Step 1: Divide the number into separate digits:
61890 => 6 1 8 9 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 1 => 5
1 to 8 => 7
8 to 9 => 1
9 to 0 => 9
0 to 6 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
5 => N
7 => N
1 => C
9 => C
6 => N
Step 4: Combine the filters to a string:
N N C C N = NNCCN