The insight here is that, for unsigned values, rounding down to a multiple of N is cheaper than rounding up to a multiple of N.
In addition to simpler arithmetic, rounding down always succeeds - 0 is a multiple of all N - while rounding up may fail if a larger multiple does not exist. So you need to check in the round-up case.
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The insight here is that, for unsigned values, rounding down to a multiple of N is cheaper than rounding up to a multiple of N.
In addition to simpler arithmetic, rounding down always succeeds - 0 is a multiple of all N - while rounding up may fail if a larger multiple does not exist. So you need to check in the round-up case.