Bitwise not

[luna.judah]

What

[cymplecy]

Snap converts the input into a 32bit signed integer

So 0 gets converted to binary 0b000000000000000000000000 (can also be represented by hexadecimal 0x00000000)

Then each bit is inverted to produce 0b11111111111111111111111111111111 or 0xFFFFFF

This is how -1 is represented using 32bits and that's why Snap! returns -1

Have a google for 32bit signed numbers to find out more about this and to learn about the two's complement encoding system