Recently on an Etihad Airways flight, I tried their onboard sudoku program and picked the hardest option available. This was the initial state of the puzzle:
| 5 | 6 | 8 | 7 | |||||
| 9 | 2 | 1 | 3 | 5 | 6 | |||
| 3 | 1 | 4 | 9 | |||||
| 9 | ||||||||
| 7 | ||||||||
| 5 | 9 | 8 | 4 | |||||
| 1 | 2 | 6 | 5 | 9 | 4 | |||
| 3 | 4 | 9 | 1 |
By the end of the flight, I'd made little progress towards completing it. I took a picture of the partial solution to write down on paper and continue later.
| 4 | 5 | 9 | 6 | 2 | 8 | 7 | ||
| 9 | 2 | 1 | 4 | 3 | 5 | 6 | ||
| 3 | 1 | 4 | 9 | |||||
| 9 | ||||||||
| 7 | ||||||||
| 5 | 9 | 8 | 4 | |||||
| 1 | 2 | 6 | 5 | 9 | 4 | |||
| 3 | 4 | 7 | 9 | 1 |
As it turns out, this was about as far I was going to get (there were a few more deterministic numbers but not many). After a few days of frustration and learning various new sudoku solving techniques, I gave in and put it into an online solver. This particular puzzle has 10 unique solutions (according to sudokuspoiler.com), which means it's not a valid sudoku puzzle. Unsolvable is maybe not the correct word to describe a sudoku like this, as it's possible to brute force a solution, but it is certainly invalid.
On the return flight, I tried again on an easier setting hoping to avoid any guesswork, only to be faced with this state:
| 4 | 3 | 1 | 5 | 9 | 6 | 2 | 8 | 7 |
| 9 | 7 | 2 | 8 | 1 | 4 | 3 | 5 | 6 |
| 6 | 8 | 5 | 2 | 7 | 3 | 9 | ||
| 2 | 6 | 9 | 7 | 8 | 5 | 3 | ||
| 7 | 5 | 4 | 1 | 3 | 9 | 8 | 6 | 2 |
| 8 | 1 | 3 | 6 | 4 | 2 | 7 | 9 | 5 |
| 5 | 9 | 8 | 4 | 2 | 7 | 6 | 3 | 1 |
| 1 | 2 | 6 | 3 | 5 | 8 | 9 | 7 | 4 |
| 3 | 4 | 7 | 9 | 6 | 1 | 5 | 2 | 8 |
It was only on writing this blog did I realise that this puzzle is in fact just an easier version of the harder puzzle! Does this program just have a single sudoku and provides more or less clues depending on the difficulty setting? Etihad Airways neglected to comment on this, as I neglected to tell them about it.
