Step 1: Divide the number into separate digits:
87903 => 8 7 9 0 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 7 => 1
7 to 9 => 2
9 to 0 => 9
0 to 3 => 3
3 to 8 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
2 => N
9 => C
3 => N
5 => N
Step 4: Combine the filters to a string:
C N C N N = CNCNN