diff --git a/Cargo.toml b/Cargo.toml
index 0249f0c..14bfb68 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -46,3 +46,7 @@ path = "src/day10.rs"
 [[bin]]
 name = "day11"
 path = "src/day11.rs"
+
+[[bin]]
+name = "day12"
+path = "src/day12.rs"
diff --git a/inputs/day12.txt b/inputs/day12.txt
new file mode 100644
index 0000000..a409ba5
--- /dev/null
+++ b/inputs/day12.txt
@@ -0,0 +1,21 @@
+TR-start
+xx-JT
+xx-TR
+hc-dd
+ab-JT
+hc-end
+dd-JT
+ab-dd
+TR-ab
+vh-xx
+hc-JT
+TR-vh
+xx-start
+hc-ME
+vh-dd
+JT-bm
+end-ab
+dd-xx
+end-TR
+hc-TR
+start-vh
diff --git a/src/day12.rs b/src/day12.rs
new file mode 100644
index 0000000..254f011
--- /dev/null
+++ b/src/day12.rs
@@ -0,0 +1,151 @@
+use std::collections::HashMap;
+use std::str::FromStr;
+
+#[derive(Clone)]
+struct CavePath {
+    caves: (String, String),
+}
+
+impl CavePath {
+    fn from(&self, cave: &str) -> Option<&str> {
+        if cave == self.caves.0 {
+            Some(&self.caves.1)
+        } else if cave == self.caves.1 {
+            Some(&self.caves.0)
+        } else {
+            None
+        }
+    }
+}
+
+impl FromStr for CavePath {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<CavePath, Self::Err> {
+        let caves = s.split_once('-').unwrap();
+        Ok(CavePath {
+            caves: (caves.0.to_string(), caves.1.to_string()),
+        })
+    }
+}
+
+fn is_small_cave(cave: &str) -> bool {
+    if cave == "start" {
+        false
+    } else {
+        cave.chars().all(|c| matches!(c, 'a'..='z'))
+    }
+}
+
+fn count_paths(input: &str, allow_twice: bool) -> usize {
+    let paths = input
+        .lines()
+        .map(|line| line.parse().unwrap())
+        .collect::<Vec<CavePath>>();
+    count_paths_recursive("start", &paths, &HashMap::from([("start", 2)]), allow_twice)
+}
+
+fn count_paths_recursive(
+    root: &str,
+    paths: &[CavePath],
+    visited: &HashMap<&str, usize>,
+    allow_twice: bool,
+) -> usize {
+    if root == "end" {
+        return 1;
+    }
+
+    let mut new_visited = visited.clone();
+    if is_small_cave(root) {
+        let counter = new_visited.entry(root).or_insert(0);
+        *counter += 1;
+    }
+
+    let mut sum = 0;
+    for path in paths {
+        if let Some(cave) = path.from(root) {
+            let visited_count = visited.get(cave);
+            let is_twice = allow_twice && is_small_cave(cave) && visited_count == Some(&1);
+            if visited_count.is_none() || is_twice {
+                sum += count_paths_recursive(cave, paths, &new_visited, allow_twice && !is_twice);
+            }
+        }
+    }
+    sum
+}
+
+fn main() {
+    const INPUT: &str = include_str!("../inputs/day12.txt");
+    println!("solution {}", count_paths(INPUT, false));
+    println!("solution {}", count_paths(INPUT, true));
+}
+
+#[test]
+fn test_count_single_paths() {
+    const INPUT: &str = "start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end";
+    assert_eq!(count_paths(INPUT, false), 10);
+}
+
+#[test]
+fn test_count_double_paths() {
+    const INPUT: &str = "start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end";
+    assert_eq!(count_paths(INPUT, true), 36);
+}
+
+#[test]
+fn test_count_larger_single_paths() {
+    const INPUT: &str = "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";
+    assert_eq!(count_paths(INPUT, false), 226);
+}
+
+#[test]
+fn test_count_larger_double_paths() {
+    const INPUT: &str = "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";
+    assert_eq!(count_paths(INPUT, true), 3509);
+}