Hugo Peixoto

Advent of code 2020, week 1

Published on December 07, 2020

Table of contents

Advent of code 2020 is ongoing. I’ve been solving the problems daily using rust. So far so good. I have a private leaderboard with a few folks, anyone is free to join, here’s the code: 9779-59e7cb46.

My schedule has been kind of compatible with the time of release, so I’ve been able to start working on them almost immediately. So far I haven’t got any points (rank <= 100), though. Here are my personal stats so far:

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      -------Part 1--------   -------Part 2--------
Day       Time  Rank  Score       Time  Rank  Score
  7   00:17:27   801      0   00:30:15  1022      0
  6   00:08:04  2745      0   00:13:41  2145      0
  5   00:23:43  4339      0   00:28:12  3544      0
  4   00:20:52  4184      0   00:56:14  3896      0
  3   00:56:04  8977      0   01:04:08  8165      0
  2   00:07:32  1519      0   00:10:52  1224      0
  1   01:10:16  6419      0   01:16:28  6046      0

Days 1-6 were straightforward, while day 7 was a bit more involved. Last year I wrote some tests. This year, I’m trying to get the solutions out of the door as fast as I can, so there’s none of that. I’m mostly using println!s.

Here’s what I found interesting so far each day:

Day 1

I learned about using break with labels:

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'part1: for a in lines.iter() {
  for b in lines.iter() {
    if a + b == 2020 {
      println!("{}", a * b);
      break 'part1;
    }
  }
}

Day 2

I tried to speedrun this one to see if I could get some leaderboard points, so I did this in ruby instead, but it still took me 7 minutes to get the initial solution. I rewrote it in rust afterwards. It was a good problem to exercise using regular expressions.

Day 3

I read the whole input into a string and worked with it without any conversion. So far I’ve been using a buffered reader to read lines.

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use std::fs::read_to_string;

pub fn main() {
  let area = read_to_string("inputs/day3.in").unwrap();
}

Day 4

I used a bunch of regular expressions, even for numeric range checking. Things like this:

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let hgt = Regex::new(r"^hgt:(1([5-8]\d|9[0-3])cm|(59|6\d|7[0-6])in)$").unwrap();

It took me a while to get some of these right. Maybe I should have gone with something simpler.

Day 5

This problem was basically implementing parsing numbers in base 2:

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i32::from_str_radix(
  &x
    .replace("F", "0")
    .replace("B", "1")
    .replace("R", "1")
    .replace("L", "0"),
  2,
)

For part two, I kind of missed ruby’s Enumerable#each_cons. Thankfully, the itertools crate has tuple_windows:

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let missing = lines
  .iter()
  .tuple_windows()
  .filter(|&(prev, current)| current - prev > 1)
  .map(|(_, current)| current - 1)
  .next()
  .unwrap();

Day 6

This day I learned that trim doesn’t return a copy of the string, but something that references it. That means I can’t do something like this:

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# this doesn't work
let text = read_to_string("inputs/day6.in").unwrap().trim();

I would need to either call to_string() on it, to make a copy, or to keep the full text around:

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# either copy it
let text = read_to_string("inputs/day6.in").unwrap().trim().to_string();
# or keep it around
let text = read_to_string("inputs/day6.in").unwrap();
let trimmed = text.trim();

Day 7

This was the first problem this year that looked like a graph problem. I wrote a couple of DFS-like functions. I expected to have some more trouble with ownership of the arguments, but it was kind of easy. I used a bunch of std::collections::HashMaps.