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

[David Laight <david.laight.linux@gmail.com>]

On Tue, 4 Nov 2025 11:48:57 +0200
Andy Shevchenko <andriy.shevchenko@intel.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@gmail.com> wrote:  
> > > On Mon, Nov 03, 2025 at 11:24:35AM +0100, Andy Shevchenko wrote:  
> 
> ...
> 
> > > Since I believe many people test and care about W=1 builds, I think we
> > > need to find another way to avoid this warning? Perhaps we could
> > > consider what you suggested:
> > > 
> > > #define BASE64_REV_INIT(val_plus, val_comma, val_minus, val_slash, val_under) { \
> > > 	[ 0 ... '+'-1 ] = -1, \
> > > 	[ '+' ] = val_plus, val_comma, val_minus, -1, val_slash, \
> > > 	[ '0' ] = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, \
> > > 	[ '9'+1 ... 'A'-1 ] = -1, \
> > > 	[ 'A' ] = 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, \
> > > 		  23, 24, 25, 26, 27, 28, 28, 30, 31, 32, 33, 34, 35, \
> > > 	[ 'Z'+1 ... '_'-1 ] = -1, \
> > > 	[ '_' ] = val_under, \
> > > 	[ '_'+1 ... 'a'-1 ] = -1, \
> > > 	[ 'a' ] = 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, \
> > > 		  49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, \
> > > 	[ 'z'+1 ... 255 ] = -1 \
> > > }  
> > 
> > I've a slightly better version:
> > 
> > #define INIT_62_63(ch, ch_62, ch_63) \
> > 	[ ch ] = ch == ch_62 ? 62 : ch == ch_63 ? 63 : -1
> > 
> > #define BASE64_REV_INIT(ch_62, ch_63) { \
> > 	[ 0 ... '0' - 6 ] = -1, \
> > 	INIT_62_63('+', ch_62, ch_63), \
> > 	INIT_62_63(',', ch_62, ch_63), \
> > 	INIT_62_63('-', ch_62, ch_63), \
> > 	INIT_62_63('.', ch_62, ch_63), \
> > 	INIT_62_63('/', ch_62, ch_63), \
> > 	[ '0' ] = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, \
> > 	[ '9' + 1 ... 'A' - 1 ] = -1, \
> > 	[ 'A' ] = 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, \
> > 		  23, 24, 25, 26, 27, 28, 28, 30, 31, 32, 33, 34, 35, \
> > 	[ 'Z' + 1 ... '_' - 1 ] = -1, \
> > 	INIT_62_63('_', ch_62, ch_63), \
> > 	[ '_' + 1 ... 'a' - 1 ] = -1, \
> > 	[ 'a' ] = 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, \
> > 		  49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, \
> > 	[ 'z' + 1 ... 255 ] = -1 \
> > }
> > 
> > that only requires that INIT_62_63() be used for all the characters
> > that are used for 62 and 63 - it can be used for extra ones (eg '.').
> > If some code wants to use different characters; the -1 need replacing
> > with INIT_62_63() but nothing else has to be changed.
> > 
> > I used '0' - 6 (rather than '+' - 1 - or any other expression for 0x2a)
> > to (possibly) make the table obviously correct without referring to the
> > ascii code table.  
> 
> Still it's heavily depends on the values of '+,-./_' as an index that
> makes it not so flexible.

How about this one?
#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.

> 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.

	David

> 
> The above is simply an unreadable mess.
>