Step 1: Divide the number into separate digits:
55406 => 5 5 4 0 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 5 => 0
5 to 4 => 1
4 to 0 => 4
0 to 6 => 6
6 to 5 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
0 => E
1 => C
4 => N
6 => N
1 => C
Step 4: Combine the filters to a string:
E C N N C = ECNNC