Step 1: Divide the number into separate digits:
42 => 4 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 2 => 2
2 to 4 => 2
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
2 => N
Step 4: Combine the filters to a string:
N N = NN