Advent of Code 2021 🎄

Solutions for the 2021 edition of Advent of Code.

This year, I try to do this in Rust 🦀. I discover the language, so do not take this as an idiomatic implementation.

What I learned:

• Day01
  • organization of the project
  • using the lib.rs to expose modules
  • referencing them in main.rs.
  • Read a file with fs::read_to_string,
  • iterate the lines with .lines().
  • parse().unwrap() for a quick parsing of the line based on declared type in the function.
• Day02
  • I used split_whitespace, but split_once(" ") would have been better, because I could unpack the result directly.
  • If you iter a borrowed vector, you get a double reference for arguments. You can unpack them with for &(x, y) to access the value as (here) &str, otherwise you get &&ref and you cannot compare those to string literals. Another solution is to use x.as_ref.
• Day03
  • HashMap default value: hashmap.get(x).unwrap_or(0)
  • HashMap mutating with default value: hashmap.entry(w).or_insert(0) += 1
• Day04
  • Converting a Vec into an array with try_into().unwrap().
• Day08
  • Regex with the regex crate (along with lazy_static to avoid duplicate compilation of regexes).
  • HashSet exists.
• Day09
  • Mutating a ref in a recursive function (say fn func(&mut input)) can be called internally with fn(input), fn(&mut input) is not necessary.
• Day10
  • When building in release, integer will overflow with no warnings, u32 was not giving the right answer, u64 does...
• Day11
  • You can use get_mut on an HashMap to get a mutable reference.