Step 1: Divide the number into separate digits:
8429 => 8 4 2 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 4 => 4
4 to 2 => 2
2 to 9 => 7
9 to 8 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
4 => N
2 => N
7 => N
1 => C
Step 4: Combine the filters to a string:
N N N C = NNNC