Step 1: Divide the number into separate digits:
7800 => 7 8 0 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 8 => 1
8 to 0 => 8
0 to 0 => 0
0 to 7 => 7
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
8 => N
0 => E
7 => N
Step 4: Combine the filters to a string:
C N E N = CNEN