Re: Straw Poll: Restore Header Table and Static Table Indices

[Willy Tarreau <[e-mail]>]

On Thu, Oct 16, 2014 at 09:09:55AM +0000, RUELLAN Herve wrote:
> > -----Original Message-----
> > From: Willy Tarreau [mailto:[e-mail]]
> > Sent: mercredi 15 octobre 2014 22:16
> > To: Amos Jeffries
> > Cc: [e-mail]
> > Subject: Re: Straw Poll: Restore Header Table and Static Table Indices
> > 
> > On Wed, Oct 15, 2014 at 06:20:00PM +1300, Amos Jeffries wrote:
> > > -----BEGIN PGP SIGNED MESSAGE-----
> > > Hash: SHA1
> > >
> > > On 15/10/2014 6:11 p.m., Adrian Cole wrote:
> > > >> If an argument can be made that 2 byte encodings are still too
> > > >> large for dynamic headers, then instead of flipping back let's
> > > >> investigate how the 1 byte slots can be shared between static and
> > > >> dynamic.
> > > > FWIW, I'm happy to implement an alternate approach, if one comes
> > > > out. Thanks, Greg.
> > > >
> > >
> > > Alternative approach has already been proposed. That the first bit of
> > > the index is used as a flag to indicate static or dynamic table for
> > > the remaning 7+ bits.
> > >
> > > That not only puts both on an even bias, but expands the range of
> > > values getting 1-byte encodings in either table and removes the need
> > > for the math complexity people are disliking.
> > >
> > > 1 stone, 3 birds.
> > 
> > I totally agree with this except that I don't see where you take that
> > spare bit from, that's what initially led me to rethink the encoding.
> > So if you konw where to find one bit, +1 for me obviously!
> 
> Everywhere where there's an index into the static/dynamic table, reduce the
> index size by 1 bit and use this bit as a flag to find which table is used.

OK that way it's clear. Initially I didn't want to do it that way (when
I tried to adjust the encoding to save more bits) because I believed we
needed to have all the static headers in 1 byte. But assuming the most
common ones have the lowest indexes, it makes sense to do so, as we can
encode the first 32 of them with 1 byte, same for the dynamic ones.

Thus +1 for me.

Willy