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Pentominoes: First Working Code

It works !! Moving on from the previous starting code.

  • I added a routine which would select one of the possible solutions from those available. All possible solutions are found and stored. Currently we select a solution for each piece randomly, later we might select the possible solutions one by one or high grade the solutions based on some algorithms we might define to obtain quicker convergence. At the moment once a solution is selected for a given piece, we move onto the next piece. If no solution is found for the next piece then we go back to the beginning again. A final solution, and associated graphics, is only produced once a solution is produced for all 5 pieces. Currently when we go back to the beginning we restart the searches each time and carry on with random selections. This is a bit wasteful, but it will work eventually.
  • I started with 3 pieces, with the first hard coded, and then checked the code such that solutions were found in the order 1,2,3 and 1,3,2. Next we freed up the first piece and tested orders 1,2,3 and 3,2,1. Ultimately all 12 pieces must be tested in random orders.
  • Next we made the code slightly slicker, and moved onto four pieces. Interestingly, despite previous testing, this uncovered a bug in which piece 3 found no solutions but the code could carry on and find a solution for piece 4. The logic had to be changed slightly to get around this problem resulting in some messy, but working code.
  • The final extension to 5 pieces was straightforward as was adding in the final screen graphics.

My 5 pieces are currently hard coded into orientations which I know will produce a solution for testing. The next level of complexity will be to add in rotations and reflections of the pieces. At the moment the piece shapes are hard coded – it would be possible to carry on with this method and hard code reflections and rotations for each piece. There are 63 possible combinations. I plan to investigate a more efficient way of storing the pieces in arrays and then using array manipulations to achieve the rotations where required. This will be the result of the next post.

Remember you can subscribe to the posts with a link to the right of the main post. I’m also experimenting linking these posts to my linkedin profile.

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Pentominoes: First Code Attempts

As I’ve mentioned, the actual coding is slightly ahead of the blog pages, because I’ve spent ages trying to configure the blog and the online Tonnta Store. All this while supervising two recalcitrant daughters for zoom school during covid lockdown.

I generally write code in chunks, testing each section and building in more detail. Well, I havn’t coded anything in 20+ years, so maybe I shouldn’t generalise ! The first code hardcodes the first piece, an “L” shape and runs a loop around a 5×5 box to fit in the second piece. Arrays are used to store both the solution and each piece with a trial being produced by adding the arrays together. There is code to check that the piece doesn’t fall outside of the array of the solution (in this case 5×5. I’m still thinking I might pad the solution area to avoid these checks, but then that would give me a much bigger search box which could be wasteful. I also wrote a text printing subroutine for debugging purposes since it would be quicker than the graphics shown earlier. The code cycles through possible solutions printing various diagnostic information together with the current position of the pieces. I used the wooden blocks that I bought to help visualise and check the various solutions that the code produced. In the above diagram the two pieces overlap (1+1)=2 so this would be rejected.

The next modification was a routine to check for isolated holes and thereby reject possible solutions. It searches around the potential hole (in this case top left) and if the hole is completely isolated then the potential solution is rejected. This is the first stage in “high grading” the possible piece positions. I’ve yet to decide whether this type, or further position grading will be required. It may be that too many checks will slow everything down – such a solution will be “found out” soon enough because the last pieces will ultimately not fit and the code will have to go back to the beginning anyway !

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More on Pentominoes

I was thinking about making a set of the 12 Pentominoes for my kids using salt dough or lego. However during lockdown we only have a few lego sets here and the pieces are tied up. I couldn’t bring myself to dismantle part of the Death Star for pieces, even though my daughters have removed most of the guns and fitted the Death Star out as bedrooms for Harry Potter lego minifigures. A few have commented on my linkedin post on different ways they have made Pentominoes in the past.

I found the following pictured game set online at Cogs the Brain Shop in Dublin. There are some quite nice things on the website and the delivery was fast, only a few days. My daughters went to work to find a solution…..and I was using the set to help visualise algorithms for my computer solution. Would we find a solution manually or my computer first ? The race was on…..

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As in all good projects, I decided to start with the final display, that is the graphics. I was able to locate the BBC microcomputer user guide in my loft. My guide is dated 1982 and acknowledges, among others, Richard Russell who wrote the BBCSDE interface that I decided to use in the previous post. The user guide contains a list of BBC BASIC commands and covers basic graphics with some sample programs including a classic Moon Lander game. I started by flicking through the book.

It didn’t take to long to write a simple BASIC code that would plot the grid and display Pentomino pieces in different colours. The code itself is quite useless, but it was a good start and will come in handy for displaying solutions (assuming I find any) and debugging algorithms that aim to fit the pieces together. I also found that the Teletext graphics mode contains several of the Pentomino pieces and that it is possible to define more as characters. An algorithm could probably be made directly using graphics commands and direct memory addressing or just based on the colours. However I decided against that approach since it would be difficult to port to other codes in the future.

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The Software Platform

This blog is already lagging a few days behind actual events, but that is probably of no consequence to the few readers likely to encounter it. Since I had decided to code in BBC basic, I was faced with two options

  1. Get the old BBC micro from the loft. I know it works, but it needs to be connected to a TV screen via a coaxial cable, so that’s marginally inconvenient to my TV watching family. Somewhere I have a 12″ B&W TV that I originally used for the BBC micro, but I’d have to find that. Also I’d have to get the floppy drive working. My DAD had a 512Kb hard disk, but I’ve never used that….all in all it seems a bit too retro even for a retro project like this.
  2. Use BeebEM – a PC based BBC emulator which I used in the past to play classic BBC games such as Elite. I had a little bit of a fiddle around with this and wrote a few lines of code. However it wasn’t easy to save the code in any kind of text file.

This is a screen shot of BeebEm running on Windows10 PC.

  1. Whilst searching around in BeebEm for something to save the code in a way I can use in other applications, I found BBCSDL a version of BBC basic for the PC ! This website is well worth checking out for a wealth of historical information and has been recently updated. The code window itself features two basic editors and can save the code as ASCII text (my original aim).

So, decision made, I decided to use BBCSDL to produce code.

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I discovered the existence of Pentominoes quite recently while reading Arthur C Clarke’s “Imperial Earth”. Checking out the Wikipedia page I discovered that these were a quite recent concept (well, 1953) and had originally been made popular by Martin Gardener in his Scientific American column. Somewhere in the loft I have Gardener’s “Mathematical Puzzles and Diversions” which probably includes a section on Polyominoes, of which the Pentomino is but a 5 cell subset. Another famous subset is the Tetromino which was used in the video game Tetris. I think in the loft I have an old gameboy with Tetris, I wonder if it still works. I recall I bought Gardener’s great book to learn more about hexaflexagons which I had seen featured in the 1974 Royal Institute Christmas Lectures by Eric Laithwaite. Somewhere in the same loft (the loft gets quite a few mentions but I can’t hyperlink to it), I also have a letter from Laithwaite. Since I was only 9 at the time I probably didn’t understand most of the concepts of the lectures or the books, but they probably had an influence on my scientific career. Someday soon I might seek out the book and see if I understand it now.

I introduced the Pentominoes to my children (8, 10) using coloured paper squares in pretty much the same way that Clarke introduces them in his books to the fictional child, Duncan. In 15 minutes my children had found more of the possible 12 shapes (ignoring reflections and rotations) than I did. The paper versions I had made, even when improved with light cardboard and blu tack didn’t really work out that well, but did provide a few hours of non screen related diversion for the family after covid lockdown school had finished.

Later, whilst out walking the dog, I decided that it would do my brain good to write a computer program which searched for solutions to a Pentomino type problem. Of course there are doubtless whole websites dedicated to such containing algorithms, discussions and free programs to download. However, for once, I wanted to develop something from scratch like we used to before the internet ! I also decided to use BBC basic which is also a throwback to the 80’s for me (these is also an old BBC microcomputer in the loft).

I first setup a website in 1999. However once I set it up I couldn’t think of much to populate it with that would be of interest to anyone else. I think I added song lyrics of tunes I could play on the guitar, a few pretty photos and a thought for the day section. I imagine this is what early Facebook pages looked like and it turned out that I didn’t have many thoughts. In 2001 I changed the site into which contained some basic training material, but I was never very much attracted by the blogging concept. When I setup this store to enable sales of training courses and other services the blog part came for free, since I used WordPress as the basis for the ecommerce part of the website. I had the idea that visitors to the website could use the blog posts to file problems or curiosities with the seismic data which I (or other users) could then comment on. The purpose of this particular blog, though, is to record my thought process and progress towards the goal of writing Pentomino code which will find a solution to a given problem and provide appropriate graphical printout.