Step 1: Divide the number into separate digits:
07548 => 0 7 5 4 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 7 => 7
7 to 5 => 2
5 to 4 => 1
4 to 8 => 4
8 to 0 => 8
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
7 => N
2 => N
1 => C
4 => N
8 => N
Step 4: Combine the filters to a string:
N N C N N = NNCNN