Consider adding more methods that work on a single string.

[RokIvansek]

A method for finding the longest repeated substring would be nice.
https://en.wikipedia.org/wiki/Longest_repeated_substring_problem

And one for counting all nontrivial repeating substrings.
http://stackoverflow.com/questions/37499968/finding-all-repeated-substrings-in-a-string-and-how-often-they-appear

[tonetto]

#9 implements the longest repeated substring.