This calculates the similarity between two texts in percentage. It is an implementation as described in Programming Classics: Implementing the World's Best Algorithms by Ian Oliver). Note that this implementation does not use a stack as in Oliver's pseudo code, but recursive calls which may or may not speed up the whole process. Note also that the complexity of this algorithm is O(N**3) where N is the length of the longest string.
For more details: https://en.wikipedia.org/wiki/Approximate_string_matching