day11: part2

README.md

@@ -1,6 +1,6 @@
 <!-- AOC TILES BEGIN -->
 <h1 align="center">
-  2025 - 18 ⭐ - Rust
+  2025 - 20 ⭐ - Rust
 </h1>
 <a href="src/day1.rs">
   <img src=".aoc_tiles/tiles/2025/01.png" width="161px">
@@ -32,4 +32,7 @@
 <a href="src/day10.rs">
   <img src=".aoc_tiles/tiles/2025/10.png" width="161px">
 </a>
+<a href="src/day11.rs">
+  <img src=".aoc_tiles/tiles/2025/11.png" width="161px">
+</a>
 <!-- AOC TILES END -->

src/day11.rs

@@ -0,0 +1,140 @@
+use aoc_runner_derive::{aoc, aoc_generator};
+use std::collections::{HashMap, HashSet};
+
+type Edges = HashMap<String, Vec<String>>;
+
+#[aoc_generator(day11)]
+fn parse(input: &str) -> Edges {
+    let mut edges: Edges = HashMap::new();
+    for l in input.lines() {
+        let (k, rest) = l.split_once(": ").unwrap();
+        for v in rest.split_ascii_whitespace() {
+            edges.entry(k.to_string()).or_default().push(v.to_string());
+        }
+    }
+
+    edges
+}
+
+fn find_path(cur: &str, goal: &str, edges: &Edges) -> u64 {
+    if cur == goal {
+        return 1;
+    }
+    if let Some(nexts) = edges.get(cur) {
+        nexts.iter().map(|n| find_path(n, goal, edges)).sum()
+    } else {
+        0
+    }
+}
+
+fn mark_paths<'a>(
+    cur: &'a str,
+    edges: &'a Edges,
+    mut reachable: HashSet<&'a str>,
+) -> HashSet<&'a str> {
+    reachable.insert(cur);
+    if let Some(nexts) = edges.get(cur) {
+        for n in nexts {
+            if !reachable.contains(&n.as_str()) {
+                // reachable.insert(n);
+                reachable = mark_paths(n, edges, reachable);
+            }
+        }
+    }
+    reachable
+}
+
+#[aoc(day11, part1)]
+fn part1(fwd: &Edges) -> u64 {
+    find_path("you", "out", fwd)
+}
+
+#[aoc(day11, part2)]
+fn part2(edges: &Edges) -> u64 {
+    let mut rev = Edges::from_iter(edges.keys().map(|k| (k.to_owned(), vec![])));
+    for (from, tos) in edges {
+        for to in tos {
+            rev.entry(to.to_owned()).or_default().push(from.to_owned());
+        }
+    }
+    let mut reachable_dac = mark_paths("dac", edges, HashSet::new());
+    reachable_dac = mark_paths("dac", &rev, reachable_dac);
+    let mut reachable_fft = mark_paths("fft", edges, HashSet::new());
+    reachable_fft = mark_paths("fft", &rev, reachable_fft);
+
+    let reachable: HashSet<&str> = reachable_dac
+        .intersection(&reachable_fft)
+        .copied()
+        .collect();
+
+    let unreachable: HashSet<&str> = HashSet::from_iter(edges.keys().map(|k| k.as_str()))
+        .difference(&reachable)
+        .copied()
+        .collect();
+
+    let mut reachable_edges = edges.clone();
+
+    for k in &unreachable {
+        reachable_edges.remove(*k);
+    }
+    for (_k, v) in reachable_edges.iter_mut() {
+        for ur in &unreachable {
+            if let Some(idx) = v.iter().position(|s| s == ur) {
+                v.remove(idx);
+            }
+        }
+    }
+
+    // This is a bit of a cheat from viewing the graph and realizing all paths are ordered this way.
+    // However, knowing that fact that we can partition in this way, we could use a shortest-path algo
+    // to determine which order fft and dac are in.
+    //
+    // The assumption holds on my data and the example, so I think it's probably true of all inputs.
+    find_path("svr", "fft", &reachable_edges)
+        * find_path("fft", "dac", &reachable_edges)
+        * find_path("dac", "out", &reachable_edges)
+}
+
+#[cfg(test)]
+mod tests {
+    use rstest::rstest;
+
+    use super::*;
+
+    const EXAMPLE: &str = "aaa: you hhh
+you: bbb ccc
+bbb: ddd eee
+ccc: ddd eee fff
+ddd: ggg
+eee: out
+fff: out
+ggg: out
+hhh: ccc fff iii
+iii: out";
+
+    const EXAMPLE2: &str = "svr: aaa bbb
+aaa: fft
+fft: ccc
+bbb: tty
+tty: ccc
+ccc: ddd eee
+ddd: hub
+hub: fff
+eee: dac
+dac: fff
+fff: ggg hhh
+ggg: out
+hhh: out";
+
+    #[rstest]
+    #[case(EXAMPLE, 5)]
+    fn part1_example(#[case] input: &str, #[case] expected: u64) {
+        assert_eq!(part1(&parse(input)), expected);
+    }
+
+    #[rstest]
+    #[case(EXAMPLE2, 2)]
+    fn part2_example(#[case] input: &str, #[case] expected: u64) {
+        assert_eq!(part2(&parse(input)), expected);
+    }
+}

src/lib.rs

@@ -1,5 +1,6 @@
 mod day1;
 mod day10;
+mod day11;
 mod day2;
 mod day3;
 mod day4;