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SubjectRe: [PATCH 1/4] exfat: Simplify exfat_utf8_d_hash() for code points above U+FFFF
On Friday 03 April 2020 02:18:15 Kohada.Tetsuhiro@dc.MitsubishiElectric.co.jp wrote:
> > I guess it was designed for 8bit types, not for long (64bit types) and
> > I'm not sure how effective it is even for 16bit types for which it is
> > already used.
>
> In partial_name_hash (), when 8bit value or 16bit value is specified,
> upper 8-12bits tend to be 0.
>
> > So question is, what should we do for either 21bit number (one Unicode
> > code point = equivalent of UTF-32) or for sequence of 16bit numbers
> > (UTF-16)?
>
> If you want to get an unbiased hash value by specifying an 8 or 16-bit value,

Hello! In exfat we have sequence of 21-bit values (not 8, not 16).

> the hash32() function is a good choice.
> ex1: Prepare by hash32 () function.
> hash = partial_name_hash (hash32 (val16,32), hash);
> ex2: Use the hash32() function directly.
> hash + = hash32 (val16,32);

Did you mean hash_32() function from linux/hash.h?

> > partial_name_hash(unsigned long c, unsigned long prevhash)
> > {
> > return (prevhash + (c << 4) + (c >> 4)) * 11;
> > }
>
> Another way may replace partial_name_hash().
>
> return prevhash + hash32(c,32)
>

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