Step 1: Divide the number into separate digits:
439 => 4 3 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 3 => 1
3 to 9 => 6
9 to 4 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
6 => N
5 => N
Step 4: Combine the filters to a string:
C N N = CNN