The string similarity problem

For two strings A and B (in the ASCII [a-z] range), we define the similarity of the strings to be the length of the longest prefix common to both strings. For example, the similarity of strings "abc" and "abd" is 2, while the similarity of strings "aaa" and "aaab" is 3.

The reader is asked to calculate the sum of similarities of a string S with each of its suffixes. Reference (https://www.hackerrank.com/challenges/string-similarity/problem)