From cf9b8f0321fe12edb41909417da3de5a5e75711a Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Sun, 12 Dec 2021 12:09:53 +0100
Subject: [PATCH] Day 12

---
 input/12.txt  |  21 ++++++
 src/bin/12.rs | 193 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 214 insertions(+)
 create mode 100644 input/12.txt
 create mode 100644 src/bin/12.rs

diff --git a/input/12.txt b/input/12.txt
new file mode 100644
index 0000000..5a75dd8
--- /dev/null
+++ b/input/12.txt
@@ -0,0 +1,21 @@
+mx-IQ
+mx-HO
+xq-start
+start-HO
+IE-qc
+HO-end
+oz-xq
+HO-ni
+ni-oz
+ni-MU
+sa-IE
+IE-ni
+end-sa
+oz-sa
+MU-start
+MU-sa
+oz-IE
+HO-xq
+MU-xq
+IE-end
+MU-mx
diff --git a/src/bin/12.rs b/src/bin/12.rs
new file mode 100644
index 0000000..c66af7d
--- /dev/null
+++ b/src/bin/12.rs
@@ -0,0 +1,193 @@
+use std::collections::HashMap;
+
+fn main() {
+    let puzzle = Network::from_str(include_str!("../../input/12.txt"));
+    println!("First solution: {}", puzzle.solve(Path::new(), false));
+    println!("Second solution: {}", puzzle.solve(Path::new(), true));
+}
+
+type Cave = &'static str;
+
+struct Network {
+    connections: HashMap<Cave, Vec<Cave>>,
+}
+
+#[derive(Clone, Debug)]
+struct Path {
+    caves: Vec<Cave>,
+}
+
+impl Network {
+    // Recursively count the number of paths.
+    fn solve(&self, current: Path, part2: bool) -> usize {
+        if current.end() == "end" {
+            return 1;
+        }
+
+        let mut n = 0;
+
+        for &next in &self.connections[current.end()] {
+            let can_visit = if part2 {
+                current.can_visit_plus(next)
+            } else {
+                current.can_visit(next)
+            };
+
+            if can_visit && next != "start" {
+                let mut next_path = current.clone();
+                next_path.visit(next);
+                n += self.solve(next_path, part2);
+            }
+        }
+        n
+    }
+
+    fn from_str(s: &'static str) -> Self {
+        let mut connections: HashMap<Cave, Vec<Cave>> = HashMap::new();
+        for (from, to) in s.lines().filter_map(|l| l.trim().split_once('-')) {
+            connections.entry(from).or_default().push(to);
+            connections.entry(to).or_default().push(from);
+        }
+
+        Self { connections }
+    }
+}
+
+impl Path {
+    fn new() -> Self {
+        Self {
+            caves: vec!["start"],
+        }
+    }
+    fn can_visit(&self, c: Cave) -> bool {
+        if c.to_lowercase() == c {
+            // Small caves may be visitied at most once
+            !self.caves.iter().any(|&cc| c == cc)
+        } else {
+            // Big caves can be visited any number of times
+            true
+        }
+    }
+    // Allows a single small cave to be visited twice.
+    fn can_visit_plus(&self, c: Cave) -> bool {
+        if c.to_lowercase() == c {
+            let already_visited = self.caves.iter().any(|&cc| c == cc);
+            if already_visited {
+                // We have already been to this small cave. Find out if we visited any single
+                // cave twice already.
+                let mut only_small_caves: Vec<Cave> = self
+                    .caves
+                    .clone()
+                    .into_iter()
+                    .filter(|&s| s.to_lowercase() == s)
+                    .collect();
+                only_small_caves.sort_unstable();
+                let a = only_small_caves.len();
+                only_small_caves.dedup();
+                let b = only_small_caves.len();
+                a == b
+            } else {
+                true
+            }
+        } else {
+            true
+        }
+    }
+    fn visit(&mut self, c: Cave) {
+        self.caves.push(c);
+    }
+    fn end(&self) -> Cave {
+        self.caves.last().unwrap()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::{Network, Path};
+
+    #[test]
+    fn input10() {
+        assert_eq!(
+            Network::from_str(
+                r"start-A
+        start-b
+        A-c
+        A-b
+        b-d
+        A-end
+        b-end
+        "
+            )
+            .solve(Path::new(), false),
+            10
+        );
+    }
+
+    #[test]
+    fn input10plus() {
+        assert_eq!(
+            Network::from_str(
+                r"start-A
+        start-b
+        A-c
+        A-b
+        b-d
+        A-end
+        b-end
+        "
+            )
+            .solve(Path::new(), true),
+            36
+        );
+    }
+
+    #[test]
+    fn input19() {
+        assert_eq!(
+            Network::from_str(
+                r"dc-end
+        HN-start
+        start-kj
+        dc-start
+        dc-HN
+        LN-dc
+        HN-end
+        kj-sa
+        kj-HN
+        kj-dc
+        "
+            )
+            .solve(Path::new(), false),
+            19
+        );
+    }
+
+    #[test]
+    fn input226() {
+        assert_eq!(
+            Network::from_str(
+                r"fs-end
+        he-DX
+        fs-he
+        start-DX
+        pj-DX
+        end-zg
+        zg-sl
+        zg-pj
+        pj-he
+        RW-he
+        fs-DX
+        pj-RW
+        zg-RW
+        start-pj
+        he-WI
+        zg-he
+        pj-fs
+        start-RW
+        "
+            )
+            .solve(Path::new(), false),
+            226
+        );
+    }
+}