diff --git a/input/day12 b/input/day12
new file mode 100644
index 0000000..012ca7e
--- /dev/null
+++ b/input/day12
@@ -0,0 +1,21 @@
+KF-sr
+OO-vy
+start-FP
+FP-end
+vy-mi
+vy-KF
+vy-na
+start-sr
+FP-lh
+sr-FP
+na-FP
+end-KF
+na-mi
+lh-KF
+end-lh
+na-start
+wp-KF
+mi-KF
+vy-sr
+vy-lh
+sr-mi
\ No newline at end of file
diff --git a/src/day11.rs b/src/day11.rs
index 9701fc6..2db219b 100644
--- a/src/day11.rs
+++ b/src/day11.rs
@@ -81,6 +81,7 @@ fn do_flash(mut hm: &mut FlashMap, mut flashing: &mut Vec<(i32, i32)>, mut flash
     }
 }
 
+#[allow(unused)]
 fn print_flashes(map: &FlashMap, x_max: i32, y_max: i32) {
     let mut v: Vec<Vec<i32>> = Vec::new();
     for y in 0..y_max {
diff --git a/src/day12.rs b/src/day12.rs
new file mode 100644
index 0000000..a0b75aa
--- /dev/null
+++ b/src/day12.rs
@@ -0,0 +1,275 @@
+use std::collections::HashMap;
+use crate::Day;
+
+pub struct Day12(HashMap<String, Cave>);
+
+impl Day for Day12 {
+    fn init(content: String) -> anyhow::Result<Self> {
+        let mut caves = HashMap::new();
+
+        for line in content.lines() {
+            let z = line.split('-').collect::<Vec<_>>();
+            let first = z[0];
+            let second = z[1];
+
+            {
+                let first_cave = caves.entry(first.to_string()).or_insert(Cave::new(first.into()));
+                first_cave.add_connection(second);
+            }
+            {
+                let second_cave = caves.entry(second.to_string()).or_insert(Cave::new(second.into()));
+                second_cave.add_connection(first);
+            }
+        }
+
+        Ok(Self(caves))
+    }
+
+    fn part1(&self) -> anyhow::Result<String> {
+        fn get_all_paths(caves: &HashMap<String, Cave>, complete_paths: &mut Vec<Vec<String>>, current_path: &Vec<String>, current_cave_idx: &str) {
+            if current_cave_idx == "end" {
+                complete_paths.push(current_path.clone());
+                return;
+            }
+
+            let current_cave = caves.get(current_cave_idx).expect("This cave does not exist!").clone();
+
+            for connected_cave in current_cave.connections {
+                let current = caves.get(&connected_cave).unwrap();
+                match current.cave_type {
+                    CaveType::Small => {
+                        if current_path.iter().find(|&cave_pair| cave_pair == &connected_cave) != None {
+                            continue;
+                        }
+                    }
+                    _ => (),
+                }
+
+                let mut new_path = current_path.clone();
+                new_path.push(connected_cave.clone());
+                get_all_paths(&caves, complete_paths, &new_path, &connected_cave);
+            }
+        }
+
+        let mut complete_paths = Vec::new();
+        let current_path = vec!["start".into()];
+
+        get_all_paths(&self.0, &mut complete_paths, &current_path, "start".into());
+
+        Ok(format!("{}", complete_paths.len()))
+    }
+
+    fn part2(&self) -> anyhow::Result<String> {
+        let mut small_caves_thing: HashMap<Vec<String>, Vec<String>> = HashMap::<Vec<String>, Vec<String>>::new();
+        fn get_all_paths(caves: &HashMap<String, Cave>, complete_paths: &mut Vec<Vec<String>>, current_path: &Vec<String>, current_cave_idx: &str, small_caves_thing: &mut HashMap<Vec<String>, Vec<String>>) {
+            if current_cave_idx == "end" {
+                complete_paths.push(current_path.clone());
+                return;
+            }
+
+
+            let current_cave = caves.get(current_cave_idx).expect("This cave does not exist!").clone();
+
+            let small_caves = {
+                let e = small_caves_thing.entry(current_path.clone()).or_insert(small_caves_visited_at_least_twice(caves, current_path));
+                e.clone()
+            };
+
+            for connected_cave in current_cave.connections {
+                if connected_cave == "start" {
+                    continue;
+                }
+                if !small_caves.is_empty() {
+                    let current = caves.get(&connected_cave).unwrap();
+                    match current.cave_type {
+                        CaveType::Small => {
+                            if !(small_caves.len() == 1 && small_caves[0] != connected_cave) {
+                                continue;
+                            }
+                        }
+                        _ => (),
+                    }
+                }
+
+                let mut new_path = current_path.clone();
+                new_path.push(connected_cave.clone());
+                get_all_paths(&caves, complete_paths, &new_path, &connected_cave, small_caves_thing);
+            }
+        }
+
+        let mut complete_paths = Vec::new();
+        let current_path = vec!["start".into()];
+
+        get_all_paths(&self.0, &mut complete_paths, &current_path, "start", &mut small_caves_thing);
+
+        Ok(format!("{}", complete_paths.len()))
+    }
+}
+
+fn small_caves_visited_at_least_twice(caves: &HashMap<String, Cave>, current_path: &Vec<String>) -> Vec<String> {
+    let mut small_cave_indices = HashMap::new();
+    for cave_name in current_path {
+        let c = caves.get(cave_name).expect("how?");
+        if c.cave_type == CaveType::Small {
+            let e = small_cave_indices.entry(cave_name).or_insert(0);
+            *e += 1;
+        }
+    }
+    return small_cave_indices.iter().filter(|(_, &b)| b > 1).map(|(&a, _)| a.clone()).collect();
+}
+
+#[derive(Ord, PartialOrd, Eq, PartialEq, Copy, Clone)]
+enum CaveType {
+    Large,
+    Small,
+}
+
+#[derive(Ord, PartialOrd, Eq, PartialEq, Clone)]
+struct Cave {
+    connections: Vec<String>,
+    cave_type: CaveType,
+}
+
+impl Cave {
+    fn add_connection<T>(&mut self, name: T) where T: ToString {
+        self.connections.push(name.to_string());
+        self.connections.sort();
+        self.connections.dedup();
+    }
+
+    fn new(cave_name: String) -> Self {
+        let cave_type = if cave_name.chars().nth(0).unwrap().is_uppercase() {
+            CaveType::Large
+        } else {
+            CaveType::Small
+        };
+        Self {
+            cave_type,
+            connections: vec![],
+        }
+    }
+}
+
+impl Day12 {
+    fn search(&self, cur: &str, had: &HashMap<String, i32>, part1: bool) -> i32 {
+        let mut had = had.clone();
+        if had.values().any(|&i| i > 0) || part1 {
+            if had.contains_key(cur) {
+                return 0;
+            }
+        }
+
+        if cur == "start" && had.contains_key("start") {
+            return 0;
+        }
+        if cur == "end" {
+            return 1;
+        }
+        if cur.chars().all(|x| x.is_lowercase()) {
+            let e = had.entry(cur.into()).or_insert(0);
+            *e += 1;
+        }
+        let n = self.0.get(cur).unwrap();
+        return n.connections.iter().map(|c| self.search(c, &mut had, part1)).sum::<i32>();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{Day, Day12};
+
+    const INPUT_1: &str = r"start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end";
+
+    const INPUT_2: &str = r"dc-end
+HN-start
+start-kj
+dc-start
+dc-HN
+LN-dc
+HN-end
+kj-sa
+kj-HN
+kj-dc";
+
+    const INPUT_3: &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";
+
+    #[test]
+    fn part1_test_1() -> anyhow::Result<()> {
+        let d = Day12::init(INPUT_1.to_string())?;
+        assert_eq!("10", d.part1()?);
+        Ok(())
+    }
+
+    #[test]
+    fn part1_test_2() -> anyhow::Result<()> {
+        let d = Day12::init(INPUT_2.to_string())?;
+        assert_eq!("19", d.part1()?);
+        Ok(())
+    }
+
+    #[test]
+    fn part1_test_3() -> anyhow::Result<()> {
+        let d = Day12::init(INPUT_3.to_string())?;
+        assert_eq!("226", d.part1()?);
+        Ok(())
+    }
+
+    // #[test]
+    // fn part1_real() -> anyhow::Result<()> {
+    //     let d = Day12::init(crate::load_input("12")?)?;
+    //     assert_eq!("4885", d.part1()?);
+    //     Ok(())
+    // }
+
+    #[test]
+    fn part2_test_1() -> anyhow::Result<()> {
+        let d = Day12::init(INPUT_1.to_string())?;
+        assert_eq!("36", d.part2()?);
+        Ok(())
+    }
+
+    #[test]
+    fn part2_test_2() -> anyhow::Result<()> {
+        let d = Day12::init(INPUT_2.to_string())?;
+        assert_eq!("103", d.part2()?);
+        Ok(())
+    }
+
+    // These are very slow, so they are commented out.
+    // #[test]
+    // fn part2_test_3() -> anyhow::Result<()> {
+    //     let d = Day12::init(INPUT_3.to_string())?;
+    //     assert_eq!("3509", d.part2()?);
+    //     Ok(())
+    // }
+
+    #[test]
+    fn part2_real() -> anyhow::Result<()> {
+        let d = Day12::init(crate::load_input("12")?)?;
+        assert_eq!("117095", d.part2()?);
+        Ok(())
+    }
+}
\ No newline at end of file
diff --git a/src/main.rs b/src/main.rs
index fa76862..b81e939 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -14,6 +14,7 @@ use crate::day08::Day08;
 use crate::day09::Day09;
 use crate::day10::Day10;
 use crate::day11::Day11;
+use crate::day12::Day12;
 
 mod day;
 mod day01;
@@ -27,6 +28,7 @@ mod day08;
 mod day09;
 mod day10;
 mod day11;
+mod day12;
 
 fn load_input(day: &str) -> Result<String> {
     read_to_string(format!("./input/day{}", day)).map_err(|x| x.into())
@@ -72,7 +74,8 @@ fn main() -> anyhow::Result<()> {
                                        Box::new(Day08::init(load_input("08")?)?),
                                        Box::new(Day09::init(load_input("09")?)?),
                                        Box::new(Day10::init(load_input("10")?)?),
-                                       Box::new(Day11::init(load_input("11")?)?),];
+                                       Box::new(Day11::init(load_input("11")?)?),
+                                       Box::new(Day12::init(load_input("12")?)?),];
 
     let _verbosity = matches.occurrences_of("v");
     if matches.is_present("all") {