diff --git a/src/compressor.rs b/src/compressor.rs
index 07d7dec..b9cdf5e 100644
--- a/src/compressor.rs
+++ b/src/compressor.rs
@@ -36,6 +36,7 @@ use string_cache::DefaultAtom as Atom;
 use super::{collapse_state_maps, StateGroupEntry};
 
 /// Holds information about a particular level.
+#[derive(Debug)]
 struct Level {
     /// The maximum size this level is allowed to be
     max_length: usize,
@@ -67,7 +68,7 @@ impl Level {
             // If we're referencing the previous head then increment our chain
             // length estimate
             if !self.has_space() {
-                panic!("Tried to add to a already full level");
+                panic!("Tried to add to an already full level");
             }
 
             self.current_chain_length += 1;
@@ -248,6 +249,89 @@ impl<'a> Compressor<'a> {
     }
 }
 
+#[cfg(test)]
+mod level_tests {
+    use crate::compressor::Level;
+    #[test]
+    fn new_produces_empty_level() {
+        let l = Level::new(15);
+        assert_eq!(l.max_length, 15);
+        assert_eq!(l.current_chain_length, 0);
+        assert_eq!(l.current, None);
+    }
+
+    #[test]
+    fn update_adds_to_non_full_level() {
+        let mut l = Level::new(10);
+        l.update(7, true);
+        assert_eq!(l.max_length, 10);
+        assert_eq!(l.current_chain_length, 1);
+        assert_eq!(l.current, Some(7));
+    }
+
+    #[test]
+    #[should_panic(expected = "Tried to add to an already full level")]
+    fn update_panics_if_adding_and_too_full() {
+        let mut l = Level::new(5);
+        l.update(1, true);
+        l.update(2, true);
+        l.update(3, true);
+        l.update(4, true);
+        l.update(5, true);
+        l.update(6, true);
+    }
+
+    #[test]
+    fn update_resets_level_correctly() {
+        let mut l = Level::new(5);
+        l.update(1, true);
+        l.update(2, true);
+        l.update(3, true);
+        l.update(4, true);
+        l.update(5, true);
+        l.update(6, false);
+        assert_eq!(l.max_length, 5);
+        assert_eq!(l.current_chain_length, 1);
+        assert_eq!(l.current, Some(6));
+    }
+
+    #[test]
+    fn get_current_returns_current() {
+        let mut l = Level::new(5);
+        assert_eq!(l.get_current(), None);
+        l.update(23, true);
+        assert_eq!(l.get_current(), Some(23));
+    }
+
+    #[test]
+    fn has_space_returns_true_if_empty() {
+        let l = Level::new(15);
+        assert_eq!(l.has_space(), true);
+    }
+
+    #[test]
+    fn has_space_returns_true_if_part_full() {
+        let mut l = Level::new(15);
+        l.update(12, true);
+        l.update(234, true);
+        l.update(1, true);
+        l.update(143, true);
+        l.update(15, true);
+        assert_eq!(l.has_space(), true);
+    }
+
+    #[test]
+    fn has_space_returns_false_if_full() {
+        let mut l = Level::new(5);
+        l.update(1, true);
+        l.update(2, true);
+        l.update(3, true);
+        l.update(4, true);
+        l.update(5, true);
+        assert_eq!(l.has_space(), false);
+    }
+}
+
 #[test]
 fn test_new_map() {
     let mut initial: BTreeMap<i64, StateGroupEntry> = BTreeMap::new();