matroska: implement support for ALAC

Support Matroska native formatting.

On demuxing reconstruct the 36-bytes QuickTime atom that the ALAC
decoder expects by prepending the "atom size", "tag" and
"tag version" fields missing from the Matroska's CodecPrivate
element.

On muxing remove the initial 12 bytes

Sample files are available:
http://www.bunkus.org/videotools/mkvtoolnix/samples/alac/alac-in-matroska.mka
and the CoreAudio file it was created from with today's mkvmerge:
http://www.bunkus.org/videotools/mkvtoolnix/samples/alac/alac-in-matroska-source.caf

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index ff2a6c29dc..69a8c7f06e 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1528,6 +1528,19 @@ static int matroska_read_header(AVFormatContext *s)
                 extradata_size = 5;
             } else
                 extradata_size = 2;
+        } else if (codec_id == AV_CODEC_ID_ALAC && track->codec_priv.size) {
+            /* Only ALAC's magic cookie is stored in Matroska's track headers.
+               Create the "atom size", "tag", and "tag version" fields the
+               decoder expects manually. */
+            extradata_size = 12 + track->codec_priv.size;
+            extradata = av_mallocz(extradata_size + FF_INPUT_BUFFER_PADDING_SIZE);
+            if (extradata == NULL)
+                return AVERROR(ENOMEM);
+            AV_WB32(extradata, extradata_size);
+            memcpy(&extradata[4], "alac", 4);
+            AV_WB32(&extradata[8], 0);
+            memcpy(&extradata[12], track->codec_priv.data,
+                                   track->codec_priv.size);
         } else if (codec_id == AV_CODEC_ID_TTA) {
             extradata_size = 30;
             extradata = av_mallocz(extradata_size);