Step 1: Divide the number into separate digits:
3698 => 3 6 9 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 6 => 3
6 to 9 => 3
9 to 8 => 1
8 to 3 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
3 => N
3 => N
1 => C
5 => N
Step 4: Combine the filters to a string:
N N C N = NNCN