Step 1: Divide the number into separate digits:
6192 => 6 1 9 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 1 => 5
1 to 9 => 8
9 to 2 => 7
2 to 6 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
5 => N
8 => N
7 => N
4 => N
Step 4: Combine the filters to a string:
N N N N = NNNN