huffman: fix for inputs with one repeated byte

src/huffman.rs

@@ -53,7 +53,12 @@ pub enum Node {
 
 impl Node {
     fn build(counts: &HashMap<u8, u32>) -> Option<Self> {
-        WeightedNode::build(counts).map(WeightedNode::unburden)
+        let branch = match WeightedNode::build(counts).map(WeightedNode::unburden)? {
+            leaf @ Node::Leaf { .. } => Self::join(leaf.clone(), leaf),
+            branch @ Node::Branch { .. } => branch,
+        };
+
+        Some(branch)
     }
 
     /// Write to `buf` the sequence of bits that this tree has assigned `byte`.
@@ -120,6 +125,13 @@ impl Node {
             }
         }
     }
+
+    fn join(path0: Self, path1: Self) -> Self {
+        Self::Branch {
+            path0: Box::new(path0),
+            path1: Box::new(path1),
+        }
+    }
 }
 
 impl fmt::Debug for Node {