AoC 2022 Day 4: Overlapinator

Part 1

Given a list of pairs of spans (of the form a-b,x-y) count how many spans have one span entirely contained within the other.

Fair enough. Let’s make a Span:

struct Span {
min: isize,
max: isize,
}

impl Span {
fn new(s: &str) -> Span {
let (min, max) = s.split_at(s.find("-").expect("missing dash in span"));

Span {
min: min.parse::<isize>().expect("min is not an integer"),
max: max
.strip_prefix("-")
.expect("malformed prefix, missing dash")
.parse::<isize>()
.expect("max is not an integer"),
}
}

fn contains(&self, other: &Span) -> bool {
self.min <= other.min && self.max >= other.max
}
}

fn parse(lines: &Vec<String>) -> Vec<(Span, Span)> {
let mut result = Vec::new();

for line in lines {
let (left, right) = line.split_at(line.find(",").expect("missing comma in line"));
result.push((
Span::new(left),
Span::new(
right
.strip_prefix(",")
.expect("malformed prefix, missing comma"),
),
))
}

result
}


This time I’m going to use split_at and find instead of split, since I know there will always be exactly two values (first separated by a comma and then a dash within the span). That should be enough to parse everything.

So far as contains, it’s easy enough since they’re ordered. For one Span to contain another, it’s min must be greater and it’s max smaller (inclusive in both cases). This gives us the completely disjoin cases, since one will be true and the other false in all of those.

Now to count them, a nice filter and len:

fn part1(filename: &Path) -> String {

let containing: Vec<&(Span, Span)> = span_pairs.iter().filter(
|pair| pair.0.contains(&pair.1) || pair.1.contains(&pair.0)
).collect();

containing.len().to_string()
}


That works well enough, but one downside is that we’re allocating memory for the collected results when… we don’t really care what the values are. Just that they exist. You can’t directly use len on an iterator (such as a Filter)… but you can use count!

fn part1(filename: &Path) -> String {
.iter()
.filter(|pair| pair.0.contains(&pair.1) || pair.1.contains(&pair.0))
.count()
.to_string()
}


That’s pretty minimal. Is it actually more or less readable? I think it’s pretty decent at least.

Part 2

Instead, count how many spans have one overlapping the other at all.

Okay, expand the implementation of Span:

impl Span {
fn overlaps(&self, other: &Span) -> bool {
(self.min >= other.min && self.min <= other.max)
|| (self.max >= other.min && self.max <= other.max)
|| (other.max >= self.min && other.max <= self.max)
|| (other.max >= self.min && other.max <= self.max)
}
}


A bit messier, but it needs to be to deal properly with the single value edge cases (6-6). And it’ll all short circuit nicely if they don’t overlap, so it works. The part2 itself just subs out the function:

fn part2(filename: &Path) -> String {
.iter()
.filter(|pair| pair.0.overlaps(&pair.1))
.count()
.to_string()
}


Performance

Quick.

$./target/release/04-overlapinator 1 data/04.txt 466 took 442.458µs$ ./target/release/04-overlapinator 2 data/04.txt

865
took 362.166µs


We’re still really not doing anything outside of O(n) .