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blog(aoc-2022): add 4th week
Signed-off-by: Matej Focko <me@mfocko.xyz>
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blog/aoc-2022/04-week-4.md
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---
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title: 4th week of Advent of Code '22 in Rust
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description: Surviving fourth week in Rust.
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date: 2023-07-07T15:14
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slug: aoc-2022/4th-week
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authors: mf
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tags:
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- aoc-2022
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- advent-of-code
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- rust
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hide_table_of_contents: false
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---
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Let's go through the fourth week of [_Advent of Code_] in Rust.
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<!--truncate-->
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## [Day 22: Monkey Map](https://adventofcode.com/2022/day/22)
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:::info tl;dr
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Simulating a movement on a 2D map with given instructions. Map becomes a cube in
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the 2nd part…
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:::
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:::caution Rant
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This was the most obnoxious problem of this year… and a lot of Rust issues have
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been hit.
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:::
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### Solution
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It seems like a very simple problem to solve, but with very obnoxious changes in
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the 2nd part and also it's relatively hard to decompose »properly«.
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#### Column iterator
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In the first part of the problem it was needed to know the boundaries of each
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row and column, since I stored them in `Vec<Vec<char>>` and padded with spaces
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to ensure I have a rectangular 2D “array”. However when you wanted to go through
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each row and column to determine the boundaries, it was very easy to do for the
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rows (cause each row is a `Vec` element), but not for the columns, since they
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span multiple rows.
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For this use case I have implemented my own _column iterator_:
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```rust
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pub struct ColumnIterator<'a, T> {
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map: &'a [Vec<T>],
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column: usize,
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i: usize,
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}
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impl<'a, T> ColumnIterator<'a, T> {
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pub fn new(map: &'a [Vec<T>], column: usize) -> ColumnIterator<'a, T> {
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Self { map, column, i: 0 }
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}
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}
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impl<'a, T> Iterator for ColumnIterator<'a, T> {
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type Item = &'a T;
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fn next(&mut self) -> Option<Self::Item> {
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if self.i >= self.map.len() {
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return None;
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}
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self.i += 1;
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Some(&self.map[self.i - 1][self.column])
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}
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}
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```
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Given this piece of an iterator, it is very easy to factor out the common
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functionality between the rows and columns into:
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```rust
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let mut find_boundaries = |constructor: fn(usize) -> Orientation,
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iterator: &mut dyn Iterator<Item = &char>,
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upper_bound,
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i| {
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let mut first_non_empty = iterator.enumerate().skip_while(|&(_, &c)| c == ' ');
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let start = first_non_empty.next().unwrap().0 as isize;
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let mut last_non_empty = first_non_empty.skip_while(|&(_, &c)| c != ' ');
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let end = last_non_empty.next().unwrap_or((upper_bound, &'_')).0 as isize;
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boundaries.insert(constructor(i), start..end);
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};
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```
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And then use it as such:
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```rust
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// construct all horizontal boundaries
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(0..map.len()).for_each(|row| {
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find_boundaries(
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Orientation::horizontal,
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&mut map[row].iter(),
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map[row].len(),
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row,
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);
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});
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// construct all vertical boundaries
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(0..map[0].len()).for_each(|col| {
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find_boundaries(
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Orientation::vertical,
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&mut ColumnIterator::new(&map, col),
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map.len(),
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col,
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);
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});
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```
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#### Walking around the map
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Once the 2nd part got introduced, you start to think about a way how not to
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copy-paste a lot of stuff (I haven't avoided it anyways…). In this problem, I've
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chosen to introduce a trait (i.e. _interface_) for 2D and 3D walker.
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```rust
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trait Wrap: Clone {
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type State;
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// simulation
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fn is_blocked(&self) -> bool;
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fn step(&mut self, steps: isize);
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fn turn_left(&mut self);
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fn turn_right(&mut self);
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// movement
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fn next(&self) -> (Self::State, Direction);
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// final answer
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fn answer(&self) -> Output;
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}
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```
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Each walker maintains its own state and also provides the functions that are
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used during the simulation. The “promised” methods are separated into:
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* _simulation_-related: that are used during the simulation from the `.fold()`
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* _movement_-related: just a one method that holds most of the logic differences
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between 2D and 3D
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* _final answer_: which extracts the _proof of solution_ from the
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implementation-specific walker
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Both 2D and 3D versions borrow the original input and therefore you must
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annotate the lifetime of it:
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```rust
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struct Wrap2D<'a> {
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input: &'a Input,
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position: Position,
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direction: Direction,
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}
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impl<'a> Wrap2D<'a> {
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fn new(input: &'a Input) -> Wrap2D<'a> {
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// …
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```
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#### Problems
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I have used a lot of closures for this problem and once I introduced a parameter
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that was of unknown type (apart from the fact it implements a specific trait), I
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got suggested a “fix” for the compilation error that resulted in something that
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was not possible to parse, cause it, more than likely, violated the grammar.
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In a similar fashion, I have been suggested changes that led to a code that
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didn't make sense by just looking at it (there was no need to try the changes),
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for example one suggested change in the closure parameter caused disapperance of
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the parameter name. :smile:
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#### Clippy
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I have to admit that Clippy was rather helpful here, I'll include two examples
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of rather smart suggestions.
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When writing the parsing for this problem, the first thing I have spotted on the
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`char` was the `.is_digit()` function that takes a radix as a parameter. Clippy
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noticed that I use `radix = 10` and suggested switching to `.is_ascii_digit()`
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that does exactly the same thing:
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```diff
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- .take_while(|c| c.is_digit(10))
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+ .take_while(|c| c.is_ascii_digit())
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```
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Another useful suggestion appeared when working with the iterators and I wanted
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to get the $n$-th element from it. You know the `.skip()`, you know the
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`.next()`, just “slap” them together and we're done for :grin: Well, I got
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suggested to use `.nth()` that does exactly the combination of the two mentioned
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methods on iterators:
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```diff
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- match it.clone().skip(skip).next().unwrap() {
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+ match it.clone().nth(skip).unwrap() {
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```
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## [Day 23: Unstable Diffusion](https://adventofcode.com/2022/day/23)
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:::info tl;dr
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Simulating movement of elves around with a set of specific rules.
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:::
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### Solution
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There's not much to mention since it's just a cellular automaton simulation
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(even though the AoC rules for cellular automatons usually get out of hand
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:wink:).
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Although I had a need to determine boundaries of the elves' positions and ended
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up with a nasty DRY violation. Knowing that you you're looking for maximum and
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minimum that are, of course, exactly the same except for initial values and
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comparators, it looks like a rather simple fix, but typing in Rust is something
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else, right? In the end I settled for a function that computes both boundaries
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without any duplication while using a closure:
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```rust
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fn get_bounds(positions: &Input) -> (Vector2D<isize>, Vector2D<isize>) {
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let f = |init, cmp: &dyn Fn(isize, isize) -> isize| {
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positions
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.iter()
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.fold(Vector2D::new(init, init), |acc, elf| {
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Vector2D::new(cmp(acc.x(), elf.x()), cmp(acc.y(), elf.y()))
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})
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};
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(f(isize::MAX, &min::<isize>), f(isize::MIN, &max::<isize>))
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}
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```
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This function returns a pair of 2D vectors that represent opposite points of the
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bounding rectangle of all elves.
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You might ask why would we need a closure and the answer is that `positions`
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cannot be captured from within the nested function, only via closure. One more
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fun fact on top of that is the type of the comparator
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```rust
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&dyn Fn(isize, isize) -> isize
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```
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Once we remove the `dyn` keyword, compiler yells at us and also includes a way
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how to get a more thorough explanation of the error by running
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$ rustc --explain E0782
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which shows us
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Trait objects must include the `dyn` keyword.
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Erroneous code example:
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|
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```
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trait Foo {}
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fn test(arg: Box<Foo>) {} // error!
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```
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|
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Trait objects are a way to call methods on types that are not known until
|
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runtime but conform to some trait.
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|
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Trait objects should be formed with `Box<dyn Foo>`, but in the code above
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`dyn` is left off.
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This makes it harder to see that `arg` is a trait object and not a
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simply a heap allocated type called `Foo`.
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|
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To fix this issue, add `dyn` before the trait name.
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|
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```
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trait Foo {}
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fn test(arg: Box<dyn Foo>) {} // ok!
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```
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|
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This used to be allowed before edition 2021, but is now an error.
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:::danger Rant
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Not all of the explanations are helpful though, in some cases they might be even
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more confusing than helpful, since they address _very simple_ use cases.
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As you can see, even in this case there are two sides to the explanations:
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* it explains why you need to use `dyn`, but
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* it still mentions that trait objects need to be heap-allocated via `Box<T>`
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that, as you can see in my snippet, **does not** apply here :smile: IMO it's
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caused by the fact that we are borrowing it and therefore we don't need to
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care about the size or whereabouts of it.
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|
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:::
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:::info C++ parallel
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|
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If you dive into the explanation above, you can notice that the `Box<dyn Trait>`
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pattern is very helpful for using types that are not known during compile-time.
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You would use a very similar approach in C++ when parsing some data structure
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from input (let's say JSON for example).
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On the other hand, in this case, it doesn't really make much sense, cause you
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can clearly see that the types **are known** during the compile-time, which in
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C++ could be easily resolved by templating the helper function.
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:::
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## [Day 24: Blizzard Basin](https://adventofcode.com/2022/day/24)
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:::info tl;dr
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Navigating your way through a basin with series of blizzards that move around
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you as you move.
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:::
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:::caution
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|
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It's second to last day and I went “_bonkers_” on the Rust :smile: Proceed to
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read _Solution_ part on your own risk.
|
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|
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:::
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### Solution
|
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|
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You are given a map with blizzards all over the place and you're supposed to
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find the minimum time it requires you to walk through the basin without getting
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in any of the blizzards.
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#### Breakdown
|
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Relatively simple, yet a bit annoying, approach can be taken. It's technically
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a shortest-path algorithm implementation with some relaxation restrictions and
|
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being able to stay on one position for some time, so each _vertex_ of the graph
|
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is determined by the position on the map and the _timestamp_. I have chosen to
|
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use `Vector3D<usize>`, since `x` and `y` attributes can be used for the position
|
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and, well, let's use `z` for a timestamp, cause why not, right? :wink:
|
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|
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#### Evaluating the blizzards
|
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|
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:::caution
|
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|
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I think that this is the most perverted abuse of the traits in the whole 4 weeks
|
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of AoC in Rust…
|
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|
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:::
|
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|
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The blizzards move along their respective directions in time and loop around in
|
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their respective row/column. Each vertex holds position **and** time, so we can
|
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_just_ index the basin with the vertex itself, right? Yes, we can :smiling_imp:
|
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|
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:::tip Fun fact
|
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|
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While writing this part, I've recognized unnecessary verbosity in the code and
|
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cleaned it up a bit. The changed version is shown here and the original was just
|
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more verbose.
|
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|
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:::
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|
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I'll skip the boring parts of checking bounds and entry/exit of the basin :wink:
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We can easily calculate positions of the blizzards using a modular arithmetics:
|
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```rust
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impl Index<Position> for Basin {
|
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type Output = char;
|
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|
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fn index(&self, index: Position) -> &Self::Output {
|
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// ‹skipped boring parts›
|
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|
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// We need to account for the loops of the blizzards
|
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let width = self.cols - 2;
|
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let height = self.rows - 2;
|
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|
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let blizzard_origin = |size, d, t, i| ((i - 1 + size + d * (t % size)) % size + 1) as usize;
|
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[
|
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(
|
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index.y() as usize,
|
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blizzard_origin(width, -1, index.z(), index.x()),
|
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'>',
|
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),
|
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(
|
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index.y() as usize,
|
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blizzard_origin(width, 1, index.z(), index.x()),
|
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'<',
|
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),
|
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(
|
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blizzard_origin(height, -1, index.z(), index.y()),
|
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index.x() as usize,
|
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'v',
|
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),
|
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(
|
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blizzard_origin(height, 1, index.z(), index.y()),
|
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index.x() as usize,
|
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'^',
|
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),
|
||||
]
|
||||
.iter()
|
||||
.find_map(|&(y, x, direction)| {
|
||||
if self.map[y][x] == direction {
|
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Some(&self.map[y][x])
|
||||
} else {
|
||||
None
|
||||
}
|
||||
})
|
||||
.unwrap_or(&'.')
|
||||
}
|
||||
}
|
||||
```
|
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|
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As you can see, there is an expression for calculating the original position and
|
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it's used multiple times, so why not take it out to a lambda, right? :wink:
|
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|
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I couldn't get the `rustfmt` to format the `for`-loop nicely, so I've just
|
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decided to go with iterating over an elements of a slice. I have used, once
|
||||
again, a combination of two functions (`find_map` in this case) to do 2 things
|
||||
at once and at the end, if we haven't found any blizzard, we just return the
|
||||
empty space.
|
||||
|
||||
I think it's a very _nice_ (and naughty) way how to use the `Index` trait, don't
|
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you think?
|
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|
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#### Shortest-path algorithm
|
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|
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For the shortest path you can choose and adjust any of the common shortest-path
|
||||
algorithms, in my case, I have decided to use [_A\*_] instead of Dijkstra's
|
||||
algorithm, since it better reflects the _cost_ function.
|
||||
|
||||
:::info Comparison of costs
|
||||
|
||||
With the Dijkstra's algorithm I would proceed with the `time` attribute used as
|
||||
a priority for the queue.
|
||||
|
||||
Whereas with the _A\*_, I have chosen to use both time and Manhattan distance
|
||||
that promotes vertices closer to the exit **and** with a minimum time taken.
|
||||
|
||||
:::
|
||||
|
||||
Cost function is, of course, a closure :wink:
|
||||
```rust
|
||||
let cost = |p: Position| p.z() as usize + exit.y().abs_diff(p.y()) + exit.x().abs_diff(p.x());
|
||||
```
|
||||
|
||||
And also for checking the possible moves from the current vertex, I have
|
||||
implemented, yet another, closure that yields an iterator with the next moves:
|
||||
```rust
|
||||
let next_positions = |p| {
|
||||
[(0, 0, 1), (0, -1, 1), (0, 1, 1), (-1, 0, 1), (1, 0, 1)]
|
||||
.iter()
|
||||
.filter_map(move |&(x, y, t)| {
|
||||
let next_p = p + Vector3D::new(x, y, t);
|
||||
|
||||
if basin[next_p] == '.' {
|
||||
Some(next_p)
|
||||
} else {
|
||||
None
|
||||
}
|
||||
})
|
||||
};
|
||||
```
|
||||
|
||||
Rest is just the algorithm implementation which is not that interesting.
|
||||
|
||||
## [Day 25: Full of Hot Air](https://adventofcode.com/2022/day/25)
|
||||
|
||||
:::info tl;dr
|
||||
|
||||
Playing around with a numbers in a _special_ base.
|
||||
|
||||
:::
|
||||
|
||||
Getting flashbacks to the _IB111 Foundations of Programming_… Very nice „problem“
|
||||
with a rather easy solution, as the last day always seems to be.
|
||||
|
||||
### Solution
|
||||
|
||||
Implementing 2 functions, converting from the _SNAFU base_ and back to the _SNAFU_
|
||||
_base_ representation. Let's do a bit more though! I have implemented two functions:
|
||||
* `from_snafu`
|
||||
* `to_snafu`
|
||||
|
||||
Now it is apparent that all I do is number to string and string to number. Hmm…
|
||||
that sounds familiar, doesn't it? Let's introduce a structure for the SNAFU numbers
|
||||
and implement the traits that we need.
|
||||
|
||||
Let's start with a structure:
|
||||
```rust
|
||||
#[derive(Debug, PartialEq, Eq, PartialOrd, Ord)]
|
||||
struct SNAFU {
|
||||
value: i64,
|
||||
}
|
||||
```
|
||||
|
||||
#### Converting from `&str`
|
||||
|
||||
We will start by implementing the `FromStr` trait that will help us parse our input.
|
||||
This is rather simple, I can just take the `from_snafu` function, copy-paste it
|
||||
into the `from_str` method and the number I get will be wrapped in `Result` and
|
||||
`SNAFU` structure.
|
||||
|
||||
#### Converting to `String`
|
||||
|
||||
This is more fun. In some cases you need to implement only one trait and others
|
||||
are automatically implemented using that one trait. In our case, if you look in
|
||||
the documentation, you can see that `ToString` trait is automatically implemented
|
||||
for any type that implements `Display` trait.
|
||||
|
||||
Let's implement the `Display` trait then. We should be able to use the `to_snafu`
|
||||
function and just take the `self.value` from the `SNAFU` structure.
|
||||
|
||||
And for the convenience of tests, we can also implement a rather simple `From<i64>`
|
||||
trait for the `SNAFU`.
|
||||
|
||||
#### Adjusting the code
|
||||
|
||||
After those changes we need to adjust the code and tests.
|
||||
|
||||
Parsing of the input is very easy, before we have used the lines, now we parse
|
||||
everything:
|
||||
```diff
|
||||
fn parse_input<P: AsRef<Path>>(pathname: P) -> Input {
|
||||
- file_to_lines(pathname)
|
||||
+ file_to_structs(pathname)
|
||||
}
|
||||
```
|
||||
|
||||
Part 1 needs to be adjusted a bit too:
|
||||
```diff
|
||||
fn part_1(input: &Input) -> Output {
|
||||
- to_snafu(input.iter().map(|s| from_snafu(s)).sum())
|
||||
+ SNAFU::from(input.iter().map(|s| s.value).sum::<i64>()).to_string()
|
||||
}
|
||||
```
|
||||
|
||||
You can also see that it simplifies the meaning a bit and it is more explicit than
|
||||
the previous versions.
|
||||
|
||||
And for the tests:
|
||||
```diff
|
||||
#[test]
|
||||
fn test_from() {
|
||||
- for (n, s) in EXAMPLES.iter() {
|
||||
- assert_eq!(from_snafu(s), *n);
|
||||
+ for (&n, s) in EXAMPLES.iter() {
|
||||
+ assert_eq!(s.parse::<SNAFU>().unwrap().value, n);
|
||||
}
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn test_to() {
|
||||
- for (n, s) in EXAMPLES.iter() {
|
||||
- assert_eq!(to_snafu(*n), s.to_string());
|
||||
+ for (&n, s) in EXAMPLES.iter() {
|
||||
+ assert_eq!(SNAFU::from(n).to_string(), s.to_string());
|
||||
}
|
||||
```
|
||||
|
||||
## Summary
|
||||
|
||||
Let's wrap the whole thing up! Keeping in mind both AoC and the Rust…
|
||||
|
||||
### Advent of Code
|
||||
|
||||
This year was quite fun, even though most of the solutions and posts came in
|
||||
later on (*cough* in '23 *cough*). Day 22 was the most obnoxious one… And also
|
||||
it feels like I used priority queues and tree data structures **a lot** :eyes:
|
||||
|
||||
### with Rust
|
||||
|
||||
I must admit that a lot of compiler warnings and errors were very useful. Even
|
||||
though I still found some instances where they didn't help at all or cause even
|
||||
worse issues than I had. Compilation times have been addressed with the caching.
|
||||
|
||||
Building my first tree data structure in Rust has been a very “interesting”
|
||||
journey. Being able to write a more generic BFS algorithm that allows you to not
|
||||
duplicate code while still mantaining the desired functionality contributes to
|
||||
a very readable code.
|
||||
|
||||
I am definitely much more aware of the basic things that bloated Python is
|
||||
missing, yet Rust has them…
|
||||
|
||||
Using explicit types and writing down placeholder functions with `todo!()`
|
||||
macros is very pleasant, since it allows you to easily navigate the type system
|
||||
during the development when you don't even need to be sure how are you going to
|
||||
put the smaller pieces together.
|
||||
|
||||
I have used a plethora of traits and also implemented some of them to either be
|
||||
idiomatic, or exploit the syntactic sugar they offer. Deriving the default trait
|
||||
implementation is also very helpful in a lot of cases, e.g. debugging output,
|
||||
copying, equality comparison, etc.
|
||||
|
||||
I confess to touching more “cursed” parts of the Rust, such as macros to
|
||||
declutter the copy-paste for tests or writing my own structures that need to
|
||||
carry a lifetime for their own fields.
|
||||
|
||||
tl;dr Relatively pleasant language until you hit brick wall :wink:
|
||||
|
||||
---
|
||||
|
||||
See you next year! Maybe in Rust, maybe not :upside_down_face:
|
||||
|
||||
[_Advent of Code_]: https://adventofcode.com
|
||||
[_A\*_]: https://en.wikipedia.org/wiki/A*_search_algorithm
|
Loading…
Reference in a new issue