On 11/20/2013 01:04 PM, Andrea Mazzoleni wrote:
> Hi Peter,
>
>>> static inline uint64_t d2_64(uint64_t v)
>>> {
>>> uint64_t mask = v & 0x0101010101010101U;
>>> mask = (mask << 8) - mask;
>>
>> (mask << 7) I assume...
> No. It's "(mask << 8) - mask". We want to expand the bit at position 0
> (in each byte) to the full byte, resulting in 0xFF if the bit is at 1,
> and 0x00 if the bit is 0.
>
> (0 << 8) - 0 = 0x00
> (1 << 8) - 1 = 0x100 - 1 = 0xFF
>
Oh, right... it is the same as (v << 1) - (v >> 7) except everything is
shifted over one.
-hpa
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