Step 1: Divide the number into separate digits:
2056 => 2 0 5 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 0 => 2
0 to 5 => 5
5 to 6 => 1
6 to 2 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
5 => N
1 => C
4 => N
Step 4: Combine the filters to a string:
N N C N = NNCN