Day 9 (Advent of Code 2020)

This article is part of the Advent of Code 2020 series.

Day 9's problem statement is convoluted - the "ah maybe that's why I don't usually do Advent of Code" kind of convoluted, but let's give it a go anyway.

So, we have a series of numbers, like so:

35 20 15 25 47 40 62 55 65 95 102 117 150 182 127 219 299 277 309 576

And uh the first N numbers are a "preamble" and every number that comes after that must be the sum of any two of the numbers that come before it.

For the example above, N is 5. So, again, there's probably a smart and fast way to solve this, but I'll go again for a simple and correct solution instead.

One thing I like about this problem is that it lets me showcase a bunch of cool methods.

We're going to iterate over windows of size n+1 - so here, elements 0..=5 (inclusive), then 1..=6, then 2..=7, and so on. Then we're going to get all possible combinations of elements 0..5 (exclusive), 1..6, 2..7, and see if the sum of any of those combinations is equal to the last element of our window.

Rust code
use itertools::Itertools; fn main() { let numbers = include_str!("input.txt") .lines() .map(|x| x.parse::<usize>().unwrap()) .collect::<Vec<_>>(); let n = 5; let answer = + 1).find_map(|s| { if (&s[..n]) .iter() .tuple_combinations() .any(|(a, b)| a + b == s[n]) { None } else { Some(s[n]) } }); println!("answer = {:?}", answer); }
Shell session
$ cargo run --quiet answer = Some(127)

Cool, this matches the example! I guess we're already done?

Let's try it with n = 25 and the actual input:

Shell session
$ cargo run --quiet answer = Some(26134589)

Hey, that's the correct answer!


Part 2

The next part asks us to find "a contiguous set of at least two numbers in our list which sum to the invalid number from step 1".

Well, that doesn't seem too hard either. One thing we can do is to sum all the windows of size 2, then all the windows of size 3, and so on - and sum the items in all of these. As soon as we reach the answer, we're done!

Rust code
let answer = answer.unwrap(); let answer2 = (2..numbers.len()) .into_iter() .map(|n||s| s.iter().sum::<usize>())) .flatten() .find(|&n| n == answer); println!("answer2 = {:?}", answer2);
Shell session
$ cargo run --quiet answer2 = Some(26134589)

Ok, so we did find a contiguous set of numbers whose sum is the same as the answer we found in part 1, but we don't know where or how large the set was.

Let's address that:

Rust code
let answer2 = (2..numbers.len()) .into_iter() .map(|n| { numbers .windows(n) .enumerate() .map(move |(i, s)| (n, i, s.iter().sum::<usize>())) }) .flatten() .find(|&(_, _, sum)| sum == answer); let (n, i, _) = answer2.unwrap(); let set = &numbers[i..][..n]; println!("sum({:?}) = {}", set, answer);
Shell session
$ cargo run --quiet sum([1503494, 978652, 1057251, 1142009, 1239468, 1407633, 1048040, 1484541, 1164289, 1432864, 1792914, 2556472, 2464510, 1750429, 1753116, 1673488, 1685419]) = 26134589

That's better.

Now we need to add together the smallest and largest number in this contiguous range:

Rust code
let answer3 = set.iter().max().unwrap() + set.iter().min().unwrap(); dbg!(answer3);
Shell session
$ cargo run --quiet [src/] answer3 = 3535124

Aaand we're done! That was easy, I don't know what I was worried about.

This article was made possible thanks to my patrons: Vicente Bosch, Chirag Jain, Ville Mattila, Marie Janssen, Cameron Clausen, spike grobstein, Daniel Franklin, Jon Gjengset, Andrei Dyldin, Paul Marques Mota, Jakub Fijałkowski, Mitchell Hamilton, Brad Luyster, Max von Forell, Jake S, Dimitri Merejkowsky, Chris Biscardi, René Ribaud, Alex Doroshenko, Vincent, Steven McGuire, Chad Birch, Chris Emery, Bob Ippolito, John Van Enk, metabaron, Isak Sunde Singh, Philipp Gniewosz, Mads Johansen, lukvol, Ives van Hoorne, Jan De Landtsheer, Daniel Strittmatter, Evgeniy Dubovskoy, Alex Rudy, Shane Lillie, Romet Tagobert, Douglas Creager, Gus W, Corey Alexander, Molly Howell, knutwalker, Zachary Dremann, Sebastian Ziebell, Julien Roncaglia, Amber Kowalski, T, Juniper Wilde, Paul Kline, Kristoffer Ström, Astrid Bek, Yoh Deadfall, Justin Ossevoort, taziden, Harsh Shandilya, Tomáš Duda, Jeremy Banks, Rasmus Larsen, Torben Clasen, Sam Rose, C J Silverio, Walther, Pete Bevin, Shane Sveller, Clara Schultz, Ivan Dubrov, jer, Wonwoo Choi, Hawken Rives, João Veiga, Richard Pringle, Adam Perry, Benjamin Röjder Delnavaz, Matt Jadczak, tavr, Mara Bos, Jonathan Knapp, Maximilian, Seth Stadick, brianloveswords, Sean Bryant, Ember, Sebastian Zimmer, Fernando, Makoto Nakashima, Geert Depuydt, Geoff Cant, Geoffroy Couprie, Michael Alyn Miller, o0Ignition0o, Zaki, Raphael Gaschignard, Romain Ruetschi, Ignacio Vergara, Pascal, Jane Lusby, Nicolas Goy, Ted Mielczarek, Ryszard Sommefeldt, Jesús Higueras, Aurora.

This article is part 9 of the Advent of Code 2020 series.

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