Step 1: Divide the number into separate digits:
90245 => 9 0 2 4 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 0 => 9
0 to 2 => 2
2 to 4 => 2
4 to 5 => 1
5 to 9 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
9 => C
2 => N
2 => N
1 => C
4 => N
Step 4: Combine the filters to a string:
C N N C N = CNNCN