Step 1: Divide the number into separate digits:
28609 => 2 8 6 0 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 8 => 6
8 to 6 => 2
6 to 0 => 6
0 to 9 => 9
9 to 2 => 7
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
2 => N
6 => N
9 => C
7 => N
Step 4: Combine the filters to a string:
N N N C N = NNNCN