Step 1: Divide the number into separate digits:
1942 => 1 9 4 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 9 = 8
9 to 4 = 5
4 to 2 = 2
2 to 1 = 1
Step 3: Combine the filters to a string:
8 5 2 1 = 8521