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

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Message-ID: <TY1PR01MB15782019FA3094015950830590C70@TY1PR01MB1578.jpnprd01.prod.outlook.com>
Date:   Fri, 3 Apr 2020 02:18:15 +0000
From:   "Kohada.Tetsuhiro@...MitsubishiElectric.co.jp" 
        <Kohada.Tetsuhiro@...MitsubishiElectric.co.jp>
To:     "'pali@...nel.org'" <pali@...nel.org>
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


> 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,
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);

> 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)