c++ - Explanation needed for round_down macro -


in

#define round_down(x, s) ((x) & ~((s)-1))

i don't quite understand part ((x) & ~((s)-1)). thankful explanation.

so, usage of is:

round_down(152, 128);

let's @ happens:

~(128 - 1) == ~(127)            == ~(0x7f)            == 0xffffff80

so, appears masking out lower bits of number.

round_down(152, 128) == (152 & 0xffffff80) == 128 round_down(300, 128) == (300 & 0xffffff80) == 256

also, s must power of 2. macro doesn't make mathematical sense otherwise.


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