Re: Add support for base64url variant

[Eli Zaretskii <eliz@gnu.org>]

> From: pierre.techoueyres@free.fr (Pierre Téchoueyres)
> Cc: emacs-devel@gnu.org
> Date: Thu, 23 May 2019 19:51:00 +0200
> 
> > I think an optional argument will be fine.
> 
> Except here I've added two, and one that doesn't really mean anything if
> the second isn't set.

Then perhaps switch their order.

> >> - I would like to improve base64-decode* in a way it could detect the 
> >> variant but actually don't know how to do that.
> >
> > Maybe someone else will have an idea.  Is such an algorithmic
> > detection described someplace?
> >
> None I'm aware of.  I was thinking to something like that :
> - define two boolean.  One saying you're on crude base64 another saying
>   you're on base64url variant. 
> - initialize them as false.
> - start decoding.
> - when finding crude base64  chars (/ or +) set the base64 to true,
> - when finding specific url variant chars (- or _) set base64url to true,

Doesn't sound worth it.  Just an argument passed by the caller should
be fine.  The caller will have to know how the text is encoded.

> I would also bring your attention on the part where I dynamically assign
> pointers on specialized arrays for encoding (resp. decoding).

I don't see a problem with that.

> Before my change there were static const, so I suppose compiler could
> have inlined them or at least stored on some cache.  But now ...
> So I'm a little scarried by the possible lost of performance.  If anyone
> has some hint on how I could benchmark this (other than by the naive way
> which could result in my data where all in cache ...)

Emacs has benchmarking facilities, so you could time the code and see
if there's some performance penalty.  If the performance suffers too
much, we could always have a helper function which receives the array
as a parameter.