Step 1: Divide the number into separate digits:
0679 => 0 6 7 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 6 = 6
6 to 7 = 1
7 to 9 = 2
9 to 0 = 9
Step 3: Combine the filters to a string:
6 1 2 9 = 6129