Function str_index_of_f

pub fn str_index_of_f(s1: &str, s2: &str, s3: usize) -> i64

Finds the zero-based index of the nth occurrence of a substring s2 within the string s1, starting the search at position s3.

The function iteratively searches for the substring s2 in s1, updating the starting position with each found occurrence while ensuring that it does not exceed the length of s1. If the search exceeds the bounds of s1 or if s2 is not found, the function returns -1. Otherwise, it returns the index of the nth occurrence of s2, offset by 1 due to incrementing result.