Step 1: Divide the number into separate digits:
7901 => 7 9 0 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 9 => 2
9 to 0 => 9
0 to 1 => 1
1 to 7 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
9 => C
1 => C
6 => N
Step 4: Combine the filters to a string:
N C C N = NCCN