linux-kernel - Re: [PATCH 1/4] exfat: Simplify exfat_utf8_d

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Date:   Fri, 3 Apr 2020 22:40:37 +0200
From:   Pali Rohár <pali@...nel.org>
To:     "Kohada.Tetsuhiro@...MitsubishiElectric.co.jp" 
        <Kohada.Tetsuhiro@...MitsubishiElectric.co.jp>
Cc:     "'linux-fsdevel@...r.kernel.org'" <linux-fsdevel@...r.kernel.org>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        "'namjae.jeon@...sung.com'" <namjae.jeon@...sung.com>,
        "'sj1557.seo@...sung.com'" <sj1557.seo@...sung.com>,
        "'viro@...iv.linux.org.uk'" <viro@...iv.linux.org.uk>
Subject: Re: [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@...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)
>