diff --git a/.gitignore b/.gitignore
index 1313560..b8f3ecb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,4 +9,4 @@ target/
 *.gcda
 
 # Specific days
-!day10/src/input.rs
+!day10/src/*.rs
diff --git a/day10/Cargo.toml b/day10/Cargo.toml
index 416ae24..7ac6edf 100644
--- a/day10/Cargo.toml
+++ b/day10/Cargo.toml
@@ -8,10 +8,12 @@ edition = "2018"
 
 [features]
 
-part2=[]
+part2=["num_cpus", "semaphore"]
 test = []
 
 [dependencies]
 ad-hoc-iter = "0.2.2"
 cfg-if = "1.0.0"
+num_cpus = {version = "1.13.0", optional = true}
+semaphore = {version = "0.4.0", optional = true}
 smallmap = "1.2.1"
diff --git a/day10/src/part2.rs b/day10/src/part2.rs
new file mode 100644
index 0000000..846281c
--- /dev/null
+++ b/day10/src/part2.rs
@@ -0,0 +1,136 @@
+use super::*;
+use std::{
+    sync::{
+	Arc,
+	RwLock,
+    },
+    thread,
+};
+use semaphore::Semaphore;
+
+#[derive(Debug)]
+pub struct Cache(RwLock<Map<NonZeroU8, usize>>);
+
+impl Cache
+{
+    pub fn new() -> Self
+    {
+	Self(RwLock::new(Map::new()))
+    }
+    pub fn get_or_insert_with_owned<F>(self: Arc<Self>, key: u8, with: F) -> usize
+    where F: FnOnce() -> usize
+    {
+	match Arc::try_unwrap(self)
+	{
+	    Ok(this) => {
+		debug_assert!(key>0);
+		let key = unsafe {NonZeroU8::new_unchecked(key)};
+
+		let map = this.0.into_inner().unwrap();
+		if let Some(&value) = map.get(&key) {
+		    value
+		} else {
+		    // No need to insert, the cache will be dropped after this
+		    with()
+		}
+	    },
+	    Err(this) => this.get_or_insert_with(key, with)
+	}
+    }
+    #[inline] pub fn insert(&self, key: u8, value: usize) -> usize
+    {
+	debug_assert!(key>0);
+	let key = unsafe {NonZeroU8::new_unchecked(key)};
+	self.0.write().unwrap().insert(key, value);
+	value
+    }
+    pub fn get(&self, key: u8) -> Option<usize>
+    {
+	debug_assert!(key>0);
+	let key = unsafe {NonZeroU8::new_unchecked(key)};
+	self.0.read().unwrap().get(&key).copied()
+    }
+    pub fn get_or_insert_with<F>(&self, key: u8, with: F) -> usize
+    where F: FnOnce() -> usize
+    {
+	debug_assert!(key>0);
+	let key = unsafe {NonZeroU8::new_unchecked(key)};
+
+	{
+	    let read = self.0.read().unwrap();
+	    if let Some(&value) = read.get(&key) {
+		return value;
+	    }
+	}
+	// Insert needed
+	{
+	    let value = with();
+	    let mut lock = self.0.write().unwrap();
+	    lock.insert(key, value);
+	    value
+	}
+    }
+}
+
+#[derive(Debug)]
+enum Deffered
+{
+    Known(usize),
+    Yielded(thread::JoinHandle<usize>),
+}
+
+impl Deffered
+{
+    #[inline] pub fn into_value(self) -> usize
+    {
+	match self {
+	    Self::Known(v) => v,
+	    Self::Yielded(v) => v.join().unwrap(),
+	}
+    }
+}
+
+fn rec_part2(map: Arc<Adaptors>, cache: Arc<Cache>, lock: Semaphore<()>, max: u8, f: u8) -> usize
+{
+    if f == max {
+	return 1;
+    }
+    debug_assert!(f < max);
+
+    match iterate_adaptor_chain(&map, None, f).map(|next| {
+	
+	if next == max {
+	    Deffered::Known(1)
+	} else {
+	    match cache.get(next) {
+		Some(value) => Deffered::Known(value),
+		None => {
+		    let map = Arc::clone(&map);
+		    let cache = Arc::clone(&cache);
+		    let lock = lock.clone();
+		    if let Ok(_guard) = lock.try_access() {
+			Deffered::Yielded(thread::spawn(move || {
+			    cache.clone().insert(next, rec_part2(map, cache, lock, max, next))
+			}))
+		    } else {
+			Deffered::Known(cache.clone().insert(next, rec_part2(map, cache, lock, max, next)))
+		    }
+		}
+	    }
+	}
+    }).map(Deffered::into_value)
+	.sum::<usize>() {
+	    #[cold] 0 => panic!("eh"),
+	    x => x,
+	}
+}
+
+fn gensem() -> Semaphore<()>
+{
+    Semaphore::new(num_cpus::get()+1, ())
+}
+
+#[inline] pub fn solve(a: Adaptors, max: u8) -> usize
+{
+    rec_part2(Arc::new(a), Arc::new(Cache::new()), gensem(), max, 0)
+}