Step 1: Divide the number into separate digits:
0845 => 0 8 4 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 8 = 8
8 to 4 = 4
4 to 5 = 1
5 to 0 = 5
Step 3: Combine the filters to a string:
8 4 1 5 = 8415