Step 1: Divide the number into separate digits:
65371 => 6 5 3 7 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 5 => 1
5 to 3 => 2
3 to 7 => 4
7 to 1 => 6
1 to 6 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
2 => N
4 => N
6 => N
5 => N
Step 4: Combine the filters to a string:
C N N N N = CNNNN