matroskadec: Implement support for ALAC

This patch implements support reading ALAC from Matroska files. The
only non-trivial thing about it is that only the ALAC magic cookie is
stored in Matroska's CodecPrivate element but not the "atom size",
"tag" and "tag version" fields that FFMPEG's ALAC decoder
expects. However, those are trivial to re-create.

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: Michael Niedermayer <michaelni@gmx.at>

2 files changed, 13 insertions, 0 deletions

diff --git a/libavformat/matroska.c b/libavformat/matroska.c
index 5e9a6cade3..ea91ed7494 100644
--- a/libavformat/matroska.c
+++ b/libavformat/matroska.c
@@ -24,6 +24,7 @@
 const CodecTags ff_mkv_codec_tags[]={
     {"A_AAC"            , CODEC_ID_AAC},
     {"A_AC3"            , CODEC_ID_AC3},
+    {"A_ALAC"           , CODEC_ID_ALAC},
     {"A_DTS"            , CODEC_ID_DTS},
     {"A_EAC3"           , CODEC_ID_EAC3},
     {"A_FLAC"           , CODEC_ID_FLAC},
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 2c954afa05..0f1808773b 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1580,6 +1580,18 @@ static int matroska_read_header(AVFormatContext *s)
                    && (track->codec_priv.data != NULL)) {
             fourcc = AV_RL32(track->codec_priv.data);
             codec_id = ff_codec_get_id(ff_codec_movvideo_tags, fourcc);
+        } else if (codec_id == 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);
+            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 == CODEC_ID_PCM_S16BE) {
             switch (track->audio.bitdepth) {
             case  8:  codec_id = CODEC_ID_PCM_U8;     break;