使用线程计算所有出现的 substring

Question

Supposing that I have t threads, what is the optimal solution for counting all non-overlapping occurrences of a substring T in a string S?

This is a chunk of code that does the normal count, but I'm not sure how to implement it concurrently. What would happen if t is smaller than length of the substring?

public class Substrings {
public int countOccurrences(String S, String T) {
  int count = 0, offset = 0, index;
  while((index = S.indexOf(T, offset)) != -1) {
    offset = index + T.length();
            count++;
  }
  return count;
}

}

Answer 1

I'm not sure why you want to do this as operations like this are pretty fast, unless you have alot of giant Strings. The optimal solution is not something I want to think about, but there is a simple way to do this that's about twice as slow as the optimal. Split the String up into sections and run countOccurrences on each section using a thread. Put all the indexes you found into a Set. Then slide the sections forward half the length of a section and do it again. This second part will find any occurrences that span sections. Of course you could limit this second search by the length of the String on both sides, but this would complicate the code. As an exercise, perhaps you can do this with Boyer-Moore.