Fixed bug in phone error rate

phone_distance.py

@@ -82,6 +82,33 @@ Examples:
 
 """
 
+def phone_error_rate(prediction:str, reference: str, distance_computer:panphon.distance.Distance):
+    """Computes phone error rates. This is similar to the Distance.phoneme_error_rate function, but 
+    is more efficient and fixes some bugs related to normalization in the original function.
+
+    Args:
+        distance_computer (panphon.distance.Distance): computes edit distance and resolves characters to phones
+
+    Returns:
+        float:  the phone error rate
+    """
+    if reference: # Can only be computed when the length of the reference greater than 0 
+        pred_phones = distance_computer.fm.ipa_segs(prediction)
+        ref_phones = distance_computer.fm.ipa_segs(reference)
+
+        phone_edits = distance_computer.min_edit_distance(
+            lambda x: 1, # deletion cost
+            lambda x: 1, # insertion cost
+            lambda x, y: 0 if x == y else 1, # substitution cost,
+            [[]], 
+            pred_phones, 
+            ref_phones
+        )
+
+        return phone_edits / len(ref_phones)
+    else: 
+        return 0.0
+        
 
 @evaluate.utils.file_utils.add_start_docstrings(_DESCRIPTION, _KWARGS_DESCRIPTION)
 class PhoneDistance(evaluate.Metric):
@@ -128,7 +155,7 @@ class PhoneDistance(evaluate.Metric):
             else:
                 hd = distance_computer.hamming_feature_edit_distance(p, r)
             hamming_distances.append(hd)
-            per = distance_computer.phoneme_error_rate(p, r)
+            per = phone_error_rate(p, r, distance_computer)
             phone_error_rates.append(per)
             fer = distance_computer.feature_error_rate(p, r)
             feature_error_rates.append(fer)