Step 1: Divide the number into separate digits:
2190 => 2 1 9 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 => 1
1 to 9 => 8
9 to 0 => 9
0 to 2 => 2
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
8 => N
9 => C
2 => N
Step 4: Combine the filters to a string:
C N C N = CNCN