Step 1: Divide the number into separate digits:
1858 => 1 8 5 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 8 => 7
8 to 5 => 3
5 to 8 => 3
8 to 1 => 7
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
7 => N
3 => N
3 => N
7 => N
Step 4: Combine the filters to a string:
N N N N = NNNN