Step 1: Divide the number into separate digits:
21864 => 2 1 8 6 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 8 = 7
8 to 6 = 2
6 to 4 = 2
4 to 2 = 2
Step 3: Combine the filters to a string:
1 7 2 2 2 = 17222