Step 1: Divide the number into separate digits:
58021 => 5 8 0 2 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 8 => 3
8 to 0 => 8
0 to 2 => 2
2 to 1 => 1
1 to 5 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
3 => N
8 => N
2 => N
1 => C
4 => N
Step 4: Combine the filters to a string:
N N N C N = NNNCN