Step 1: Divide the number into separate digits:
87520 => 8 7 5 2 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 7 => 1
7 to 5 => 2
5 to 2 => 3
2 to 0 => 2
0 to 8 => 8
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
2 => N
3 => N
2 => N
8 => N
Step 4: Combine the filters to a string:
C N N N N = CNNNN