that will take a whole lot of memory that Look-up-Table stuff
A lookup table like the one we are discussing here does not need be full, I have, in some such cases, used a table such as

0, 3
7,2
14,1
19,0
27,-1
33,-2

The first column in the table is scanned and when the first value above the raw value is found, the value from the second column is added.

Erik