refactor: add harmony calculation to Chord class

pytheory/chords.py

@@ -6,9 +6,19 @@ class Chord:
         l = tuple([tone.full_name for tone in self.tones])
         return f"<Chord tones={l!r}>"
 
-    # @property
-    # def harmony(self):
-    #     pass
+    @property
+    def harmony(self):
+        if len(self.tones) < 2:
+            return 0  # No harmony for a single tone or empty chord
+
+        # Calculate intervals between adjacent tones
+        intervals = [abs(self.tones[i].pitch() - self.tones[i-1].pitch()) for i in range(1, len(self.tones))]
+
+        # Simple harmony calculation: sum of inverse intervals
+        # Smaller intervals contribute more to harmony
+        harmony_value = sum(1 / interval for interval in intervals if interval != 0)
+
+        return harmony_value
 
 class NamedChord:
     def __init__(self, *, name, system):