As I was thinking about how to implement the
[BITOP][https://redis.io/commands/bitop] command for Redis in Ruby Chapter
10, I started to wonder how to implement the wrap-around logic.
For instance if the type is set to u4, 15 + 1 should result in 0, which isn't too hard, we could do something like:
current_value + increment & 2**number_of_bits
But what about signed integer, with i4, 7 + 1 should result in -8. That's because 7 (0111) + 1 is 1000, which is the representation of -8 in two's complement. So I was wondering how to get to that and ...
~
The not operator!
~7 # => -8, expected result for i4
~-8 # => 7
~3 # => -4 expected result for i3
~-4 # => 3
Looking at 7 as an example, 7 is 0111, if we flip all the bits we end up with 1000, which is -8, TADA, done!