[FFmpeg-devel] [RFC] properly document av_crc

Michael Niedermayer michaelni
Sun Aug 19 14:10:06 CEST 2007


On Sun, Aug 19, 2007 at 01:20:45PM +0200, Reimar D?ffinger wrote:
> Hello,
> currently the crc functions in libavutil IMO have a very peculiar API
> that in addition is also undocumented.
> Attached patch documents it as good as I could come up with.
> Though since it is not yet part of the public API I think it might be
> worth changing it, e.g. make av_crc do the necessary bswaps...

no, iam against this
the API is the way it is because its the most efficient
the shift+bswap you propose would be unneeded in nearly all cases
for checking the crc during reading its a matter of crc!=0 the
order doesnt matter
for writing its a matter of little vs. big endian writing functions
and if multiple blocks would be handled your suggestion would have
to do the shift bswap twice thus slowing the code down for small

> Index: libavutil/crc.c
> ===================================================================
> --- libavutil/crc.c	(revision 10141)
> +++ libavutil/crc.c	(working copy)
> @@ -37,6 +37,14 @@
>   * Inits a crc table.
>   * @param ctx must be an array of sizeof(AVCRC)*257 or sizeof(AVCRC)*1024
>   * @param cts_size size of ctx in bytes
> + * @param le if 1, lowest bit represents coefficient for highest exponent
> + *           of corresponding polynomial (both for poly and actual CRC).

> + *           If 0, you must swap the crc parameter and the result of av_crc
> + *           (can be simplified in most cases to e.g. bswap16):
> + *           bswap_32(crc << (32-bits))

its not really "must" its only needed if you need the crc in a non "bswaped"

> @@ -70,6 +78,13 @@
>      return 0;
>  }
> +/**
> + * Calculate the CRC of a block
> + * @param crc CRC of previous blocks if any or initial value for CRC.
> + * @return CRC updated with the data from the given block
> + *
> + * Please read the explanation of the le parameter to av_crc_init

i think theres a @see tag or so for this

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Good people do not need laws to tell them to act responsibly, while bad
people will find a way around the laws. -- Plato
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