Tilde operation

good question: https://stackoverflow.com/questions/8305199/the-tilde-operator-in-python

besides, an amazing website that I found: Bit-Twiddling-Hacks

In a nutshell, ~x == (-x)-1, which might be useful in list index operation,

array = [0, 1, 2, 3, 4, 5, 6, 7, 8]
         ^     ^           ^     ^
# i      0     2          ~2    ~0