diff --git a/aoc2021/input/day12.txt b/aoc2021/input/day12.txt
new file mode 100644
index 0000000..731cd4b
--- /dev/null
+++ b/aoc2021/input/day12.txt
@@ -0,0 +1,22 @@
+GC-zi
+end-zv
+lk-ca
+lk-zi
+GC-ky
+zi-ca
+end-FU
+iv-FU
+lk-iv
+lk-FU
+GC-end
+ca-zv
+lk-GC
+GC-zv
+start-iv
+zv-QQ
+ca-GC
+ca-FU
+iv-ca
+start-lk
+zv-FU
+start-zi
diff --git a/aoc2021/input/day12_provided1.txt b/aoc2021/input/day12_provided1.txt
new file mode 100644
index 0000000..6fd8c41
--- /dev/null
+++ b/aoc2021/input/day12_provided1.txt
@@ -0,0 +1,7 @@
+start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end
diff --git a/aoc2021/input/day12_provided2.txt b/aoc2021/input/day12_provided2.txt
new file mode 100644
index 0000000..62cc714
--- /dev/null
+++ b/aoc2021/input/day12_provided2.txt
@@ -0,0 +1,10 @@
+dc-end
+HN-start
+start-kj
+dc-start
+dc-HN
+LN-dc
+HN-end
+kj-sa
+kj-HN
+kj-dc
diff --git a/aoc2021/input/day12_provided3.txt b/aoc2021/input/day12_provided3.txt
new file mode 100644
index 0000000..65f3833
--- /dev/null
+++ b/aoc2021/input/day12_provided3.txt
@@ -0,0 +1,18 @@
+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
diff --git a/aoc2021/src/day12.rs b/aoc2021/src/day12.rs
new file mode 100644
index 0000000..c92b979
--- /dev/null
+++ b/aoc2021/src/day12.rs
@@ -0,0 +1,140 @@
+use std::collections::{HashMap, HashSet};
+use std::fmt::Write;
+
+use anyhow::{Context, Result};
+
+const INPUT: &str = include_str!("../input/day12.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+fn part1(input: &str) -> Result<usize> {
+    let cave_map: CaveMap = input.try_into()?;
+
+    cave_map.count_paths()
+}
+
+#[derive(Clone, Copy)]
+struct Cave<'a> {
+    name: &'a str,
+    small: bool,
+}
+
+impl<'a> Cave<'a> {
+    fn is_end(&self) -> bool {
+        self.name == "end"
+    }
+
+    fn is_start(&self) -> bool {
+        self.name == "start"
+    }
+
+    fn is_small(&self) -> bool {
+        self.small
+    }
+}
+
+impl<'a> From<&'a str> for Cave<'a> {
+    fn from(s: &'a str) -> Self {
+        Cave {
+            name: s,
+            small: s.chars().all(char::is_lowercase),
+        }
+    }
+}
+
+impl<'a> std::hash::Hash for Cave<'a> {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.name.hash(state);
+    }
+}
+
+impl<'a> std::cmp::PartialEq for Cave<'a> {
+    fn eq(&self, other: &Self) -> bool {
+        self.name == other.name
+    }
+}
+impl<'a> std::cmp::Eq for Cave<'a> {}
+
+struct CaveMap<'a> {
+    connections: HashMap<Cave<'a>, Vec<Cave<'a>>>,
+}
+
+impl<'a> CaveMap<'a> {
+    fn count_paths(&self) -> Result<usize> {
+        let start = *self
+            .connections
+            .keys()
+            .find(|cave| cave.is_start())
+            .context("couldn't find starting cave")?;
+        Ok(self.count_paths_rec(start, HashSet::new()))
+    }
+
+    fn count_paths_rec(&self, from: Cave<'a>, mut small_seen: HashSet<Cave<'a>>) -> usize {
+        if from.is_end() {
+            return 1;
+        }
+
+        if from.is_small() {
+            if small_seen.contains(&from) {
+                return 0;
+            }
+            small_seen.insert(from);
+        }
+
+        let mut paths = 0;
+        for dst in &self.connections[&from] {
+            paths += self.count_paths_rec(*dst, small_seen.clone());
+        }
+
+        paths
+    }
+}
+
+impl<'a> TryFrom<&'a str> for CaveMap<'a> {
+    type Error = anyhow::Error;
+
+    fn try_from(s: &'a str) -> Result<Self> {
+        let mut map = HashMap::new();
+
+        for line in s.lines().map(str::trim) {
+            let (src, dst) = line
+                .split_once('-')
+                .context("couldn't parse cave connection")?;
+            map.entry(src.into())
+                .or_insert_with(Vec::new)
+                .push(dst.into());
+            map.entry(dst.into())
+                .or_insert_with(Vec::new)
+                .push(src.into());
+        }
+
+        Ok(CaveMap { connections: map })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED1: &str = include_str!("../input/day12_provided1.txt");
+    const PROVIDED2: &str = include_str!("../input/day12_provided2.txt");
+    const PROVIDED3: &str = include_str!("../input/day12_provided3.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED1).unwrap(), 10);
+        assert_eq!(part1(PROVIDED2).unwrap(), 19);
+        assert_eq!(part1(PROVIDED3).unwrap(), 226);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 5252);
+    }
+}
diff --git a/aoc2021/src/lib.rs b/aoc2021/src/lib.rs
index 5d8eeba..cc37626 100644
--- a/aoc2021/src/lib.rs
+++ b/aoc2021/src/lib.rs
@@ -11,4 +11,5 @@ pub mod day08;
 pub mod day09;
 pub mod day10;
 pub mod day11;
+pub mod day12;
 pub mod day13;
diff --git a/aoc2021/src/main.rs b/aoc2021/src/main.rs
index c21a96f..1fa15e3 100644
--- a/aoc2021/src/main.rs
+++ b/aoc2021/src/main.rs
@@ -13,6 +13,7 @@ use aoc2021::day08;
 use aoc2021::day09;
 use aoc2021::day10;
 use aoc2021::day11;
+use aoc2021::day12;
 use aoc2021::day13;
 
 fn main() -> Result<()> {
@@ -28,6 +29,7 @@ fn main() -> Result<()> {
         day09::run,
         day10::run,
         day11::run,
+        day12::run,
         day13::run,
     ];