Step 1: Divide the number into separate digits:
9100 => 9 1 0 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 1 => 8
1 to 0 => 1
0 to 0 => 0
0 to 9 => 9
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
8 => N
1 => C
0 => E
9 => C
Step 4: Combine the filters to a string:
N C E C = NCEC