Sunday, 2019-12-01

Advent of Code 2019

This blog post is a work in progress

Project website: Advent of Code 2019.

Previous years: 2015, 2016, 2017, 2018.

I use Perl for all the solutions.

Most assume the input data is in a file called input.txt in the same directory as the file.

A note on scoring

Current score (2020-12-25): 46. I’m aiming for a final of 44+1.

I score my problems to mark where I’ve finished a solution myself or given up and looked for hints. A score of 2 means I solved both the daily problems myself, a score of 1 means I looked up a hint for one of the problems, and a zero score means I didn’t solve any of the problems myself.

My goals for this year (in descending order of priority):

  • get 38 stars or more (75%)
  • solve all problems within 24 hours of release

Link to Github repo.


  • complete day 18

Day 1 - Day 2 - Day 3 - Day 4 - Day 5 - Day 6 - Day 7 - Day 8 - Day 9 - Day 10 - Day 11 - Day 12 - Day 13 - Day 14 - Day 15 - Day 16 - Day 17 - Day 19 - Day 20 - Day 21 - Day 22 - Day 23 - Day 24 - Day 25

Day 1 - The Tyranny of the Rocket Equation

Day 1 - complete solution

A nice and simple problem to kick off this year.

Score: 2

Day 2 - 1202 Program Alarm

Day 2 - complete solution

An earlier appearance of register rodeo than expected! I think we’ll see more of this going forward.

[intcode part 1]

Score: 2

Day 3 - Crossed Wires

Day 3 - complete solution

This took longer than it had to. I messed up adding the paths, and only managed to get the correct answer to part 1 by chance. Once I plotted the example data I could correct the code, then add the logic for part 2.

I’m not entirely happy with the duplicated direction subroutines. Some people have used complex numbers to simplify this but that would require a separate Perl module to implement.

Score: 2.

Day 4 - Secure Container

Day 4 - complete solution

I blanked on this one and dreaded combinatorics. Turns out brute force is eminently doable. Credits to /u/andreyrmg in the daily solutions thread, and A_D and sim642 in the IRC channels for help in inspiration.

I still think my solution is my own though (and pretty Perlish), so full score today.

Score: 2.

Day 5 - Sunny with a Chance of Asteroids

Day 5 - complete solution

After struggling with the convoluted problem description I was pleasantly surprised to find my code ran flawlessly first try. I still have some niggling issues with the test data, and need to clear that up before the inevitable next intcode problem.

[intcode part 2]

Score: 2.

Day 6 - Universal Orbit Map

Day 6 - complete solution

I bailed on this one and sought inspiration in the daily solutions subreddit. Credit in source!

Score: 0.

Day 7 - Amplification Circuit

Day 7 - part 1 Day 7 - part 2

A tough, but fun one. There were a lot of subtleties in the second part, and I got some pointers from the subreddit.

I got the chance to clean up my intcode implementation, and learned a new facet of Perl.

[intcode part 3]

Score: 2.

Day 8 - Space Image Format

Day 8 - complete solution

Defying expectations (and maybe fears), this Sunday problem was not that complicated.

Of course, it helps if you confirm that what you think is input actually is the same as the problem input. Not that I’d have anything other than theoretical knowledge of this situation…

Score: 2.

Day 9 - intcode test suite

Day 9 - complete solution Day 9 - complete solution

So the Intcode computer is done, and we’ve amassed a number of test cases to ensure it works. I’m kinda sorta happy with my code. It’s not the most elegantly put together but it works fine.

[intcode part 4]

Score: 2.

Day 10 - Monitoring Station

Day 10 - complete solution

This was a fun one, even though I got sidetracked by my incorrect assumptions and got lost in a hallway of indices, all alike.

Part 2 was found by inspecting the output, but hey, a star is a star.

Score: 2.

Day 11 - Space Police

Day 11 - complete solution

Ah, the return of Langton’s ant. Always nice to see an old friend.

Nothing too complex here, although I’m quite proud of the line noise for the dispatch table for movement:

my %directions = (
    '^'=>sub{!$_[0]?['<', 0,-1 ]:['>', 0, 1 ]},
    '<'=>sub{!$_[0]?['v', 1, 0 ]:['^',-1, 0 ]},
    'v'=>sub{!$_[0]?['>', 0, 1 ]:['<', 0,-1 ]},
    '>'=>sub{!$_[0]?['^',-1, 0 ]:['v', 1, 0 ]},

[intcode part 5]

Score: 2.

Day 12 - The N-Body Problem

Day 12 - complete solution

A fun little problem.

Score: 2.

Day 13 - Care Package

Day 13 - complete solution

I am in awe of what the creator of Advent of Code has wrought in the form of intcode.

[intcode part 6]

Score: 2.

Day 14 - Space Stoichiometry

Day 14 - complete solution

A hard problem that was satisfying to solve.

Score: 2.

Day 15 - Oxygen System

Day 15 - complete solution

Not my proudest moment. I’m happy my intcode implementation works well enough for this kind of application now, but my utter inability to code a BFS routine is humiliating. In the end I had to use a modified Djikstra’s that I cribbed for last year’s day 22.

[intcode part 7]

Score: 2.

Day 16 - Flawed Frequency Transmission

Day 16 - part 1 Day 16 - part 2

I had a lot of trouble with part 2, mostly due to indexing errors.

Runtime is 2m16s for part 2, which is just barely acceptable.

Score: 2

Day 17 - Set and Forget

Day 17 - complete solution

This one was a slog!

I was very worried that my intcode interpreter was incorrect, but it was actually just me not being able to read the input specification correctly.

[intcode part 8]

Score: 2.

Day 19 - Tractor Beam

Day 19 - complete solution

This was supposed to be a breather…

From the beginning I realized that this problem is best expressed as x in terms of y, instead of the more usual y in term of x, and I made a mental note not to mix them up.

Of course, many hours later I realized I had done that just that.

[intcode part 9]

Score: 2.

Day 20 - Donut Maze

Day 20 - part 1

Part 1 yields easily to a modified BFS approach.

Update 2020-12-26

Part 2 is similar, only adding the “dimension” of the recursive level. There some really scruffy code to figure out the “edges” of the transition.

Score: 1.

Day 21 - Springdroid Adventure

Day 21 - complete solution

I felt zero interest in trying to puzzle this out so found some closed forms on the subreddit.

[intcode part 10]

Score: 0.

Day 22 - Slam Shuffle

Day 22 - part 1

Part one only done for now, part 2 requires way too much weird (read modular) math for me. Damnit, Cap’n, I’m continuous, not discrete!

Score: 1.

Day 23 - Category Six

Day 23 - complete solution

A remarkably straight-forward puzzle.

[intcode part 11]

Score: 2.

Day 24 - Planet of Discord

Day 24 - part 1 Day 24 - part 2

An interesting problem.

Update 2020-12-25 I revisited this after the multiple Games of Life in 2020. Instead of trying to be clever figuring out how to calculate the neighbors, I just created a giant lookup table.

This Reddit post contains all iterations of the example input and was helpful for debugging.

Score: 2

Day 25 - Cryostasis

Day 25 - complete solution

A fitting end to a good edition of Advent of Code!

Score: 2.