Logic to mirror byte value around 128

Question

Hey,

I have a need to mirror a byte's value around the centre of 128. So, example outputs of this function include:

In  0     Out  255
In  255   Out  0
In  128   Out  128
In  127   Out  129
In  30    Out  225
In  225   Out  30

I'm driving myself nuts with this, I'm sure I'll kick myself when I read the answers.

Cheers

Answer 1

There is no obvious rule that matches your example output.

The first two and the last result could be explained by 255-n, which might be a normal interpretation of ‘mirroring’ a byte. But 128->128 would have to be 256-n, and 127->1 is seemingly inexplicable.

Answer 2

The simple answer, which doesn't quite match your table, is 255 - v.

Answer 3

How about this?

In  0     Out  255
In  255   Out  0
In  128   Out  127
In  127   Out  128
In  30    Out  225
In  225   Out  30

It will be 255-n

Answer 4

If you must stay within a byte, use (255-v) + (v==255)?0:1. Otherwise, use 256-v. That mirrors around 128. ~v (bitwise not) may be faster than 255-v if you want to mirror around 127.5.