Step 1: Divide the number into separate digits:
21789 => 2 1 7 8 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 7 = 6
7 to 8 = 1
8 to 9 = 1
9 to 2 = 7
Step 3: Combine the filters to a string:
1 6 1 1 7 = 16117