Step 1: Divide the number into separate digits:
3958 => 3 9 5 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 => 6
9 to 5 => 4
5 to 8 => 3
8 to 3 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
4 => N
3 => N
5 => N
Step 4: Combine the filters to a string:
N N N N = NNNN