Step 1: Divide the number into separate digits:
9133 => 9 1 3 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 1 => 8
1 to 3 => 2
3 to 3 => 0
3 to 9 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
8 => N
2 => N
0 => E
6 => N
Step 4: Combine the filters to a string:
N N E N = NNEN