Step 1: Divide the number into separate digits:
67598 => 6 7 5 9 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 7 => 1
7 to 5 => 2
5 to 9 => 4
9 to 8 => 1
8 to 6 => 2
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
2 => N
4 => N
1 => C
2 => N
Step 4: Combine the filters to a string:
C N N C N = CNNCN