Fixed single character bug

src/hufftree/base.rs

@@ -44,6 +44,14 @@ impl Hufftree {
     pub fn get_character_code(&self, character: char) -> Result<BitVec, &str> {
         match self.root.get_character_code(character) {
             Ok(code) => {
+                if code.is_empty() {
+                    // Single-character alphabet: the root is the only node, so the
+                    // recursive walk returns no bits. Assign a 1-bit code so the
+                    // encoded text is non-empty and decodable.
+                    let mut single = BitVec::new();
+                    single.push(false);
+                    return Ok(single);
+                }
                 let code = code.iter().rev().collect();
                 Ok(code)
             }
@@ -116,6 +124,19 @@ mod test {
         assert_eq!(c_code.to_string(), "00");
     }
 
+    #[test]
+    fn single_character_alphabet_roundtrip() {
+        let mut chars_and_freq: HashMap<char, i32> = HashMap::new();
+        chars_and_freq.insert('a', 5);
+
+        let huff = Hufftree::new(chars_and_freq);
+        let code = huff.get_character_code('a').unwrap();
+        assert_eq!(code.len(), 1);
+
+        let encoded = huff.convert_text("aaaaa".to_string()).unwrap();
+        assert_eq!(encoded.len(), 5);
+    }
+
     #[test]
     fn get_charcter_freq() {
         let text = String::from("aaaaaabb cc");