[FFmpeg-devel] [PATCH v2 2/2] aadec: add chapters and seeking

[Karsten Otto]

read_packet reads content in chunks. Thus seek must be clamped to valid
chunk positions in the file, which in turn are relative to chapter start
positions.

So in read_header, scan for chapter headers once by skipping through the
content. Set stream time_base to bitrate in bytes/s, for easy timestamp to
position conversion.

Then in read_seek, find the chapter containing the seek position, calculate
the nearest chunk position, and reinit the read_seek state accordingly.
---
Update addresses comment by wm4, honoring AVSEEK_FLAG_BACKWARD when seeking.

 libavformat/aadec.c | 80 +++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 77 insertions(+), 3 deletions(-)

diff --git a/libavformat/aadec.c b/libavformat/aadec.c
index 4db71b1939..d0123da264 100644
--- a/libavformat/aadec.c
+++ b/libavformat/aadec.c
@@ -35,6 +35,7 @@
 #define MAX_TOC_ENTRIES 16
 #define MAX_DICTIONARY_ENTRIES 128
 #define TEA_BLOCK_SIZE 8
+#define CHAPTER_HEADER_SIZE 8
 
 typedef struct AADemuxContext {
     AVClass *class;
@@ -46,6 +47,7 @@ typedef struct AADemuxContext {
     struct AVTEA *tea_ctx;
     uint8_t file_key[16];
     int64_t current_chapter_size;
+    int64_t content_start;
     int64_t content_end;
 } AADemuxContext;
 
@@ -70,7 +72,7 @@ static int aa_read_header(AVFormatContext *s)
     uint32_t nkey, nval, toc_size, npairs, header_seed = 0, start;
     char key[128], val[128], codec_name[64] = {0};
     uint8_t output[24], dst[8], src[8];
-    int64_t largest_size = -1, current_size = -1;
+    int64_t largest_size = -1, current_size = -1, chapter_pos;
     struct toc_entry {
         uint32_t offset;
         uint32_t size;
@@ -172,19 +174,23 @@ static int aa_read_header(AVFormatContext *s)
     if (!strcmp(codec_name, "mp332")) {
         st->codecpar->codec_id = AV_CODEC_ID_MP3;
         st->codecpar->sample_rate = 22050;
+        st->time_base = av_make_q(8, 32000);
         st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
-        st->start_time = 0;
     } else if (!strcmp(codec_name, "acelp85")) {
         st->codecpar->codec_id = AV_CODEC_ID_SIPR;
         st->codecpar->block_align = 19;
         st->codecpar->channels = 1;
         st->codecpar->sample_rate = 8500;
+        st->codecpar->bit_rate = 8500;
+        st->time_base = av_make_q(8, 8500);
         st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
     } else if (!strcmp(codec_name, "acelp16")) {
         st->codecpar->codec_id = AV_CODEC_ID_SIPR;
         st->codecpar->block_align = 20;
         st->codecpar->channels = 1;
         st->codecpar->sample_rate = 16000;
+        st->codecpar->bit_rate = 16000;
+        st->time_base = av_make_q(8, 16000);
         st->need_parsing = AVSTREAM_PARSE_FULL_RAW;
     }
 
@@ -198,7 +204,30 @@ static int aa_read_header(AVFormatContext *s)
     }
     start = TOC[largest_idx].offset;
     avio_seek(pb, start, SEEK_SET);
+
+    // extract chapter positions. since all formats have constant bit rate, use it
+    // as time base in bytes/s, for easy stream position <-> timestamp conversion
+    st->start_time = 0;
+    c->content_start = start;
     c->content_end = start + largest_size;
+
+    while ((chapter_pos = avio_tell(pb)) >= 0 && chapter_pos < c->content_end) {
+        int chapter_idx, chapter_size;
+        chapter_idx = s->nb_chapters;
+        chapter_pos -= start + CHAPTER_HEADER_SIZE * chapter_idx;
+        chapter_size = avio_rb32(pb);
+        if (chapter_size == 0) break;
+        avio_skip(pb, 4);
+        avio_skip(pb, chapter_size);
+        if (!avpriv_new_chapter(s, chapter_idx, st->time_base,
+            chapter_pos, chapter_pos + chapter_size, NULL))
+                return AVERROR(ENOMEM);
+    }
+
+    st->duration = largest_size - CHAPTER_HEADER_SIZE * s->nb_chapters;
+
+    ff_update_cur_dts(s, st, 0);
+    avio_seek(pb, start, SEEK_SET);
     c->current_chapter_size = 0;
 
     return 0;
@@ -271,6 +300,50 @@ static int aa_read_packet(AVFormatContext *s, AVPacket *pkt)
     return 0;
 }
 
+static int aa_read_seek(AVFormatContext *s,
+                        int stream_index, int64_t timestamp, int flags)
+{
+    AADemuxContext *c = s->priv_data;
+    AVChapter *ch;
+    int64_t chapter_pos, chapter_start, chapter_size;
+    int chapter_idx = 0;
+
+    // find chapter containing seek timestamp
+    if (timestamp < 0)
+        timestamp = 0;
+
+    while (chapter_idx < s->nb_chapters && timestamp >= s->chapters[chapter_idx]->end) {
+        ++chapter_idx;
+    }
+
+    if (chapter_idx >= s->nb_chapters) {
+        chapter_idx = s->nb_chapters - 1;
+        if (chapter_idx < 0) return -1; // there is no chapter.
+        timestamp = s->chapters[chapter_idx]->end;
+    }
+
+    ch = s->chapters[chapter_idx];
+
+    // sync by clamping timestamp to nearest valid block position in its chapter
+    chapter_size = ch->end - ch->start;
+    chapter_pos = av_rescale_rnd(timestamp - ch->start, 1, c->codec_second_size,
+        (flags & AVSEEK_FLAG_BACKWARD) ? AV_ROUND_DOWN : AV_ROUND_UP)
+        * c->codec_second_size;
+    if (chapter_pos >= chapter_size)
+        chapter_pos = chapter_size;
+    chapter_start = c->content_start + ch->start + CHAPTER_HEADER_SIZE * (1 + chapter_idx);
+
+    // reinit read state
+    avio_seek(s->pb, chapter_start + chapter_pos, SEEK_SET);
+    c->current_codec_second_size = c->codec_second_size;
+    c->current_chapter_size = chapter_size - chapter_pos;
+    c->chapter_idx = 1 + chapter_idx;
+
+    ff_update_cur_dts(s, s->streams[0], ch->start + chapter_pos);
+
+    return 1;
+}
+
 static int aa_probe(AVProbeData *p)
 {
     uint8_t *buf = p->buf;
@@ -316,6 +389,7 @@ AVInputFormat ff_aa_demuxer = {
     .read_probe     = aa_probe,
     .read_header    = aa_read_header,
     .read_packet    = aa_read_packet,
+    .read_seek      = aa_read_seek,
     .read_close     = aa_read_close,
-    .flags          = AVFMT_GENERIC_INDEX,
+    .flags          = AVFMT_NO_BYTE_SEEK | AVFMT_NOGENSEARCH,
 };
-- 
2.14.3 (Apple Git-98)