Step 1: Divide the number into separate digits:
9556 => 9 5 5 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 5 => 4
5 to 5 => 0
5 to 6 => 1
6 to 9 => 3
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
4 => N
0 => E
1 => C
3 => N
Step 4: Combine the filters to a string:
N E C N = NECN