Advent of code, written in Rust. Going into this challenge, I had no experience writing Rust, but I've found this to be a good way to get familiar with the language.
My main confusion on day one had to do with object ownership in Rust. Specifically, with these lines:
for line in reader.lines() {
let lstr = line.unwrap().clone();At first, I wasn't cloning the line, which meant that I couldn't perform certain operations on it later on in the loop body. I'm not sure if there's a better/cleaner way of handling this.
Things got significantly more complicated with part 2, because I needed to keep an array of the largest snack counts. While I was at it, I tried to do it for the N largest snackbags. I ended up duplicating my code in the loop to properly handle the last snackbag in the file. It's not pretty, but it works for now.
I still don't know why I need ? after some function calls. I know it has to do with automatic error-handling, but I'm not familiar enough with Rust to know why.
For day 2, the main discovery was the use of match(). This allowed me to essentially perform the same steps for parts 1 and 2, just with different matches between values in the left column and values in the right column.
I'm quite proud of my day 3 implementation. I used bitwise operators to map each priority level to a bit in a u64 variable. Each line was split into left and right halves. Letters were converted to ASCII characters and then mapped to values in the range 1..52 depending on whether they were uppercase or lowercase.
For each character that occurred on the left half, I'd set the corresponding bit in my u64 bitmap. After reading the whole left side, I could see which characters were used at least once or not at all.
Then, when reading the left half, I just checked whether that character had already been entered into the bit map. If it had, I'd found the misplaced priority level, and I could simply add it to the total and break out of the loop.
For part two, I just removed the left-right sides of the bag, and treated each bag as a single entity. I had a separate bitmap for each elf in the group, giving me three total bitmaps. Then, I just used bitwise-and to find which element was common among all elves in the group.
My main discovery for part 2 was that I could use trailing_zeros() on the final bitmap to directly find the priority of the common element:
score += u64::from(shared_chars.trailing_zeros());...