This is a standard trick to find the least significant set bit in a word. Only
works for 2's complement numbers!
-x == ~x+1
For example: x = 00110000b (24), ~x+1 = 11001111+1 = 11010000. Adding them
yields 00010000; thus only the least significant set bit remains set.
Note that func LSB(x uint64) uint64 { return x&-x } works too. In your example
you get an error because in Go literal constants are untyped. It is a pragmatic
decision -- see https://blog.golang.org/constants
> On Jul 23, 2017, at 5:50 AM, Pablo Rozas Larraondo
> <p.rozas.larrao...@gmail.com> wrote:
>
> I have seen Go code using this function to find out the least significant
> byte of unsigned integers:
>
> func LSB(ci uint64) uint64 { return uint64(ci) & -uint64(ci) }
>
> This function works fine but I wonder why, if call the same AND operation, it
> results in an error: "constant -X overflows uint64"
>
> Here is a playground example to illustrate this:
> https://play.golang.org/p/_0EYtlLnmG
>
> Does anyone know what changes when -uint64() is called in a return statement?
> How a negative uint should be interpreted?
>
> Thank you,
> Pablo
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