diff --git a/shuffle3rs/Cargo.lock b/shuffle3rs/Cargo.lock
index b8cd917..ff05771 100644
--- a/shuffle3rs/Cargo.lock
+++ b/shuffle3rs/Cargo.lock
@@ -1,5 +1,7 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+version = 3
+
 [[package]]
 name = "cfg-if"
 version = "1.0.0"
@@ -82,6 +84,7 @@ dependencies = [
  "cfg-if",
  "lazy_static",
  "rand",
+ "rand_chacha",
 ]
 
 [[package]]
diff --git a/shuffle3rs/Cargo.toml b/shuffle3rs/Cargo.toml
index 0421c4b..bda77e0 100644
--- a/shuffle3rs/Cargo.toml
+++ b/shuffle3rs/Cargo.toml
@@ -6,6 +6,14 @@ edition = "2018"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[profile.test]
+# Needed for tail call opt, to not cause stack overflow.
+opt-level = 1
+
+[profile.dev]
+# Needed for tail call opt, to not cause stack overflow.
+opt-level = 1
+
 [features]
 default = ["deferred-drop"]
 
@@ -16,3 +24,6 @@ deferred-drop = ["lazy_static"]
 cfg-if = "1.0.0"
 lazy_static = {version = "1.4.0", optional = true}
 rand = "0.8.3"
+
+[dev-dependencies]
+rand_chacha = "0.3.0"
\ No newline at end of file
diff --git a/shuffle3rs/src/ext.rs b/shuffle3rs/src/ext.rs
index 04ebeb6..43204db 100644
--- a/shuffle3rs/src/ext.rs
+++ b/shuffle3rs/src/ext.rs
@@ -32,7 +32,7 @@ impl<T> ShuffleExt<T> for [T]
 	shuffle::shuffle(self, rng)
     }
     #[inline] fn unshuffle<R: Rng + ?Sized>(&mut self, rng: &mut R) {
-	todo!()
+	shuffle::unshuffle(self, rng)
     }
 }
 
@@ -41,7 +41,6 @@ impl<T> ShuffleExt<T> for [T]
 pub struct Lag<I: ?Sized, T>(Duration, I)
 where I: Iterator<Item=T>;
 
-
 impl<I: ?Sized, T>  Lag<I, T>
 where I: Iterator<Item=T>
 {
diff --git a/shuffle3rs/src/main.rs b/shuffle3rs/src/main.rs
index 834bfa5..230f3ac 100644
--- a/shuffle3rs/src/main.rs
+++ b/shuffle3rs/src/main.rs
@@ -13,4 +13,29 @@ mod shuffle;
 
 fn main() {
     println!("Hello, world!");
+
+    #[cfg(test)] {
+	let mut values: Vec<_> = (0..100000i32).collect();
+	let expected = values.clone();
+	
+	let mut rng = {
+	    use rand::prelude::*;
+	    rand_chacha::ChaChaRng::from_seed([0xfa; 32])
+	};
+	let mut nrng = rng.clone();
+	
+	println!("Start (first 10): {:?}", &values[..10]);
+
+	values.shuffle(&mut rng);
+
+	println!("Shuffled (first 10): {:?}", &values[..10]);
+
+	values.unshuffle(&mut nrng);
+
+	println!("Unshuffled (first 10): {:?}", &values[..10]);
+
+	assert_eq!(&values[..], &expected[..]);
+    }
 }
+
+#[cfg(test)] mod test;
diff --git a/shuffle3rs/src/shuffle.rs b/shuffle3rs/src/shuffle.rs
index 8d84275..6ae1b55 100644
--- a/shuffle3rs/src/shuffle.rs
+++ b/shuffle3rs/src/shuffle.rs
@@ -22,6 +22,7 @@ pub fn element_in_mut<'a, T, R: Rng + ?Sized>(slice: &'a mut (impl AsMut<[T]> +
 
 fn shuffle_slice<T, R: Rng + ?Sized>(slice: &mut [T], with: &mut R)
 {
+    //XXX: Without optimisations this recursion causes stack overflow. Cannot work without tail call optimisation
     match slice
     {
 	&mut [] | &mut [_] => {},
@@ -34,9 +35,12 @@ fn shuffle_slice<T, R: Rng + ?Sized>(slice: &mut [T], with: &mut R)
 
 fn unshuffle_slice<T, R: Rng + ?Sized>(slice: &mut [T], with: &mut R)
 {
-    let indecies: Vec<_> = (1..slice.len()).map(|idx| with.gen_range(0..idx)).collect(); 
-
-    todo!();
+    let indecies: Vec<_> = (1..slice.len()).rev().map(|x| with.gen_range(0..x)).collect();
+    
+    for (i, &idx) in (1..slice.len()).zip(indecies.iter().rev())
+    {
+	slice.swap(i, idx);
+    }
 
     drop!(indecies);
 }
@@ -45,3 +49,8 @@ fn unshuffle_slice<T, R: Rng + ?Sized>(slice: &mut [T], with: &mut R)
 {
     shuffle_slice(slice.as_mut(), with)
 }
+
+#[inline(always)] pub fn unshuffle<T, R: Rng + ?Sized>(mut slice: impl AsMut<[T]>, with: &mut R)
+{
+    unshuffle_slice(slice.as_mut(), with)
+}
diff --git a/shuffle3rs/src/test.rs b/shuffle3rs/src/test.rs
new file mode 100644
index 0000000..92f7829
--- /dev/null
+++ b/shuffle3rs/src/test.rs
@@ -0,0 +1,52 @@
+use super::*;
+
+#[test]
+pub fn shuffle_then_back()
+{
+    let mut values: Vec<_> = (0..10i32).collect();
+    let expected = values.clone();
+    
+    let mut rng = {
+	use rand::prelude::*;
+	rand_chacha::ChaChaRng::from_seed([12; 32])
+    };
+    let mut nrng = rng.clone();
+    
+    println!("Start: {:?}", values);
+
+    values.shuffle(&mut rng);
+
+    println!("Shuffled: {:?}", values);
+
+    values.unshuffle(&mut nrng);
+
+    println!("Unshuffled: {:?}", values);
+
+    assert_eq!(&values[..], &expected[..]);
+}
+
+
+#[test]
+pub fn shuffle_then_back_large()
+{
+    let mut values: Vec<_> = (0..10000i32).collect();
+    let expected = values.clone();
+    
+    let mut rng = {
+	use rand::prelude::*;
+	rand_chacha::ChaChaRng::from_seed([0xfa; 32])
+    };
+    let mut nrng = rng.clone();
+    
+    println!("Start (first 10): {:?}", &values[..10]);
+
+    values.shuffle(&mut rng);
+
+    println!("Shuffled (first 10): {:?}", &values[..10]);
+
+    values.unshuffle(&mut nrng);
+
+    println!("Unshuffled (first 10): {:?}", &values[..10]);
+
+    assert_eq!(&values[..], &expected[..]);
+}