Step 1: Divide the number into separate digits:
2401 => 2 4 0 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 4 => 2
4 to 0 => 4
0 to 1 => 1
1 to 2 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
4 => N
1 => C
1 => C
Step 4: Combine the filters to a string:
N N C C = NNCC