title: How python-Levenshtein.ratio is computed
body: <p>According to the <code>python-Levenshtein.ratio</code> source:</p>
  
  <p><a href="https://github.com/miohtama/python-Levenshtein/blob/master/Levenshtein.c#L722" rel="noreferrer">https://github.com/miohtama/python-Levenshtein/blob/master/Levenshtein.c#L722</a></p>
  
  <p>it's computed as <code>(lensum - ldist) / lensum</code>. This works for </p>
  
  <pre><code># pip install python-Levenshtein
  import Levenshtein
  Levenshtein.distance('ab', 'a') # returns 1
  Levenshtein.ratio('ab', 'a')    # returns 0.666666
  </code></pre>
  
  <p>However, it seems to break with</p>
  
  <pre><code>Levenshtein.distance('ab', 'ac')  # returns 1
  Levenshtein.ratio('ab', 'ac')     # returns 0.5
  </code></pre>
  
  <p>I feel I must be missing something very simple.. but why not <code>0.75</code>?</p>