Step 1: Divide the number into separate digits:
65517 => 6 5 5 1 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 5 => 1
5 to 5 => 0
5 to 1 => 4
1 to 7 => 6
7 to 6 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
0 => E
4 => N
6 => N
1 => C
Step 4: Combine the filters to a string:
C E N N C = CENNC