[2024] Day 23 initial solution

README.md

@@ -27,6 +27,7 @@ Thank you to [Eric Wastl](http://was.tl) for running this incredible yearly even
 - [Day 20: Race Condition](aoc_2024/src/day_20.rs)
 - [Day 21: Keypad Conundrum](aoc_2024/src/day_21.rs)
 - [Day 22: Monkey Market](aoc_2024/src/day_22.rs)
+- [Day 23: LAN Party](aoc_2024/src/day_23.rs)
 <!-- MARKER -->
 
 ## [2023](https://adventofcode.com/2023) [![aoc_2023](https://github.com/connorslade/advent-of-code/actions/workflows/aoc_2023.yml/badge.svg)](https://github.com/connorslade/advent-of-code/actions/workflows/aoc_2023.yml)

aoc_2024/src/day_23.rs

@@ -0,0 +1,145 @@
+use std::collections::{HashMap, HashSet};
+
+use common::{solution, Answer};
+use itertools::Itertools;
+
+solution!("LAN Party", 23);
+
+fn part_a(input: &str) -> Answer {
+    let graph = parse(input);
+
+    let mut triplets = HashSet::new();
+    for key in graph.keys() {
+        let neighbors = &graph[key];
+
+        for x in neighbors {
+            for y in neighbors.iter().skip(1) {
+                if graph[x].contains(y) {
+                    let mut sorted = vec![key, x, y];
+                    sorted.sort();
+                    triplets.insert((sorted[0], sorted[1], sorted[2]));
+                }
+            }
+        }
+    }
+
+    triplets
+        .iter()
+        .filter(|(a, b, c)| a.starts_with('t') || b.starts_with('t') || c.starts_with('t'))
+        .count()
+        .into()
+}
+
+fn part_b(input: &str) -> Answer {
+    let graph = parse(input);
+
+    let mut cliques = Vec::new();
+    bron_kerbosch(
+        &graph,
+        &mut HashSet::new(),
+        &mut graph.keys().cloned().collect(),
+        &mut HashSet::new(),
+        &mut cliques,
+    );
+
+    let max = cliques.iter().max_by_key(|x| x.len()).unwrap();
+    max.iter().sorted().join(",").into()
+}
+
+fn parse(input: &str) -> HashMap<&str, HashSet<&str>> {
+    let mut out: HashMap<&str, HashSet<&str>> = HashMap::new();
+
+    for line in input.lines() {
+        let (a, b) = line.split_once('-').unwrap();
+        out.entry(a).or_default().insert(b);
+        out.entry(b).or_default().insert(a);
+    }
+
+    out
+}
+
+fn bron_kerbosch<'a>(
+    graph: &HashMap<&'a str, HashSet<&'a str>>,
+    r: &mut HashSet<&'a str>,
+    p: &mut HashSet<&'a str>,
+    x: &mut HashSet<&'a str>,
+    cliques: &mut Vec<HashSet<&'a str>>,
+) {
+    if p.is_empty() && x.is_empty() {
+        cliques.push(r.clone());
+        return;
+    }
+
+    let pivot = p.iter().chain(x.iter()).next().cloned();
+
+    for &v in p.clone().difference(&pivot.map_or_else(HashSet::new, |p| {
+        graph.get(&p).unwrap_or(&HashSet::new()).clone()
+    })) {
+        let mut r = r.clone();
+        r.insert(v);
+
+        let mut p = p
+            .intersection(graph.get(&v).unwrap_or(&HashSet::new()))
+            .cloned()
+            .collect();
+        let mut x = x
+            .intersection(graph.get(&v).unwrap_or(&HashSet::new()))
+            .cloned()
+            .collect();
+
+        bron_kerbosch(graph, &mut r, &mut p, &mut x, cliques);
+
+        p.remove(&v);
+        x.insert(v);
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use indoc::indoc;
+
+    const CASE: &str = indoc! {"
+        kh-tc
+        qp-kh
+        de-cg
+        ka-co
+        yn-aq
+        qp-ub
+        cg-tb
+        vc-aq
+        tb-ka
+        wh-tc
+        yn-cg
+        kh-ub
+        ta-co
+        de-co
+        tc-td
+        tb-wq
+        wh-td
+        ta-ka
+        td-qp
+        aq-cg
+        wq-ub
+        ub-vc
+        de-ta
+        wq-aq
+        wq-vc
+        wh-yn
+        ka-de
+        kh-ta
+        co-tc
+        wh-qp
+        tb-vc
+        td-yn
+    "};
+
+    #[test]
+    fn part_a() {
+        assert_eq!(super::part_a(CASE), 7.into());
+    }
+
+    #[test]
+    fn part_b() {
+        assert_eq!(super::part_b(CASE), "co,de,ka,ta".into());
+    }
+}

aoc_2024/src/lib.rs

@@ -22,6 +22,7 @@ mod day_19;
 mod day_20;
 mod day_21;
 mod day_22;
+mod day_23;
 // [import_marker]
 
 pub const SOLUTIONS: &[Solution] = &[
@@ -47,5 +48,6 @@ pub const SOLUTIONS: &[Solution] = &[
     day_20::SOLUTION,
     day_21::SOLUTION,
     day_22::SOLUTION,
+    day_23::SOLUTION,
     // [list_marker]
 ];