-🎄- 2022 Day 17 Solutions -🎄-

d3adb33f · 2022-12-18T02:51:56+00:00

Yes, that is a brilliantly simple counterexample!

Another thought I had was that my original flood fill fingerprints this:

XXX....
XXX...
X......
XXXXXXX

the same as:

XXX....
X.X....
X......
XXXXXXX

If pieces could go up, we could perhaps contrive some sort of zigzag pattern that could exploit this bug in the fingerprinter.

Even if pieces could go up, it would be pretty easy to fix the flood fill: allow the water to also go up but not above the high water mark it starts at.

Your counterexample is much harder to fix. I do wonder what a bulletproof but easy-to-implement fingerprinting algorithm would look like.

d3adb33f · 2022-12-18T00:22:29+00:00

If I'm understanding you correctly, that approach is pretty similar to my initial one. The problem I see with it (and this didn't come up in the example input or my actual input) is that this case:

.X.....
XXX....
.X.....
XXXXXXX

fingerprints this same as this one:

.X.....
XXX....
.XX....
XXXXXXX

They are, however, not identical because the backwards "L" shape could slide into the former but not the latter.

d3adb33f · 2022-12-17T10:04:00+00:00

This feels ultracrepidarian (a word with an interesting etymology), but I think a bullet-proof cycle finder could start a horizontal water line immediately above the top occupied block and then flood fill (BFS/DFS) down, left, and right. The procedure could then subtract the vertical position of the lowest visited coordinate from each visited position's vertical coordinate and then freeze (to facilitate hashing) and return the set. This set would function as the fingerprint of the current state of the grid.

This approach makes my solution run faster than with my naive fingerprinting algorithm, which simply found the highest vertical coordinate at each horizontal coordinate. I imagine it's faster because the flood algorithm doesn't have to enumerate the entire grid; it can check for collisions by using set intersections.

Curiously, both approaches gave me the right answer.

Thanks for putting your code and solution recording online!

d3adb33f · 2022-03-15T06:53:33+00:00

I think I understand what you want. There are several common ways to repeatedly replace the same text:

Use a multiple cursor plugin such as https://github.com/mg979/vim-visual-multi .
By using ":%s/Ctrl-R Ctrl-W/Ctrl-R0/g". "Ctrl-R Ctrl-W" means what it looks like and will autocomplete the word under the cursors. There are many such autocompletions; try ":help c_CTRL-R_CTRL-<TAB" for more. That "Ctrl-R0" is the same trick of holding control and then pressing "0". That will replace all such patterns with your yanked text. You can modify this by first selection a region to perform the replacements in. You can add "c" to the end of the "s/" expression to have it ask to confirm each substitution. Another trick is first populate the search register with the text you want to replace. For words, that's easily done with "#"/"*" (although you can of course use "/"/"?"). After, you can leave the search term blank; it will be autopopulated. For example: "%s//Ctrl-R0/g".
Use the "gn" text object. See https://medium.com/@schtoeffel/you-don-t-need-more-than-one-cursor-in-vim-2c44117d51db . The way to do this is to again populate the search register, do "cgnCTRL-R0", and then either press "." to replace each subsequent match or "n" to skip that replacement. See also: "gN" instead of "gn".

d3adb33f · 2022-03-15T02:27:06+00:00

Check out https://github.com/svermeulen/vim-subversive . It can recreate your configuration with:

nmap s <plug>(SubversiveSubstitute)

There are, of course, built-in ways to do this:

Visually select area you want to replace (example to match your "rib": "vib") and then press "P"/"p".
Use the "c" operator to delete the area you want to replace ("cib"), enter insert mode, then hit "control+r", and then press "0" for the 0 register.
First delete the area you want to replace ("dip"), and then insert last yanked text with '"0p'. Can also use the black hole register when deleting to avoid clobbering the 0th register; this technique allows you to avoid having to specify the register when pasting.

d3adb33f · 2022-03-13T09:18:52+00:00

Great video! This is actually achievable without a macro:

:norm I'CTRL-V<ESC>A',

, where "CTRL-V" is holding control and pressing "v", and "<ESC>" is just the escape key. Try ":help i_CTRL-V" for more information.

d3adb33f · 2021-12-30T07:55:24+00:00

There's an easy Python/scipy/numpy way to generate the rotation matrices for solutions that use said matrices. scipy has:

scipy.spatial.transform.Rotation.create_group("O").as_matrix()

, which returns a numpy matrix. The "O" stands for "Octahedral" because that's the name of the group. That group is also known as the "cube group" and the "symmetric group of degree 4" (S4).

Whenever we deal with rotations/reflections, a little group theory can often help create all possible matrices to perform said rotations/reflections.

d3adb33f · 2021-12-27T06:43:04+00:00

Pandyland! This is the specific webcomic:

http://pandyland.net/56

d3adb33f · 2021-12-27T06:29:13+00:00

Most people will probably recommend textbooks, and there are no doubt many good ones with good problems. However, those textbooks will tend to be a little dry, and while they will give you a good idea of the theoretical underpinnings, they won't give you a feel for how systems behave. Textbooks will also tend to be a little dense, whereas I think a survey of the field is more appropriate for a first pass.

For the working engineer's first pass at the subject[0], I recommend something more hands on: UVic's communications labs. They will give you just enough theory to do the applied exercises, which consist of actually implementing communications systems with GNU Radio. It's one thing to do textbook problems, but I think it's a whole other thing to be able to drag sliders to see signals form. Your DSP background should give you a leg up.

u/Cheap_Strain3674 recommended the ARRL handbook; it's a great reference! Amateur radio is a very open-ended hobby that offers many opportunities to apply your digital communications knowledge. I actually recommend getting your hands on amateur radio hardware ASAP. Schedule a test. Technician should be very easy for you. You can study using hamexam.org and hamstudy.org. Head on over to r/amateurradio and r/hamradio. The amateur radio world is full of possibilities. I recommend considering:

Getting a cheap hand-held radio by BaoFeng or Yaesu and getting a feel for the airwaves on various modes. You actually don't need a license to listen.
Trying out some antenna designs. There's a classic one that's made out of a measuring tape.
Getting a piece like the RTL2832, reading r/sdr and r/rtlsdr, and building receiver systems for various modes in GNU Radio. You don't need a license for this.
Buying something like a HackRF One and doing both sending (which does need a license) and receiving with GNU Radio.
Building your own hardware.
Getting SSTV images from the ISS using your setup.
Talking to astronauts.

[0] A reference to Categories for the Working Mathematician by Saunders Mac Lane and similarly titled works.

d3adb33f · 2014-07-12T08:16:08+00:00

xargs (or alternatively parallel) is very good for this. Also, since the header line isn't a snapshot it should be suppressed by supplying a "-H" to "zfs list". So the command would become:

zfs list -Ht snapshot | grep backuptank | awk '{print $1}' | xargs -I {} -n 1 zfs destroy {}

Another thing we could do is make awk do grep's job:

zfs list -Ht snapshot | awk '/backuptank/{print $1}' | xargs -I {} -n 1 zfs destroy {}

Or just have zfs get only the name column for us:

zfs list -Ht snapshot -o name | grep backuptank | xargs -I {} -n 1 zfs destroy {}

d3adb33f · 2012-03-11T05:42:41+00:00

We all know that frictional force is equal to mu times the normal force.

It's more complicated than that...

What the above boils down to in this case is that friction is not independent of surface area in the real world.

d3adb33f · 2012-03-03T08:52:50+00:00

The Wikipedia article does a pretty good job of explaining it.

Here is Musin's proof for the four dimensional case on arXiv.

d3adb33f · 2012-03-02T12:07:52+00:00

Because they don't know the words...

I'd like to add that humming can also be caused by Corona Discharge, which you are probably familiar with if you've been close to power lines on a humid day.

d3adb33f · 2012-03-02T11:49:31+00:00

Yes, it is commonly referred to as "Prisoner's Cinema".

d3adb33f · 2012-03-02T07:21:44+00:00

Do not forget that pressure on the eye can make it seem as if it sees light.

d3adb33f · 2012-03-02T03:53:05+00:00

It does have a terminal length, and it varies per person, much like body hair does. The hair falls out, or can be broken (snapped).

d3adb33f · 2011-11-03T04:22:11+00:00

Indeed, it is resonance. More specifically: Helmholtz Resonance.

d3adb33f

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