Step 1: Divide the number into separate digits:
21729 => 2 1 7 2 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 => 1
1 to 7 => 6
7 to 2 => 5
2 to 9 => 7
9 to 2 => 7
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
6 => N
5 => N
7 => N
7 => N
Step 4: Combine the filters to a string:
C N N N N = CNNNN