linux-kernel - Re: [PATCH v4 0/6] lib/base64: add generic encoder/decoder, migrate users

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Message-ID: <20251105143820.11558ca8@pumpkin>
Date: Wed, 5 Nov 2025 14:38:20 +0000
From: David Laight <david.laight.linux@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Kuan-Wei Chiu <visitorckw@...il.com>, Guan-Chun Wu
 <409411716@....tku.edu.tw>, Andrew Morton <akpm@...ux-foundation.org>,
 ebiggers@...nel.org, tytso@....edu, jaegeuk@...nel.org, xiubli@...hat.com,
 idryomov@...il.com, kbusch@...nel.org, axboe@...nel.dk, hch@....de,
 sagi@...mberg.me, home7438072@...il.com, linux-nvme@...ts.infradead.org,
 linux-fscrypt@...r.kernel.org, ceph-devel@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/6] lib/base64: add generic encoder/decoder, migrate
 users

On Wed, 5 Nov 2025 16:13:45 +0200
Andy Shevchenko <andriy.shevchenko@...el.com> wrote:

> On Wed, Nov 05, 2025 at 09:48:27AM +0000, David Laight wrote:
> > On Tue, 4 Nov 2025 11:48:57 +0200
> > Andy Shevchenko <andriy.shevchenko@...el.com> wrote:  
> > > On Tue, Nov 04, 2025 at 09:03:26AM +0000, David Laight wrote:  
> > > > On Mon, 3 Nov 2025 19:07:24 +0800
> > > > Kuan-Wei Chiu <visitorckw@...il.com> wrote:    
> > > > > On Mon, Nov 03, 2025 at 11:24:35AM +0100, Andy Shevchenko wrote:    
> 
...
> > How about this one?  
> 
> Better than previous one(s) but quite cryptic to understand. Will need a
> comment explaining the logic behind, if we go this way.

My first version (of this version) had all three character ranges in the define:
so:
#define INIT_1(v, ch_62, ch_63) \
	[ v ] = (v) >= '0' && (v) <= '9' ? (v) - '0' \
		: (v) >= 'A' && (v) <= 'Z' ? (v) - 'A' + 10 \
		: (v) >= 'a' && (v) <= 'z' ? (v) - 'a' + 36 \
		: (v) == ch_62 ? 62 : (v) == ch_63 ? 63 : -1
Perhaps less cryptic - even if the .i line will be rather longer.
It could be replicated for all 256 bytes, but I think the range
initialisers are reasonable for the non-printable ranges.

I did wonder if the encode and decode lookup tables count be interleaved
and both initialisers generated from the same #define.
But I can't think of a way of generating 'x' and "X" from a #define parameter.
(I don't think "X"[0] is constant enough...)

	David

> 
> > #define INIT_1(v, ch_lo, ch_hi, off, ch_62, ch_63) \
> > 	[ v ] = ((v) >= ch_lo && (v) <= ch_hi) ? (v) - ch_lo + off \
> > 		: (v) == ch_62 ? 62 : (v) == ch_63 ? 63 : -1
> > #define INIT_2(v, ...) INIT_1(v, __VA_ARGS__), INIT_1((v) + 1, __VA_ARGS__)
> > #define INIT_4(v, ...) INIT_2(v, __VA_ARGS__), INIT_2((v) + 2, __VA_ARGS__)
> > #define INIT_8(v, ...) INIT_4(v, __VA_ARGS__), INIT_4((v) + 4, __VA_ARGS__)
> > #define INIT_16(v, ...) INIT_8(v, __VA_ARGS__), INIT_8((v) + 8, __VA_ARGS__)
> > #define INIT_32(v, ...) INIT_16(v, __VA_ARGS__), INIT_16((v) + 16, __VA_ARGS__)
> > 
> > #define BASE64_REV_INIT(ch_62, ch_63) { \
> > 	[ 0 ... 0x1f ] = -1, \
> > 	INIT_32(0x20, '0', '9', 0, ch_62, ch_63), \
> > 	INIT_32(0x40, 'A', 'Z', 10, ch_62, ch_63), \
> > 	INIT_32(0x60, 'a', 'z', 26, ch_62, ch_63), \
> > 	[ 0x80 ... 0xff ] = -1 }
> > 
> > which gets the pre-processor to do all the work.
> > ch_62 and ch_63 can be any printable characters.
> > 
> > Note that the #define names are all in a .c file - so don't need any
> > kind of namespace protection.  
> 
> > They can also all be #undef after the initialiser.  
> 
> Yes, that's too.
> 
> > > Moreover this table is basically a dup of the strings in the first array.
> > > Which already makes an unnecessary duplication.  
> > 
> > That is what the self tests are for.
> >   
> > > That's why I prefer to
> > > see a script (one source of data) to generate the header or something like
> > > this to have the tables and strings robust against typos.  
> > 
> > We have to differ on that one.
> > Especially in cases (like this) where generating that data is reasonably trivial.
> >   
> > > The above is simply an unreadable mess.  
>