2024 day 23

This solution currently assumes each node is in a single clique, which works for my input but not the example.

Part 2 output is 38 characters, which required redistributing output column widths.

Author: ictrobot

crates/aoc/src/main.rs

@@ -36,9 +36,12 @@ fn main() {
     }
 
     // FIXME support 80 character wide output (without time?)
-    println!("Puzzle  │ Part 1                           │ Part 2                           │ Time      ");
-    println!("────────┼──────────────────────────────────┼──────────────────────────────────┼───────────");
-
+    println!(
+        "Puzzle  │ Part 1               │ Part 2                                 │ Time      "
+    );
+    println!(
+        "────────┼──────────────────────┼────────────────────────────────────────┼───────────"
+    );
     let mut total = Duration::default();
     for (year, day, f) in puzzles {
         let input = match read_input(year, day) {
@@ -57,8 +60,8 @@ fn main() {
 
                 // Hack to treat "🎄" as two characters wide
                 // ("🎄" is 1 wide in Unicode 8 but 2 wide in Unicode 9+)
-                let part1_width = if part1 == "🎄" { 31 } else { 32 };
-                let part2_width = if part2 == "🎄" { 31 } else { 32 };
+                let part1_width = if part1 == "🎄" { 19 } else { 20 };
+                let part2_width = if part2 == "🎄" { 37 } else { 38 };
 
                 println!(
                     "{year:#} {day:#} │ {part1:<part1_width$} │ {part2:<part2_width$} │ {}",
@@ -73,10 +76,10 @@ fn main() {
     }
 
     println!(
-        "────────┴──────────────────────────────────┴──────────────────────────────────┼───────────"
+        "────────┼──────────────────────┼────────────────────────────────────────┼───────────"
     );
     println!(
-        "                                                                              │ {}",
+        "                                                                        │ {}",
         format_duration(total),
     );
 }

crates/year2024/src/day23.rs

@@ -0,0 +1,119 @@
+use utils::prelude::*;
+
+/// Finding the largest clique in a graph.
+#[derive(Clone, Debug)]
+pub struct Day23 {
+    nodes: Vec<Node>,
+}
+
+#[derive(Clone, Debug)]
+struct Node {
+    name: [u8; 2],
+    edges: Vec<usize>,
+}
+
+impl Day23 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let mut indexes = [None; 26 * 26];
+        let mut nodes = vec![];
+
+        for item in parser::byte_range(b'a'..=b'z')
+            .repeat_n(parser::noop())
+            .repeat_n(b'-')
+            .with_suffix(parser::eol())
+            .parse_iterator(input)
+        {
+            let [n1, n2] = item?;
+
+            let mut index_of = |n: [u8; 2]| {
+                let i = 26 * (n[0] - b'a') as usize + (n[1] - b'a') as usize;
+                match indexes[i] {
+                    Some(index) => index,
+                    None => {
+                        let index = nodes.len();
+                        nodes.push(Node {
+                            name: n,
+                            edges: Vec::new(),
+                        });
+                        indexes[i] = Some(index);
+                        index
+                    }
+                }
+            };
+
+            let index1 = index_of(n1);
+            let index2 = index_of(n2);
+
+            nodes[index1].edges.push(index2);
+            nodes[index2].edges.push(index1);
+        }
+
+        Ok(Self { nodes })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u32 {
+        let mut count = 0;
+        for (i1, n1) in self.nodes.iter().enumerate() {
+            for (i2, n2) in n1.edges.iter().map(|&i| (i, &self.nodes[i])) {
+                if i1 > i2 {
+                    continue;
+                }
+                for (i3, n3) in n2.edges.iter().map(|&i| (i, &self.nodes[i])) {
+                    if i2 < i3
+                        && n1.edges.contains(&i3)
+                        && (n1.name[0] == b't' || n2.name[0] == b't' || n3.name[0] == b't')
+                    {
+                        count += 1;
+                    }
+                }
+            }
+        }
+        count
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> String {
+        let mut assigned = vec![false; self.nodes.len()];
+        let mut node_lists = Vec::with_capacity(self.nodes.len());
+        for i in 0..self.nodes.len() {
+            if !assigned[i] {
+                let c = node_lists.len();
+                node_lists.push(Vec::new());
+                self.try_add(i, &mut assigned, &mut node_lists[c]);
+            }
+        }
+
+        let mut nodes = node_lists.into_iter().max_by_key(|l| l.len()).unwrap();
+        nodes.sort_unstable_by_key(|&n| self.nodes[n].name);
+        nodes
+            .iter()
+            .fold(String::with_capacity(nodes.len() * 3), |mut acc, &i| {
+                if !acc.is_empty() {
+                    acc.push(',');
+                }
+                acc.push(self.nodes[i].name[0] as char);
+                acc.push(self.nodes[i].name[1] as char);
+                acc
+            })
+    }
+
+    fn try_add(&self, n: usize, assigned: &mut [bool], group: &mut Vec<usize>) {
+        for &existing in group.iter() {
+            if !self.nodes[n].edges.contains(&existing) {
+                return;
+            }
+        }
+
+        group.push(n);
+        assigned[n] = true;
+
+        for &neighbour in self.nodes[n].edges.iter() {
+            if !assigned[neighbour] {
+                self.try_add(neighbour, assigned, group);
+            }
+        }
+    }
+}
+
+examples!(Day23 -> (u32, &'static str) []);

crates/year2024/src/lib.rs

@@ -24,4 +24,5 @@ utils::year!(2024 => year2024, ${
     20 => day20::Day20,
     21 => day21::Day21,
     22 => day22::Day22,
+    23 => day23::Day23,
 });