I need to convert a number to another value based on a range:

ie:

7 = "A"
106 = "I"

I have a range like this:

from to     return-val
1    17     A
17   35     B
35   38     C
38   56     D
56   72     E
72   88     F
88   98     G
98   104    H
104  115    I
115  120    J
120  123    K
123  129    L
129  infinity   M

The values are fixed and do not change.

I was thinking a lookup table would be required, but is there a way it could be done with a function on an analytics function inside of oracle?