[FFmpeg-devel] [PATCH] apedec: add ability to check CRC
Michael Niedermayer
michael at niedermayer.cc
Thu Apr 4 12:30:56 EEST 2019
On Wed, Mar 06, 2019 at 02:47:37PM +0100, Lynne wrote:
> 6 Mar 2019, 11:22 by dev at lynne.ee:
>
> > The CRC flag is only signalled once every few minutes but CRC is still
> > always present so the patch uses the file version instead.
> > CRC on 24-bit files wants non-padded samples so skip such files.
> > Some corrupt samples may have been output before the final check
> > depending on the -max_samples setting.
> >
> v2 attached
> apedec.c | 26 +++++++++++++++++++++++++-
> 1 file changed, 25 insertions(+), 1 deletion(-)
> c0cc550d77927ee0349094a4976f73d0ef671ff6 0001-apedec-add-ability-to-check-CRC-v2.patch
> From 68c25bb026761eacda3c276148e2beb34e8929fd Mon Sep 17 00:00:00 2001
> From: Lynne <dev at lynne.ee>
> Date: Wed, 6 Mar 2019 11:01:01 +0000
> Subject: [PATCH v2] apedec: add ability to check CRC
>
> The CRC flag is only signalled once every few minutes but CRC is still
> always present so the patch uses the file version instead.
> CRC on 24-bit files wants non-padded samples so skip such files.
> Some corrupt samples may have been output before the final check
> depending on the -max_samples setting.
> ---
> libavcodec/apedec.c | 26 +++++++++++++++++++++++++-
> 1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/apedec.c b/libavcodec/apedec.c
> index 15eb416ba4..a20d884606 100644
> --- a/libavcodec/apedec.c
> +++ b/libavcodec/apedec.c
> @@ -24,6 +24,7 @@
>
> #include "libavutil/avassert.h"
> #include "libavutil/channel_layout.h"
> +#include "libavutil/crc.h"
> #include "libavutil/opt.h"
> #include "lossless_audiodsp.h"
> #include "avcodec.h"
> @@ -147,7 +148,8 @@ typedef struct APEContext {
> int fset; ///< which filter set to use (calculated from compression level)
> int flags; ///< global decoder flags
>
> - uint32_t CRC; ///< frame CRC
> + uint32_t CRC; ///< signalled frame CRC
> + uint32_t CRC_state; ///< accumulated CRC
> int frameflags; ///< frame flags
> APEPredictor predictor; ///< predictor used for final reconstruction
>
> @@ -730,6 +732,7 @@ static int init_entropy_decoder(APEContext *ctx)
>
> /* Read the frame flags if they exist */
> ctx->frameflags = 0;
> + ctx->CRC_state = UINT32_MAX;
> if ((ctx->fileversion > 3820) && (ctx->CRC & 0x80000000)) {
> ctx->CRC &= ~0x80000000;
>
> @@ -1548,6 +1551,27 @@ static int ape_decode_frame(AVCodecContext *avctx, void *data,
>
> s->samples -= blockstodecode;
>
> + if ((avctx->err_recognition & AV_EF_CRCCHECK) &&
> + (s->fileversion >= 3900) && (s->bps < 24)) {
what about file versions other than this ? do they have no crc ?
what about othet bps ?
> + uint32_t crc = s->CRC_state;
> + const AVCRC *crc_tab = av_crc_get_table(AV_CRC_32_IEEE_LE);
> + for (i = 0; i < blockstodecode; i++) {
> + for (ch = 0; ch < s->channels; ch++) {
> + uint8_t *smp = frame->data[ch] + (i*(s->bps >> 3));
> + crc = av_crc(crc_tab, crc, smp, s->bps >> 3);
> + }
> + }
> +
> + if (!s->samples && ((~crc >> 1) ^ s->CRC)) {
> + av_log(avctx, AV_LOG_ERROR, "CRC mismatch! Previously decoded "
> + "frames may have been affected as well.\n");
What is the usecase for this ?
the implementation does only check CRC for a subset of files and it could
report them at a time different from where they occur
In practice for a user, that adds some unpredictability to this information
A file may contains CRC errors but no errors would be detected
A file may contain CRC errors but they would be shown too late
A file may contain CRC errors and they are shown at the correct time
thanks
[...]
--
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
Asymptotically faster algorithms should always be preferred if you have
asymptotical amounts of data
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