Step 1: Divide the number into separate digits:
2520 => 2 5 2 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 5 => 3
5 to 2 => 3
2 to 0 => 2
0 to 2 => 2
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
3 => N
3 => N
2 => N
2 => N
Step 4: Combine the filters to a string:
N N N N = NNNN