Step 1: Divide the number into separate digits:
20328 => 2 0 3 2 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 0 => 2
0 to 3 => 3
3 to 2 => 1
2 to 8 => 6
8 to 2 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
3 => N
1 => C
6 => N
6 => N
Step 4: Combine the filters to a string:
N N C N N = NNCNN