Re: JavaScript header compressor/decompressor updated to HPACK-03

Awesome!

The expiry mechanism should always be oldest item first.

-=R
On Sep 13, 2013 2:53 PM, "Fred Akalin" <akalin@google.com> wrote:

> Hey all,
>
> I updated
> http://akalin-chromium.github.io/httpbis-header-compression/compressor_test.html to
> implement the HPACK-03 draft. In particular, I tried to make it a complete
> an implementation as possible, and I added copious comments and references
> to the spec to make it easy to validate and understand.
>
> The only thing I didn't implement is UTF-8 validation for header values.
> Hopefully, the need for that will go away.
>
> Some thoughts:
>
> - There aren't any tests. I wanted to see how correct I can make the
> implementation without them (which will be measured when the compliance
> suite comes out). I'm sure there are bugs.
>
> - I didn't try very hard to make the encoder smart, but I did try to make
> it exercise all the opcodes.
>
> - I found it quite helpful that the encoding context was precisely defined
> (as a header table plus the reference set). However, I ultimately found it
> better to encode the reference set as part of the header table (by having a
> bit per entry) instead of having a separate data structure, since it
> eliminates a bunch of logic to keep the indices in the two in sync. This
> may have been obvious to some people, but not to me. I wonder if it's in
> the scope of the spec to suggest this.
>
> - I also found it helpful to have a 'touch' flag per entry since
> encoding/decoding requires processing of the untouched subset of the
> reference set.
>
> - For encoding I also needed to keep track of the number of touches
> (representing the number of times the entry would be explicitly emitted),
> and I needed to make a distinction between no touches and 0 touches
> (representing an implicit emission). This is to support duplicate headers,
> which was tricky to get right.
>
> - It would be nice to have explicit bounds for encoded integers, string
> lengths, header lengths, etc. I didn't try to make the encoder/decoder
> streaming, since that would complicate the implementation, but it seems
> difficult to guarantee memory bounds without the above explicit bounds.
>
> - It would be nice to clarify the behavior when the max header table size
> is reduced. I just implemented popping from the front until the new bound
> is satisfied.
>
> - I didn't find the need to encode index vs. index + 1 too confusing this
> time around. I feel like making the header table start at 1 would simply
> move the off-by-one bugs someplace else. I don't feel too strongly about
> this, though.
>
> Comments, pull requests, etc. welcome!
>
> -- Fred
>