Advent of Code 2019

Advent of Code 2019 solutions (spoilers, obviously, if you're not caught up).

I'm trying Elixir this year (last year I used Crystal). It's a language I like a lot, but am not yet intimately familiar with. So some after-action thoughts follow.

Day 1

• Decided to give doctests a try. Pretty great for TDD, which is how I do AoC. (I'll never place in the top 100, but when I'm done, I'm done, and I rarely submit wrong answers.) Especially great when paired with mix_test_watch.

Day 3

• Wish I'd known about MapSet! That probably would have made calculating intersections easier. I'm still glad I learned about MapSet, but it doesn't allow duplicate values, which might make it a bad fit for this particular case.

Day 4

• Was afraid my regex with backreferences might be slow, but the strings involved are so short it didn't matter.
• Forgot about the :discard option on chunk_every. Ack.
• Also forgot about chunk_by(& &1)/group_by(& &1), even better.

Day 6

• Knowing about :digraph would have made this a lot easier.

Day 7

• Briefly considered going all in on processes and messages and all that, but ended up just making the fewest changes possible.
• Revised the following day to use processes, which turned out to require even fewer changes than avoiding processes! Ha.

Day 11

• My program doesn't actually halt, but does give the right answer. Didn't have time to figure that out.

Day 14

• Started to write something for Part 2, but ended up doing it manually.

Day 15

• Did this one while watching TV. Turned in best rank yet. New strategy?

Day 16

• Finished Part 1, but couldn't get Part 2. I basically failed linear algebra in college, but was saved by a pilot math program that averaged that grade out with a subject I grasped a little better. But that means I never did learn linear algebra.
• Finally assembled a solution for Part 2 a couple days later.

Day 17

• Part 1 was very straightforward, but solved part 2 by hand. Kinda fun at first, but became tedious.

Day 18

• Another BFS problem, I reeeeally need to learn this someday. They kicked my ass last year, too.

Day 20

• Pretty sure I know what needs to be done here (BFS again, my nemesis), but... meh

Day 21

• No coding! Just boolean logic. This was an easy one.

Day 23

• Fairly easy (just got a late start).