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

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Date:   Wed, 18 Mar 2020 10:32:51 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Namjae Jeon <namjae.jeon@...sung.com>,
        Sungjong Seo <sj1557.seo@...sung.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] exfat: Simplify exfat_utf8_d_hash() for code points
 above U+FFFF

On Wednesday 18 March 2020 00:09:25 Al Viro wrote:
> On Tue, Mar 17, 2020 at 11:25:52PM +0100, Pali Rohár wrote:
> > Function partial_name_hash() takes long type value into which can be stored
> > one Unicode code point. Therefore conversion from UTF-32 to UTF-16 is not
> > needed.
> 
> Hmm...  You might want to update the comment in stringhash.h...

Well, initially I have not looked at hashing functions deeply. Used
hashing function in stringhash.h is defined as:

static inline unsigned long
partial_name_hash(unsigned long c, unsigned long prevhash)
{
	return (prevhash + (c << 4) + (c >> 4)) * 11;
}

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.

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)?

Any opinion?