Step 1: Divide the number into separate digits:
7580 => 7 5 8 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 5 = 2
5 to 8 = 3
8 to 0 = 8
0 to 7 = 7
Step 3: Combine the filters to a string:
2 3 8 7 = 2387