## SSTV decodes from the International Space Station 10/7/20

The ISS has been transmitting images using SSTV on 2m Amateur Radio for the last couple of days. Here’s a couple of my decodes from yesterday (10/7/20)

1959PST 10/7/20:

1822PST 10/7/20 – this was a low angle pass, so surprised I could receive any from this pass:

## Using my Sudoku Solver to generate new puzzles (part 2)

Following on from Part 1, I’ve been looking at some alternative approaches to generating new Sudoku puzzles. It turns out that having a working and effective solver is essential in generating new puzzles, because you need to know if you’ve generated a valid puzzle, meaning a puzzle with only a single solution.

The generally accepted approach to creating new puzzles is to generate a completed grid and then remove values from cells, checking at each step that you still have a valid puzzle and then stop at a point where you’ve generated a puzzle with the required difficulty. Grading a puzzle ‘easy’ through ‘difficult’ is another topic unto itself, so I’ll come back to this later.

Here’s a summary of possible (but not exhaustive) approaches to generate a completed grid:

### Brute force approach with backtracking

• fill each cell with a random number
• check puzzle is valid
• if puzzle is not valid, go back to previous cell and try another random number

Although this approach would work, it’s obviously not the most efficient approach. Brute force algorithms usually take advantage of the speed of today’s CPUs to offset the inefficiency of trying every combination until you get what you’re looking for.

### Running a solver against an empty grid

• run solver against empty grid
• ensure solver is picking a random candidate column from it’s exact cover matrix (see previous posts for how this is used by the solver)

This approach is interesting as it reuses the Solver, and instead of solving a puzzle with a limited number of givens, the puzzle has zero givens. My Solver uses the same approach with Knuth’s Algorithm X to ‘solve’ an empty grid, it doesn’t make much difference whether there’s a minimum of 17 givens for a valid puzzle or zero puzzle, it will still fill the grid.

The downside of my current Solver approach though is that it will always fill the grid with the same combination of numbers due to the way it always picks the next value based on the next unmet constraint with the least number of remaining possible candidates. While this approach sames significant time solving a valid puzzle, it does mean the approach has a predictable result and doesn’t produce random filled grids. This could be updated to add some randomness, but I think there’s an easier approach.

### Run solver against a partially populated grid

• Populate row 1 in the grid with a random combination of 1 through 9
• Run the solver against this ‘seeded’ grid

Like the previous approach, this reuses the solver. However, by ‘seeding’ the first row with a random combination of 1 through 9, this does result in different grids being generated. Without any closer investigation however, due to the predictable results from my Solver, I think at best this probably means this approach will only generate 9! combinations (362880). While this is enough puzzles to keep you busy for a least a few weeks (!), there still has to be a better approach.

### Conclusions

Since I already have a working solver, I’m going to use the ‘seeded’ partially populated grid approach, run my seeder against it, then remove a number of candidates, then run a ‘difficulty grader’ against it. I’ve been working on the ‘grader’ for a while now, I’ll post an update on the approach I’m taking with that in another post.

## Understanding website paywall approaches

Subscription paywalls are becoming more and more common on a number of news and other sites. Assuming that the sites are using JavaScript and/or CSS approaches in the page to popup floating DIVs obscuring the content of the page, these are surprisingly easy to bypass if you know what to look for.

There’s many other articles (e.g. here) discussing these approaches and this is not intended to be an exclusive list, but here’s a few useful observations:

• Adding a trailing ‘.’ to the domain name works on a surprising number of sites (Google if you’re interested why this works)
• If you’ve tried this and you’re still getting a popup obscuring content on the page, using your browser’s developer tools, use the ‘select element’ feature to click on a floating DIV in the page and then update the CSS to style=”display:none” is also effective. If there’s other script in the place which is adding the CSS to the DIV, right click the DIV and just select the delete element option also works
• Some sites use a combination of approaches. Often script or CSS to disable scrolling the page underneath a floating DIV is also used, so if you’ve removed the DIV but now you can’t scroll the content, look at the style on the body. If there’s an “overflow:hidden” style, remove it, or change it to “overflow:auto”

## Java 15 support in Eclipse

It used to be hit or miss and usually some delay before the latest Java version would get full new language feature support in the latest version of Eclipse. With these new Java version plugins from the Marketplace though, adding support is now as easy as installing the plugin:

The Eclipse team has been doing a great job with these plugins for latest language support.