Re: [boost] Any interest in bitstream class?

Subject: Re: [boost] Any interest in bitstream class?
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2013-06-29 10:48:27

Hi Paul,

Thanks for offering this.

Paul Long wrote:
> bool ParseRtpHeader(const BYTE *buffer, RtpHeader &rtp)
> {
>      ibitstream bin(buffer);
>      static const bitset<2> version(0x2);
>      bitset<4> csrcCount;
>      WORD extensionLength;
>
>      bin >> version >> rtp.padding >> rtp.extension.present
>          >> csrcCount >> rtp.marker >> rtp.payloadType
>          >> rtp.sequenceNumber >> rtp.timestamp >> rtp.ssrcIdentifier
>          >> setrepeat(csrcCount.to_ulong()) >> rtp.csrcIdentifier;
>
>      if (rtp.extension.present) {
>          bin >> rtp.extension.identifier >> extensionLength
>              >> setrepeat(extensionLength * sizeof(DWORD))
>              >> rtp.extension.contents;
>      }
>
>      return bin.good();
> }

Have you measured the performance of your implementation, compared to
something more like this (with the shifts etc. obviously made up here):

   version = (data[0] >> 14) & 0x07f;
   rtp.padding = (data[0] >> 6) & 0x0ff;
   rtp.extension.present = data[0] & 0x020;
   etc. etc.

?

I'm curious to know if you are doing bit-by-bit processing of the fields
with shift amounts known only at run time, compared to extracting 
groups of
contiguous bits simultaneously with shift amounts known at compile time -
and if so, how much that affects performance.

I have previously tried to do some basic MPEG stream processing on an
embedded device and started trying to write a general-purpose facility
like yours, but decided that it was not fast enough.  My conclusion at the
time was that the ideal solution would be to create a "yacc-like" tool that
would parse the bitstream spec (preferably copied straight from the MPEG
documentation) and generate stream decoding code in the style shown.
This was never more than a vague idea of mine.  Perhaps a Boost.Spirit
kind of parser would be another possibility.

Regards,  Phil.