In the previous chapter, we created a hash table with a function that can work with numerical keys. Now you will implement a hash function that will work with keys of type string.

This particular hash function must calculate the hash value by summing the ASCII values of the characters in the key string. Doing so transforms variable-length input (keys of different lengths) into a fixed-size hash value. This hash value is then modulo with the size of the hash table to determine the index where the key-value pair should be stored.

Your task is to implement this hash function by filling the gaps in the ._hash_function() method of the HashTable class.