Step 1: Divide the number into separate digits:
6450 => 6 4 5 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 4 => 2
4 to 5 => 1
5 to 0 => 5
0 to 6 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
1 => C
5 => N
6 => N
Step 4: Combine the filters to a string:
N C N N = NCNN