200e200w, thanks for your informative post!

200e200w wrote:I wrote:I don't really understand those ratings.

These ratings are:

SudokuWiki - I don't understand Andrew's rating system.

Hodoku - sum of ratings of all steps. Also used by SudoCue.

Sudoku Explainer - rating of the hardest step required to solve the puzzle.

I think SudokuWiki has a summing point system similar to Hodoku's. At least their ratings are usually in step. Sudoku Explainer, however, has sometimes quite weird ratings from a manual solver's point of view. I think the perceived difficulty of a puzzle depends mainly on three variables: the most difficult technique required (which, of course, is somewhat subjective), the number of steps required (times their difficulty), and the narrowness of the solve path. The SE ratings apparently only count the first variable, though I'm not sure how exactly it calculates the result. Even that doesn't explain some things. For example, compare the puzzle above with this other puzzle that has the same SE rating of 8.4:

000900030004700600081054002005000000000020308000090060000070800017000000400106050

- Code: Select all
`+----------------------+-------------------+--------------------+`

| 2567 2567 26 | 9 168 128 | 145 3 145 |

| 2359 2359 4 | 7 13 123 | 6 8 15 |

| 36 8 1 | 36 5 4 | 79 79 2 |

+----------------------+-------------------+--------------------+

| 1236789 234679 5 | 3468 13468 1378 | 12479 12479 1479 |

| 1679 4679 69 | 456 2 157 | 3 1479 8 |

| 12378 2347 238 | 348 9 1378 | 12457 6 1457 |

+----------------------+-------------------+--------------------+

| 23569 23569 2369 | 2345 7 359 | 8 1249 13469 |

| 235689 1 7 | 23458 348 3589 | 249 249 3469 |

| 4 239 2389 | 1 38 6 | 279 5 379 |

+----------------------+-------------------+--------------------+

I don't think their difference is just in the number of steps required! I'm pretty sure this latter puzzle requires not only many more but also much harder steps (both Hodoku and SudokuWiki seem to agree). On the other hand, the other puzzle is solvable with pretty basic AIC techniques: not even ALS-nodes are needed. The only "advanced" feature in my solution is group nodes. Still, both are SE 8.4.

I wrote:I had the same number of steps (11) as Hodoku...

That was 11 unedited steps, meaning I took some steps that weren't necessary but recorded them anyway to demonstrate a human solving process. It's hard to calculate on the go which eliminations end up being important, even though I try to have some strategy. I guess the same is true for software solvers unless they analyze and optimize the full solve path before showing it. On the go they usually seem to pick the simplest step available (depending on how they're ordered) without much strategic planning.

Anyway, my relevant original path was 8 steps. Afterwards I managed to reduce it to 7 while also simplifying the steps (no more ALS-nodes) and making the progression more logical:

1. Grouped X-Chain (Kite): (5)r6c4 = r6c79 - r5c8 = (5)r7c8 => -5 r7c4

2. Grouped AIC: (5)r9c4 = r6c4 - (5=6)r4c5 - r12c5 = (6)r2c4 => -6 r9c4

3. Grouped AIC: (5)r7c8 = r5c8 - r6c79 = r6c4 - (5=9)r9c5 - r9c9 = (9)r7c7 => -5 r7c7

4. X-Chain: (5)r2c1 = r4c1 - r5c2 = r5c8 - r7c8 = (5)r9c9 => -5 r2c9

5. Grouped AIC: (5)r2c1 = r4c1 - (5=6)r4c5 - r12c5 = r2c4 - (6=9)r2c9 => -9 r2c1

6. AIC: (9=5)r9c4 - r6c4 = r4c5 - r4c1 = (5-4)r2c1 = (4)r9c1 => -9 r9c1

7. UR Type 1 (68): r16c79 => -68 r1c7; stte

I don't know if it can be made much shorter without resorting to more complicated steps. My point is: if that puzzle can be solved with six relatively simple chains and one UR of the simplest and most obvious kind, how can it have the same SE rating as the other one??? That part I don't understand at all.

Using Hodoku's "Find all steps" tool, powered by all techniques up to Kraken Fish, I managed to reduce the number of required steps to only 3. First is a Kraken Finned X-Wing, second is a Kraken Finned Franken X-Wing, third is UR Type 1. I'll post that solution later.

Thanks for showing that! Pretty cool. While I have no immediate plans to start fishing or using krakens, your example helped me understand those concepts better. Maybe one of these days. My current manual solving skill and tool sets support finding chains pretty well, but not so well other kinds of patterns including complex fishes. Therefore I don't consider them human-friendly solving techniques at this point.

Did Hodoku show you those kraken fishes directly or did you do something else to find them? I couldn't find them in the list but maybe I didn't look hard enough (there were about 200 kraken fishes listed). How did you know which ones would be the most productive?