Step 1: Divide the number into separate digits:
64526 => 6 4 5 2 6
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 2 => 3
2 to 6 => 4
6 to 6 => 0
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
1 => C
3 => N
4 => N
0 => E
Step 4: Combine the filters to a string:
N C N N E = NCNNE