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

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Message-ID: <20200407100648.phkvxbmv2kootyt7@pali>
Date:   Tue, 7 Apr 2020 12:06:48 +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 Monday 06 April 2020 09:37:38 Kohada.Tetsuhiro@...MitsubishiElectric.co.jp wrote:
> > > 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).
> 
> hash_32() generates a less-biased hash, even for 21-bit characters.
> 
> The hash of partial_name_hash() for the filename with the following character is ...
>  - 21-bit(surrogate pair): the upper 3-bits of hash tend to be 0.
>  - 16-bit(mostly CJKV): the upper 8-bits of hash tend to be 0.
>  - 8-bit(mostly latin): the upper 16-bits of hash tend to be 0.
> 
> I think the more frequently used latin/CJKV characters are more important
> when considering the hash efficiency of surrogate pair characters.
> 
> The hash of partial_name_hash() for 8/16-bit characters is also biased.
> However, it works well.
> 
> Surrogate pair characters are used less frequently, and the hash of 
> partial_name_hash() has less bias than for 8/16 bit characters.
> 
> So I think there is no problem with your patch.

So partial_name_hash() like I used it in this patch series is enough?