Step 1: Divide the number into separate digits:
23519 => 2 3 5 1 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 3 = 1
3 to 5 = 2
5 to 1 = 4
1 to 9 = 8
9 to 2 = 7
Step 3: Combine the filters to a string:
1 2 4 8 7 = 12487