Year 2024 Day 23

Author: maneatingape

README.md

@@ -93,6 +93,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | 19 | [Linen Layout](https://adventofcode.com/2024/day/19) | [Source](src/year2024/day19.rs) | 118 |
 | 20 | [Race Condition](https://adventofcode.com/2024/day/20) | [Source](src/year2024/day20.rs) | 1354 |
 | 22 | [Monkey Market](https://adventofcode.com/2024/day/22) | [Source](src/year2024/day22.rs) | 1350 |
+| 23 | [LAN Party](https://adventofcode.com/2024/day/23) | [Source](src/year2024/day23.rs) | 43 |
 
 ## 2023
 

benches/benchmark.rs

@@ -88,5 +88,5 @@ benchmark!(year2023
 
 benchmark!(year2024
     day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13,
-    day14, day15, day16, day17, day18, day19, day20, day22
+    day14, day15, day16, day17, day18, day19, day20, day22, day23
 );

src/lib.rs

@@ -68,5 +68,5 @@ library!(year2023 "Restore global snow production."
 
 library!(year2024 "Locate the Chief Historian in time for the big Christmas sleigh launch."
     day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13,
-    day14, day15, day16, day17, day18, day19, day20, day22
+    day14, day15, day16, day17, day18, day19, day20, day22, day23
 );

src/main.rs

@@ -138,5 +138,5 @@ run!(year2023
 
 run!(year2024
     day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13,
-    day14, day15, day16, day17, day18, day19, day20, day22
+    day14, day15, day16, day17, day18, day19, day20, day22, day23
 );

src/year2024/day23.rs

@@ -0,0 +1,92 @@
+//! # LAN Party
+use crate::util::hash::*;
+
+type Input = (FastMap<usize, Vec<usize>>, Vec<[bool; 676]>);
+
+pub fn parse(input: &str) -> Input {
+    let mut nodes = FastMap::with_capacity(1_000);
+    let mut edges = vec![[false; 676]; 676];
+
+    let to_index = |b: &[u8]| 26 * to_usize(b[0]) + to_usize(b[1]);
+    let empty = || Vec::with_capacity(16);
+
+    for edge in input.as_bytes().chunks(6) {
+        let from = to_index(&edge[..2]);
+        let to = to_index(&edge[3..]);
+
+        nodes.entry(from).or_insert_with(empty).push(to);
+        nodes.entry(to).or_insert_with(empty).push(from);
+
+        edges[from][to] = true;
+        edges[to][from] = true;
+    }
+
+    (nodes, edges)
+}
+
+pub fn part1(input: &Input) -> usize {
+    let (nodes, edges) = input;
+    let mut seen = [false; 676];
+    let mut triangles = 0;
+
+    for n1 in 494..520 {
+        if let Some(neighbours) = nodes.get(&n1) {
+            seen[n1] = true;
+
+            for (i, &n2) in neighbours.iter().enumerate() {
+                for &n3 in neighbours.iter().skip(i) {
+                    if !seen[n2] && !seen[n3] && edges[n2][n3] {
+                        triangles += 1;
+                    }
+                }
+            }
+        }
+    }
+
+    triangles
+}
+
+pub fn part2(input: &Input) -> String {
+    let (nodes, edges) = input;
+    let mut seen = [false; 676];
+    let mut clique = Vec::new();
+    let mut largest = Vec::new();
+
+    for (&n1, neighbours) in nodes {
+        if !seen[n1] {
+            clique.clear();
+            clique.push(n1);
+
+            for &n2 in neighbours {
+                if clique.iter().all(|&c| edges[n2][c]) {
+                    seen[n2] = true;
+                    clique.push(n2);
+                }
+            }
+
+            if clique.len() > largest.len() {
+                largest.clone_from(&clique);
+            }
+        }
+    }
+
+    let mut result = String::new();
+    largest.sort_unstable();
+
+    for n in largest {
+        result.push(to_char(n / 26));
+        result.push(to_char(n % 26));
+        result.push(',');
+    }
+
+    result.pop();
+    result
+}
+
+fn to_usize(b: u8) -> usize {
+    (b - b'a') as usize
+}
+
+fn to_char(u: usize) -> char {
+    ((u as u8) + b'a') as char
+}

tests/test.rs

@@ -81,5 +81,5 @@ test!(year2023
 
 test!(year2024
     day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11, day12, day13,
-    day14, day15, day16, day17, day18, day19, day20, day22
+    day14, day15, day16, day17, day18, day19, day20, day22, day23
 );

tests/year2024/day23.rs

@@ -0,0 +1,47 @@
+use aoc::year2024::day23::*;
+
+const EXAMPLE: &str = "\
+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 part1_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part1(&input), 7);
+}
+
+#[test]
+fn part2_test() {
+    let input = parse(EXAMPLE);
+    assert_eq!(part2(&input), "co,de,ka,ta");
+}