Step 1: Divide the number into separate digits:
8179 => 8 1 7 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 1 = 7
1 to 7 = 6
7 to 9 = 2
9 to 8 = 1
Step 3: Combine the filters to a string:
7 6 2 1 = 7621