Step 1: Divide the number into separate digits:
5645 => 5 6 4 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 6 = 1
6 to 4 = 2
4 to 5 = 1
5 to 5 = 0
Step 3: Combine the filters to a string:
1 2 1 0 = 1210