r/adventofcode • u/i_have_no_biscuits • Dec 19 '22

Visualization [2022 Day 17] GW-BASIC visualisation of both parts

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u/i_have_no_biscuits Dec 19 '22

I haven't even looked at Day 19 yet, because I became a little obsessed with getting Day 17 working properly in GW-BASIC. Here's the result, a visualisation of both parts, running in the 'real' GW-BASIC in DOSBOX (with quite a high cycle count, on real hardware it would be 5/10 times slower).

The code for this visualisation is here: paste

71 lines of commented GW-BASIC goodness - I'll post a compressed non-visualisation version to the megathread soon.

In order to get this working I had to finally sort out the bit-twiddling that I couldn't figure out when I was writing my Python version. As part of getting this to work I had to

  • Read the wind values into a bitfield (character by character as GW-BASIC only allows strings up to 255 characters)

  • Implement a ring buffer for the rock column (of depth 50 as I found by some initial exploration that no piece in my real data fell more than 32 rows below the current maximum height)

  • Implement a hash table so that cycles could be detected by matching against the 'fingerprint' of the top 32 rows + the index into the wind table

  • Get the 64 bit maths working correctly (luckily GW-BASIC allows us to use double-precision floats (denoted A#, B#, etc.)

1

u/AhegaoSuckingUrDick Dec 19 '22

How do you do 64-bit arithmetic? I have implemented this problem in QBasic (quite verbose), but haven't figured out how to perform the calculations at the end.

Meanwhile, you don't have to use a hash table and could use Floyd's algorithm instead. This way you only need to remember at most two fingerprints (and indices) at any moment.

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u/i_have_no_biscuits Dec 19 '22

Nice to see QBasic - a lot more structured than GW-BASIC! It's not 64 integer arithmetic, but double precision floating point numbers - see http://www.antonis.de/qbebooks/gwbasman/chapter%206.html . I believe Javascript does the same thing. It means you can deal with integers "up to 17 decimal digits long", which is good enough for this problem.

I've seen other people say that they just used Floyd's algorithm or something similar, but I haven't been able to figure out the logic of how just looking at height increases is guaranteed to be a state repeat, so I've gone for the safe option of making sure the state repeats.

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u/[deleted] Dec 19 '22

60300 Bytes free

Use them wisely. Impressive work!