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Advent of Code 2019 (adventofcode.com)
552 points by scottdw 11 days ago | hide | past | web | favorite | 122 comments

Many of the comments are about the event as a whole, but I'd like to commend the author for the day 1 puzzle, which I think is very cleverly written. Many people (including myself) use these as an opportunity to try out learning a new language, and the first puzzle does a good job of making sure you know enough basic operations for some of the puzzles coming up. The puzzle itself wasn't very difficult, you could do it by hand if you really wanted to, but it required you to know how to read and parse lines in a file, handle compound data structures, iterate through, and perform basic operations (floor, casting strings to ints, etc). I thought it was really well done, and creates the basic knowledge for the puzzles ahead.

You can watch authors talk about how he prepares challenges and other challenges he had with AoC :D https://www.youtube.com/watch?v=gibVyxpi-qA&feature=emb_titl...

Learning new environments can be painful so it’s good it starts off easy.

I’m trying to solve them with Swift Playgrounds on the iPad, for example. And I can’t figure out how to create a simple text file for the data.

I started last year using Swift Playgrounds. I was lazy and just pasted the input into a multi-line string. I had to switch away from Playgrounds after a couple of days because it would crash due to memory issues. It traces every line of execution and the intermediate values of all variables. I really enjoyed doing the challenges in Swift, but I did have to switch to using my Mac to do it.

This year, I’m learning Python by using a Raspberry Pi 4 directly connected to my iPad via USB. Should be another fun month!

I solved the first part using the examples. Swift is nice.

   func calcFuel(_ mass:Int) -> Int {mass / 3 - 2}

   let z0 = massList.map(calcFuel).reduce(0, +)
or simply ...

   let z1 = massList.map({$0 / 3 - 2}).reduce(0, +)
Any idea how to read the data on the iPad?

I copy my session cookie and send a web request for the inputs. I should write some sort of caching code though..

i just copy/pasted the data into the source instead of dealing with IO.

I had to create the plain text input file in iCloud with my Mac then insert it (Big +, third option) in the iPad’s Swift Playground. Need a text app that will save to Files for a complete iPad solution. Also, a Swift Playground bug reverted my final solution to the code from my first few minutes, losing my code. That was frustrating...

Anyway, a complete Swift Playground on iPad solution:

   import UIKit

   let path = Bundle.main.url(forResource: "input01", withExtension: "txt")
   var text = try String(contentsOf: path!, encoding: .utf8)

   let massList:[Int] = text.components(separatedBy: .whitespacesAndNewlines).compactMap({Int($0)})

   let z1 = massList.map({$0 / 3 - 2}).reduce(0, +)

You can option+tap a link in Safari to download to Files. Hold down the command key to see available shortcuts.

I couldn't figure it out on a Mac. Then I Googled it, and decided I'm better off solving this in languages I know (whatever I feel like today: JS, Python, Java, perhaps even Erlang) or in a language that has a reasonable standard library (Rust) than in this mess: https://twitter.com/dmitriid/status/1201441652507844608

Last night I gave the complete solution in Swift below. Here are the two lines to read the file:

   let path = Bundle.main.url(forResource: "input01", withExtension: "txt")
   var text = try String(contentsOf: path!, encoding: .utf8)

There's no chance in hell anyone learning Swift can figure these two lines out.

Besides, there are immediate additional questions:

- what's a bundle, and why does a Bundle deal with urls?

- What's the difference between Bundle.main.url and FileManager.main.urls, and why one and not the other?

- where does the resource come from?

- how do I read a file from a path?

- Why is String responsible for reading (and writing!) contents of a file? In which world is this good design?

I use pastebin raw and get stuff from web requests sometimes rather than local files.

I've been creating multiline strings and then just using components(separatedBy:)

> Many people (including myself) use these as an opportunity to try out learning a new language

I hadn't thought about that! I'll take this as an opportunity to learn Rust!

Checking back in - have you done it, is it going OK?

I can't do it when the puzzles come out (too late in UK) but I've really enjoyed Rust so far and I got my 10 stars so far.

Results are really nice, and match and enums etc. really make the language nice.

Rust really has taken many of the best parts of Python, functional paradigms, and lower level languages, and the compiler errors - being warned about missing branches in matches! I'm hoping that finishing 2019 with Rust will lead to more in 2020!

I'm doing the same thing this year!

Thanks to these comments, in spite of already having starting advent coding, I have begun learning basic Rust, have the stack and development environment set up and I'm looking forward to doing the first two days again in Rust...

> The puzzle itself wasn't very difficult, you could do it by hand if you really wanted to, but it required you to know how to read and parse lines in a file, handle compound data structures, iterate through, and perform basic operations (floor, casting strings to ints, etc).

I don't believe that it required any compound data structures, just looping through the lines in the file.

You're right about the rest, though.

Good idea.

I have used http://www.https://cryptopals.com/ for a similar purpose, with the added benefit of learning about cryptography as a parallel outcome.

Poorly formatted link, it should be https://cryptopals.com/.

It's faster to just paste the input into your favorite editor, and massage it gently until it is a literal in the language you're using. No parsing necessary.

lol I actually did this typing into source comma down comma down comma... Only after reading this I realized how lazy I was so went back to do it properly. :)

Self promotion, but as this is my first year doing advent of code I'm solving all of the problems using awk[1], the solutions will be published here[2]

[1]: https://man.openbsd.org/awk

[2]: https://jo.ie/advent-of-awk-2019.html

I've used awk for one year of Advent of Code: https://github.com/bewuethr/advent-of-code/tree/master/2016

Some problems feel like they were designed to be solved in awk, like the assembly style ones. Others, not so much... But it was great fun overall and I learned a lot about awk.

With respect to the fact that 99% of my use of awk is done like this:

  > | awk '{print $<word>}'
The solution to the first exercise looks surprisingly clean. It will be interesting to see how they continue as the challenges get harder. Bookmarked!

As someone using a completely mainstream and normal language (well, Rust), I salute you.

I was doing a little of catching up on older problems to improve my speed to get on the leaderboard and I found that the fastest way to finish one problem was to use oeis.

After seeing the problem [0], I didn't immediately think to use oeis because the manhatten distance of a spiral is pretty easy to code or do by hand. The second stage has a harder sequence though. It prompted me to check oeis and I easily got the answer [2] within a minute of checking.

Looking back at the first stage, I found a sequence for it [1] and PARI/GP code i could run to answer the question. If I had used oeis immediately I would have a solution to both phases in 2 minutes which would be the fastest on the old leaderboard for the problem [3].





I had plenty of 20-20 hindsight about faster ways to have solved puzzles in the past. I don't know if I was ever #1 on any day on either star. I think I might've been top 10, on one star, of one day, once.

Eric, the author, says he won't accept any puzzle submissions because of attribution and legal issues. This seems like the wrong outcome for people who want to make the site better but don't want credit, compensation, or attribution.

Has anyone seen a site using an IP assignment agreement which is the equivalent of "take my work, I don't want credit, and it's just for your site"?

The Creative Commons CC0 license comes closest, but one could scrape those puzzles and create a competing site, which I'd guess Eric wants to avoid.

It’s a preference in his case that cannot be solved with licensing. Eric doesn’t want even potential ideas to influence him, and he says he has more puzzle ideas than time to implement them.

Personally, I suspect that he wants something that consumes so much of his free time to wholly be his, and I respect that.

The problem is trusting the submitter. How do you make sure derivate work is not submitted?

If you want to help, why not ask the people running the site what you can do to help? Do you actually want to help, or do you want to make puzzles? Or do you just want to discuss a perceived problem that may or may not exist?

“your site” is really complicated. Who is “you”, person, website or the company? If it’s the person can they transfer the rights? What happens if they die? If they donate the estate? If they get hired or contracted? If the website expands scope or changes in any way?

"You" would be the company, and the submitter would assign all the rights to it. Interestingly, I looked and couldn't find any terms of service or company name.

Maybe he enjoys/benefits from the work, I feel like coming up with puzzle ideas would be somewhat fun/beneficial. I would definitely assume that if this were not the case he would outsource it or maybe even open source it.

Last year was my first year and I loved every second of it.

I strongly recommend using AoC as a tool to help learn a new language. Many people, myself included, use it to learn Rust.

The best part about AoC is the community. Everyone is super helpful and kind. The daily Reddit thread has solutions in every language under the sun.

My better half is a designer and is going to participate this year to help learn JavaScript.

This is my first year. As I missed the start time I created an event to remind me and published it for anyone else who'd wish to subscribe. https://calendar.google.com/event?action=TEMPLATE&tmeid=M203...

Looking forward to the fun, the stress and the eventual euphoria.

Could not find the requested event...

An alternative with daily articles across a range of topics: https://news.ycombinator.com/item?id=21675200

This event is a really nice way to try out new programming languages. You can easily find many people doing the challenges in various languages and communities coming together to work through the problems together. It’s also very interesting seeing how each person solves the problem in their favourite language.

For example, here is the Nim communities effort: https://forum.nim-lang.org/t/5588

Always enjoy doing these but there was one last year that nearly broke me. I think it was day15. It was the worst combination of mind-numbing coding and horribly hard to debug special cases. It was purely convoluted with very little to think about which makes for the worst kind of puzzles

I can totally relate about that particular one, I remember that it destroyed a good part of a weekend for me, and I wrote that about it on Twitter, back in January of this year:

"I have thought about the particular challenges of this year's Advent of Code (which I greatly enjoyed once again, with some intermittent mixed feelings), and my conclusion is that some problems might very well be of similar difficulty levels, but with nevertheless very dissimilar frustration-inducing levels. And I think the frustration level is strongly correlated with the feedback signal one can use to debug. Case in point: Day 15, which was almost impossible to debug, once you had a working solution, versus Day 17, which was actually fun to debug, since the visual signal you could use was so strong."

I strongly disliked the puzzles that quickly devolved to correctly handling a ton of corner cases. 15 and 17 were both in this category. What's the fun in that? This is like the boring part of being a developer. Without the payment that goes with being a developer.

That one wiped me out for a week too, and caused borderline psychological problems.

Ultimately though I realised that if I followed the description word by word, and made no assumptions, then I got the right solution. Still, a nightmare: you could pass all the examples, or even some alternate real sample data, and still get it wrong if you got the ordering slightly incorrect.

Yeah, it's really frustrating when the text summary is vague and underspecified. It makes it hard to understand exactly the result the author is looking for, which isn't the fun kind of puzzle.

I think the problem was it _felt_ vague and under specified. But when I read it closely enough, it actually was exactly specific.

Yeah. That sounds like work and me begging business side for better requirements.

Day 15 took me about 8 months, but I finally discovered I had made one wrong assumption that corrupted my solution for special cases...

I hit day 15 and stopped last year. There always seems to be one puzzle which is so irritating it no longer seems worthwhile when I could be playing some music or building something in Minecraft or just lazing around on the sofa with the cat.

Day 15 was terrible for me too. The edge condition I missed was that you could both move and attack in a single turn. It only changed the outcome for me in part 2. I was only able to finally debug it by comparing debug outputs every step with a known-good solution.

If someone is interested in the previous years of Advent of Code they are available as notebooks in Peter Norvig's pytudes repository: https://github.com/norvig/pytudes

Thank you very much for this!

I was aware of Norvig's 2016 AoC solutions, but I thought he never did any of the newer editions. I know what I'll be reading today :)

I'm trying to use J and still struggle with the second part of day 1. Any other J users?

I have found a solution someone posted [0]

It looks similar to what I though I had to write, but I will have to reread it and the documentation to understand it well.

And day 2 seems a bit too complex with my meager J knowledge. OTOH with the J interpreter for android and its terseness it seems the most suited language for programming on a phone, so I will probably stick to it.

[0] https://www.reddit.com/r/adventofcode/comments/e4axxe/2019_d...

I'm actually surprise how many J/APL/K/Q solutions get posted there already. It seems to be popular among golfers.

If you haven't already found the discussion, Reddit is very active and very helpful. Seems to be the primary water cooler for the event.


I'd argue /g/ on 4chan is actually. 3-4 threads get filled a day (~300 replies each).

It's honestly the only real reason to go on /g/ (or really 4chan in general) for me.

I’m finally going to attempt it this year. Using elixir here: https://github.com/sergiotapia/advent-of-code-2019-elixir

Last year I started out using elixir, but I got filtered due to the goblins...

What does this mean?

Probably Day 15 [0] from last year. It was challenging, especially because there were so many rules to implement (in a short time). A slight misreading of the rules would let you pass all the provided examples, but fail on the real input.

[0] https://adventofcode.com/2018/day/15

Idiot-proofing is hard. Figuring out everything people will get wrong is the only reliable way to ensure people don't, but that takes more insight than is needed to set and solve problems correctly.

No doubt. I wasn't really complaining, but that was the one that seemed to cause the most headaches last year. And the text had the problem fully specified, there really was no good way to make it better other than perhaps a different presentation (also a hard problem) or a few extra test cases to tease out those potential misreadings. But you can't catch every error for the participants, coming up with tests is part of the challenge.

It's not really the instructions, but having to implement an obviously imperative ruleset using Elixir. I completed the previous two years using Javascript and Ruby, and it's honestly much easier when you have access to imperative constructs. This year I'm using F#, we'll see how it goes.

Already discussed at https://news.ycombinator.com/item?id=21660423, but since it starts today we'll give it another go.

These are daily programming puzzles? How long do they take on average?

Also, I know somebody who is just starting out at coding. Would it be too hard for them? They are brand new, learning about loops

I completed the whole thing last year; the problems start out very easy, but get dramatically more challenging over the course of the month. Check out the statistics for how many people completed each problem in 2018: https://adventofcode.com/2018/stats

The first few problems are approachable by pretty much anyone who understands how to do arithmetic, file I/O and basic flow control. Many of the later problems require advanced skills such as more sophisticated algorithms (graph theory, dynamic programming, etc.), reverse engineering, and careful debugging.

For what it's worth, I would expect anyone who knows what a loop is to be able to get the first half of today's problem without much difficulty. The second half is a bit less trivial but also pretty easy to understand.

> Check out the statistics for how many people completed each problem in 2018

I suspect you'd see a dropoff like this regardless of whether the problems get more difficult or not. Going to the gym doesn't get any harder in February but they see the same effect.

The scatterlpots [1] of the time taken for the leaderboard (top 100) to finish part 1 (silver) and part 2 (gold) are a better indicator of difficulty.

I'd like to see a plot of average (median or mode) to finish each part but only the site has all that information.

[1] http://www.maurits.vdschee.nl/scatterplot/

Those box plots use horizontal jitter within individual boxes to better show overlapping points. That's a neat technique I haven't seen before.

True; I would be interested in seeing the number of logged-in users who viewed and/or attempted each problem.

If you're medium fast, they start out in the single digit or tens of minutes range and ramp up to some 60+ minute puzzles the last handful of days (but also some 20 minute puzzles, it varies a lot).

I haven't done it since 2017, and I was pretty quick (made that year's overall leaderboard), but on average they took me about 23 minutes (for both parts).

In general the puzzles involve basic operations that any language would have. Functions, data structures, recursion, some amount of back-and-forth between strings and numbers and records. There isn't any reliance on library-like things like making HTTP requests or parsing complex file formats. I think they would be fine for someone starting out at coding.

Every day's puzzle has two parts. The first part is usually easy. The second part is then a twist that requires a change to the solution of the first part.

Apart from that, you can always skip a day if you just can't solve it. The exception is that sometimes a puzzle can reference the solution of an older day, so if you'd solved the older day you would already have an understanding of how to proceed, and might be able to reuse some code.

> I think they would be fine for someone starting out at coding.

The earlier ones, perhaps, but professional programmers struggle with the later ones. I’d never suggest that some one new to programming tries to do all of AoC.

It really helps to (1) have taken an algorithms class and (2) be aware of Python libraries that implement common graph algorithms if you're going for time. At least, that was my impression 2016-2017. The later puzzles are impossible if you don't know a good enough algorithm (they're more or less designed to take impossible amounts of time to completely brute force).

I'd suggest new programmers do any they can reasonably understand and just skip ones that seem like they'll be frustrating or require a "fast" algorithm they don't know. (OTOH, many days' first star can be gotten with a brute force algorithm. It's usually the 2nd star that takes something more clever.)

They take <15min - >2h

First couple of tasks are really easy, but difficulty increases over time and so does time needed to solve them.

Someone who is currently learning about loops might not be able to solve these on their own, but with someone else's help they might learn lots of useful concepts (e.g. linked lists, bfs, basic asm, how to code efficiently...) and try to apply them.

AoC are solving usually those who are quite familiar with programming and want to learn a new language or to compete with colleagues.

I wondered the same, also thinking about people I know who are starting to program.

In the end I think it's not a good fit, given that basic concepts are still being understood, and the mere task of reading a file and iterating over the lines, even the concept itself of what "iterating" is, is already a big challenge...

Trying is free and won't hurt, though, so why not :)

For someone just starting out they would get stuck on this very first problem, it involves recursion. People 'get' recursion, but coding it is something different.

It doesn’t need recursion, you can do a while(round(x/3)-2 > 0) type thing, but recursion is simpler.

Nothing 'needs' recursion, it's always equivalent to an iterative solution. In some languages a recursive solution is more natural and may perform better.

I know of languages that can automatically convert some kinds of recursive code into the iterative equivalent automatically, but I've never seen one that can make the recursive version faster than an iterative equivalent.

Ah true, i just naturally reach for it in elixir

I just knocked out a non-recursive solution in 9 lines of python in five minutes.

The third puzzle mentions Halt and Catch Fire, a brilliant TV show about innovation and the lives of the (fictional) people behind it.

It actually references the thing that the show is named after. If you click on the link, it's not about the show.

I always have a ton of fun with these sorts of things. I always wonder how they come up with these sorts of puzzles. That's not a skill I have.

There's a great talk by Eric Wastl that describes how he does it: https://www.youtube.com/watch?v=bS9882S0ZHs

Advent of Code is a lot of fun! Last year I wrote a little bit about all the different languages/approaches people use to solve the challenges: https://j11g.com/2018/12/03/advent-of-code/

Day 2 part 2, what is it actually asking?

I can't tell from the description what the actual puzzle is...

Is it that I need to generate a program that generates the output 19690720 when run through the function that was written in part 1?

In the first problem they told you what numbers to put in position 1 and 2. In the second problem you want to find the specific numbers for position 1 and 2 that yield the desired output.

Ah ok, thanks!

My reading comprehension obviously needs some work :P

you need to find value of memory[1] & memory[2] where memory is the program from first part, which gets you 19690720

Thanks, five minutes to write the code and get an answer, half an hour staring at it wondering what on earth I was actually supposed to do....

Feel like a bit of an idiot.

So, just like most days as a programmer?

I went to bed before trying it last night, but couldn't sleep. So I did the day one problem early this morning. It was fun, and went quicker than I thought. I guess I must've improved from last year.

Here is my solution to Day 1 Part 2 in Go: https://play.golang.com/p/SQsCdTXk_YH

One line bash solutions: https://pastebin.com/QnKJEXxt

You can do shorter in languages with functional constructs. For example Python:

  sum(map(lambda x: x // 3 - 2, numbers))

Why is that a double slash?

It rounds down the quotient

How intuitive. But thanks anyway.

In Python 2, floor division is the default for integer arguments and exact (to within machine precision) division the default for floating point arguments, but this caused a lot of unnecessary confusion: new users were surprised that 3.0/2.0 == 1.5, but 3/2 == 1.

Thus for Python 3 the new // operator was added for explicit floor division (which is often quite useful in its own right), leaving the / operator to always return an exact (to machine precision) floating point result, even with integer arguments.

cf. https://en.wikipedia.org/wiki/Principle_of_least_astonishmen...


numbers.sumBy { it / 3 - 2 }

one line ruby map reduce:

    input.map(&(-> (f) {->(g) {g.(g)}.(->(g) { f.(->(*args) {g.(g).(*args)})})}.(->(f) { ->(n) { n < 9 ? 0 : (n/3-2)+f.(n/3-2) } }))).reduce(&:+)

Surprisingly readable.

Jeez, tension is real. Specially if you are thinking about cracking top 100. Competition is a b.

Lot of fun though.

That's only possible if you live in the correct time zone, too.

Does anybody know how the scoring system works for AoC?

oh lord, I just started writing a GCODE motion planner, I forgot about AoC.

Would appreciate feedback on my Go code!

I'm sure theres a better way to approach some of these.


Wow, what a great distraction for students to have during finals season! ;)

I like everything about this except the name.

the word advent is religious.

Please don't take HN threads into religious flamewar.


The name is reminiscent of an advent calendar, which traditionally has a labeled door behind which is a piece of candy, small toy or other trinket. These calendars are intended for young children to remind them of the period of advent.

If you are religious, look upon this as a similar thing which reminds you of the season, building in anticipation of the final day.

If you are not religious, it's just a word with no special meaning other than the time of year. Same goes if you are not of a Christian religion.

This is only true when it's used as a proper noun; in which case, yes, it refers one of two things in the context of Christianity.

Otherwise, it's just a normal word which means "the arrival of a notable person, thing, or event".


if this is a religious project, it should be clearly marked as such.

if it's not, why not use a nonreligious word?

Because advent is a common term for the 4 weeks before Christmas even in non religious contexts. And that is how it's used in this non religious project.

> if this is a religious project, it should be clearly marked as such.

What an utterly strange proclamation. On every level: the notion that speech should not be free, the notion that users cannot be trusted to figure out what the project is about if it's not "clearly marked" in this way, the notion that there is some harm that can be done by such a "religious project" or a "religious word," whatever that even means.

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