Merge pull request #47 from pkuyym/fix-46

Expose edit distance for error_rate.py

Author: Yibing Liu

utils/error_rate.py

@@ -56,6 +56,62 @@ def _levenshtein_distance(ref, hyp):
  return distance[m % 2][n]
 
 
+def word_errors(reference, hypothesis, ignore_case=False, delimiter=' '):
+ """Compute the levenshtein distance between reference sequence and
+ hypothesis sequence in word-level.
+
+ :param reference: The reference sentence.
+ :type reference: basestring
+ :param hypothesis: The hypothesis sentence.
+ :type hypothesis: basestring
+ :param ignore_case: Whether case-sensitive or not.
+ :type ignore_case: bool
+ :param delimiter: Delimiter of input sentences.
+ :type delimiter: char
+ :return: Levenshtein distance and word number of reference sentence.
+ :rtype: list
+ """
+ if ignore_case == True:
+ reference = reference.lower()
+ hypothesis = hypothesis.lower()
+
+ ref_words = filter(None, reference.split(delimiter))
+ hyp_words = filter(None, hypothesis.split(delimiter))
+
+ edit_distance = _levenshtein_distance(ref_words, hyp_words)
+ return float(edit_distance), len(ref_words)
+
+
+def char_errors(reference, hypothesis, ignore_case=False, remove_space=False):
+ """Compute the levenshtein distance between reference sequence and
+ hypothesis sequence in char-level.
+
+ :param reference: The reference sentence.
+ :type reference: basestring
+ :param hypothesis: The hypothesis sentence.
+ :type hypothesis: basestring
+ :param ignore_case: Whether case-sensitive or not.
+ :type ignore_case: bool
+ :param remove_space: Whether remove internal space characters
+ :type remove_space: bool
+ :return: Levenshtein distance and length of reference sentence.
+ :rtype: list
+ """
+ if ignore_case == True:
+ reference = reference.lower()
+ hypothesis = hypothesis.lower()
+
+ join_char = ' '
+ if remove_space == True:
+ join_char = ''
+
+ reference = join_char.join(filter(None, reference.split(' ')))
+ hypothesis = join_char.join(filter(None, hypothesis.split(' ')))
+
+ edit_distance = _levenshtein_distance(reference, hypothesis)
+ return float(edit_distance), len(reference)
+
+
 def wer(reference, hypothesis, ignore_case=False, delimiter=' '):
  """Calculate word error rate (WER). WER compares reference text and
  hypothesis text in word-level. WER is defined as:
@@ -85,20 +141,15 @@ def wer(reference, hypothesis, ignore_case=False, delimiter=' '):
  :type delimiter: char
  :return: Word error rate.
  :rtype: float
- :raises ValueError: If the reference length is zero.
+ :raises ValueError: If word number of reference is zero.
  """
- if ignore_case == True:
- reference = reference.lower()
- hypothesis = hypothesis.lower()
+ edit_distance, ref_len = word_errors(reference, hypothesis, ignore_case,
+ delimiter)
 
- ref_words = filter(None, reference.split(delimiter))
- hyp_words = filter(None, hypothesis.split(delimiter))
-
- if len(ref_words) == 0:
+ if ref_len == 0:
  raise ValueError("Reference's word number should be greater than 0.")
 
- edit_distance = _levenshtein_distance(ref_words, hyp_words)
- wer = float(edit_distance) / len(ref_words)
+ wer = float(edit_distance) / ref_len
  return wer
 
 
@@ -135,20 +186,11 @@ def cer(reference, hypothesis, ignore_case=False, remove_space=False):
  :rtype: float
  :raises ValueError: If the reference length is zero.
  """
- if ignore_case == True:
- reference = reference.lower()
- hypothesis = hypothesis.lower()
+ edit_distance, ref_len = char_errors(reference, hypothesis, ignore_case,
+ remove_space)
 
- join_char = ' '
- if remove_space == True:
- join_char = ''
-
- reference = join_char.join(filter(None, reference.split(' ')))
- hypothesis = join_char.join(filter(None, hypothesis.split(' ')))
-
- if len(reference) == 0:
+ if ref_len == 0:
  raise ValueError("Length of reference should be greater than 0.")
 
- edit_distance = _levenshtein_distance(reference, hypothesis)
- cer = float(edit_distance) / len(reference)
+ cer = float(edit_distance) / ref_len
  return cer