WillAyd commented on code in PR #326:
URL: https://github.com/apache/arrow-nanoarrow/pull/326#discussion_r1407063468
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src/nanoarrow/buffer_inline.h:
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@@ -223,14 +224,11 @@ static inline int64_t _ArrowBytesForBits(int64_t bits) {
}
static inline void _ArrowBitsUnpackInt8(const uint8_t word, int8_t* out) {
- out[0] = (word & 0x1) != 0;
- out[1] = (word & 0x2) != 0;
- out[2] = (word & 0x4) != 0;
- out[3] = (word & 0x8) != 0;
- out[4] = (word & 0x10) != 0;
- out[5] = (word & 0x20) != 0;
- out[6] = (word & 0x40) != 0;
- out[7] = (word & 0x80) != 0;
+ // see https://stackoverflow.com/a/51750902/621736
+ const uint64_t magic = 0x8040201008040201ULL;
+ const uint64_t mask = 0x8080808080808080ULL;
+ const uint64_t tmp = htobe64((magic * word) & mask) >> 7;
Review Comment:
I don't expect this `htobe64` to be part of the final design. The SO post
actually suggests using a byte-reversed `magic` entry to get what we need, but
that did not work in the special-case of every bit in the `word` being set
(assumedly due to wrap-around). I left a comment on the SO post about that -
hopefully someone out there has a smarter way of handling this
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