I am confused when dealing with arithmetic around array indices, majorly around the corner cases.

For ex:

• Finding word in text ( search of needle in haystack). In following, it should be <= instead of <

    public int search(char[] haystack, char[] needle) {
            int hayStackTill = haystack.length - needle.length;
            for(int i = 0; i < hayStackTill; i++) // it should be <= instead of <
    }

• Given the size of array and count from last, Find the index from start

   int index = a.length - countFromLast; // it should be instead a.length - countFromLast + 1

Can anyone help with some practice problems around this ( Also any suggestions are welcome).