle between the two states. When happy, press OK to draw those pencil marks in the selected cell. The app ignores these marks and will not auto-update them as you continue to solve the puzzle.
Off - in this mode, no pencil marks are available.
Note that you can switch between all 3 states mid-solve. Changing from Auto to Manual preserves the latest marks, and then allows you to edit them further. Changing from any state to Auto will cause the app to take over, discarding the previous entries and redrawing all pencil marks from scratch. Changing from Manual to Off then back to Manual will preserve the previous state of Manual pencil marks.
Tip - You may wish to start off in Auto to deal with the easier cells to solve, then switch to Manual once the simpler solving strategies have been exhausted.
Q. How do Undo & Redo work?
A. A history of user grid entries is maintained during play, enabling the latest entry to be 'undone' and the grid restored to its previous state. This history extends back to the very first user entry, up to a maximum of 99 entries. Once the user has started stepping back through history, the Redo button becomes available, enabling the user to restore the next state forward, up to the latest entry. Note that deleting a cell entry counts as an input, as does using 'Solve Cell' and 'Solve All'. This can be useful to determine whether you have 'broken' the puzzle with incorrect entries - just try 'Solve All', and if it succeeds, then you are still on the right track, and a single Undo will put you back where you were so you can continue. Note also that if you step back through history and then make a new cell entry, all forward history from that point will be deleted. This is a deliberate design choice, intended for the situation where you step back through history to find an error, correct it and want to move forward from that point. You would no longer want the previous, incorrect history.
Q. What if I still get stuck?
A. You have the option to select a blank cell and press 'Solve Cell' which will show you the correct value for that cell. If you just want to see the finished solution, then you can press 'Solve All' which solves the remaining grid completely. If you have got into an impossible position which means there is no longer a single solution (eg by deleting some of the start values) then you will get a pop-up message to that effect. You can use the Undo button to successively remove user input, and test with 'Solve Cell' or 'Solve All' to see if you have recovered to a legal position, as described above. The 'Reset' button will reset the grid to be completely blank.