## An Unusual Review

This begins a review of the advanced techniques of Hodoku, Bernhard Hobiger’s puzzle generator and solver, as described on the Hodoku site’s Techniques page. The post describes the Hodoku program and the special nature of this review.

Hodoku is primarily a learning tool for advanced solving techniques. It’s a drill facility, something like a batting cage. You can dial up curves, fast balls, or sinkers. The program will compose a puzzle to challenge a user on any technique selected from its extensive advanced repertoire, and solve it up to the point where the selected technique is needed. After finding the technique and identifying eliminations, you get a color coded grid displaying the principle candidates of the technique and its removals, with a brief note of explanation. A remarkable feature of Hodoku puzzles is that they usually come out of basic solving with an example of the ordered technique ready to be found on the grid.

From the example displays on the Techniques page, you can link to the pre-solved grid on which the technique is to be discovered, and try to find it yourself. You have to spot the technique on Hodoku’s keypad pencil marked grid. You can even choose to do the pre-solving yourself. The given clues are in black on the example grids. Generally speaking, line marking is a bit tough. In this review, I transcribe all Hodoku grid displays to Sysudoku notation.

As an example of all of that, here is the line marked grid for a Hodoku unique rectangle, Type 2. It’s the same as Andrew Stuart’s Type 2, in which two candidates of the same number on one side stand guard against the deadly rectangle of a multiple solution.

You could do a personal grid searching experiment by loading the example puzzle into Hodoku. Click on the grid for directions.

The conception and execution of Hodoku is admirable. But as you see here, the batting cage solving experience is not the same as you have on a newspaper or collection puzzle. One reason is the “composed to order” characteristic of the puzzle itself, but it is more the fact that you know what to expect. A batting cage is no replacement for a talented pitcher at game time.

The descriptions and examples the Hodoku Techniques page are there to identify and explain the repertoire of the system, which I believe, reflects the general consensus of human solving experts at the time of the Hodoku launch. Hobiger’s major influence seems to be Paul Stephens, whose recent books are reviewed here. Bernhard’s aim was to create accurate examples, and solve them by technique rules and to trace solutions, for the current techniques of the day. He did an excellent job of that.

Hodoku is not a puzzle collection, and there will be no rating table of ten preselected puzzles. However, a primary reason for the review is Hobiger’s excellent selection of examples to illustrate his training system. The review is an opportunity for me to recall, and link back to additions to human solving repertoire made since the Hodoku launch. It is an opportunity also to correct misconceptions often spread by computer solver programmers, Hobiger included, about solving Sudoku puzzles with neurons rather than CPU’s.

The Hodoku active training system provides teachable moments, but I don’t recommend any solver for your general use in your Sudoku adventures, as opposed to training for them. To my mind, it’s pointless to have your computer solve a Sudoku with mysteries to be discovered. If the solution were all that important, we’d just look in the back. We get to do it for fun. With the emergence of Sysudoku basic solving, especially the bypass, I wouldn’t even consider letting my computer clutter the grid with too many starting candidates.

The Hodoku batting cage doesn’t help much with basic solving. Examples show pencil marked grids in keypad style, with the advanced technique marked. In Techniques pages, Naked and hidden subsets are defined and illustrated. No basic procedure, with stages, is suggested. Not even number scanning. The candidates appear, courtesy of Hodoku.