Step 1: Divide the number into separate digits:
06591 => 0 6 5 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 6 => 6
6 to 5 => 1
5 to 9 => 4
9 to 1 => 8
1 to 0 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
1 => C
4 => N
8 => N
1 => C
Step 4: Combine the filters to a string:
N C N N C = NCNNC