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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.

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