So given string geography and substring graph, we'd step it through like this:

1. g matches g, so j = 1
2. e does not match r, so j = 0
3. o does not match g, so j = 0
4. g does match g, so j = 1
5. r does match r, so j = 2
6. a does match a, so j = 3 and so on...

At the same time, we also need to keep track of the actual index that the match starts. The j index will help us get the matching, but we need a idxOfStart variable for just the start.

Putting it all together:

xxxxxxxxxx

let idxOfStart = 0;

let j = 0;

let str = 'digginn';

let subStr = 'inn';

​

for (i = 0; i < str.length; i++) {

  // if match, compare next character of subStr with next of string

  if (str[i] == subStr[j]) {

    j++;

  } else {

    j = 0;

  }

​

  // keep track of where match starts and ends

  if (j == 0) {

    idxOfStart = i;

  } else if (j == subStr.length) {

    console.log(idxOfStart);

    // we know this is the start of match

  }

}