Making "Rodoku"

Back in 2002, I constructed a puzzle [editor's note: dead link] for my 40th birthday. For reasons I've never been able to explain, that puzzle just sprang into my head and didn't require any technology to support it. This year I couldn't manage, by hand, to create something that even looked like a valid Sudoku that I was happy with. So I decided to get a little help.

I decided that I would write a computer program that would validate the grid as I filled it in. I wrote it in JavaScript and ran it in my web browser. Using that tool I was able to immediately see when I'd entered something that was illegal. I had just put together a layout that I liked when it hit me: Now that I have this tool, I might as well use it to create a Sudoku that can actually be solved.

Rather than write a Sudoku solver myself, I looked around on the net and found a number of them. One, in particular, was quite good at telling you whether the particular puzzle in question had more than one solution. So I rewrote it for my purposes.

What I ended up with was a Sudoku construction grid in which I could type various letter combinations and in real time I would be told whether that was enough information to produce a single legal solution. It turned out that the first legal grid that I had produced had 4 legal solutions, and now I was motivated to do better. Off and on over the course of a week I tried different layout techniques until I produced the grid that is on the card. It has one legal solution (I hope!) and, I think, looks quite elegant on the front of the card.