[FFmpeg-devel] [PATCH] avformat/hls:use EXT-X-START instead of live_start_index if it's in playlist

[Steven Liu]

少宇李 <wolfleekay at gmail.com> 于2022年6月24日周五 11:58写道：
>>
>>
>> Hi, Steven
>
>
> I got it.
>
> User could use "live_start_index" option even if #EXT-X-START in the playlist.
>
> I changed the priority about #EXT-X-START/live_start_index,
>
> this is new bellow:
>
> ---
>  libavformat/hls.c | 69 +++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 64 insertions(+), 5 deletions(-)
>
> diff --git a/libavformat/hls.c b/libavformat/hls.c
> index b736f093a9..309471efce 100644
> --- a/libavformat/hls.c
> +++ b/libavformat/hls.c
> @@ -120,6 +120,8 @@ struct playlist {
>      enum PlaylistType type;
>      int64_t target_duration;
>      int64_t start_seq_no;
> +    int time_offset_flag;
> +    int64_t start_time_offset;
>      int n_segments;
>      struct segment **segments;
>      int needed;
> @@ -741,6 +743,7 @@ static int parse_playlist(HLSContext *c, const char *url,
>      struct segment **prev_segments = NULL;
>      int prev_n_segments = 0;
>      int64_t prev_start_seq_no = -1;
> +    const char *p;
>
>      if (is_http && !in && c->http_persistent && c->playlist_pb) {
>          in = c->playlist_pb;
> @@ -889,6 +892,18 @@ static int parse_playlist(HLSContext *c, const char *url,
>                  cur_init_section->key = NULL;
>              }
>
> +        } else if (av_strstart(line, "#EXT-X-START:", &ptr)) {
> +            ret = ensure_playlist(c, &pls, url);
> +            if (ret < 0) {
> +                goto fail;
> +            }
> +            if (av_strstart(ptr, "TIME-OFFSET=", &p)) {
> +                float offset = strtof(p, NULL);
> +                pls->start_time_offset = offset * AV_TIME_BASE;
> +                pls->time_offset_flag = 1;
> +            } else {
> +                goto fail;
> +            }
>          } else if (av_strstart(line, "#EXT-X-ENDLIST", &ptr)) {
>              if (pls)
>                  pls->finished = 1;
> @@ -1721,10 +1736,54 @@ static int64_t select_cur_seq_no(HLSContext *c, struct playlist *pls)
>
>          /* If this is a live stream, start live_start_index segments from the
>           * start or end */
> -        if (c->live_start_index < 0)
> -            return pls->start_seq_no + FFMAX(pls->n_segments + c->live_start_index, 0);
> -        else
> -            return pls->start_seq_no + FFMIN(c->live_start_index, pls->n_segments - 1);
> +        if (c->live_start_index != INT_MIN) {
Maybe you should not change current logic.
You can just add  about process EXT-X-START sence,.
For example:
compute the start_seq_no and save the value to a temp varible and
don't return immediately, check the third scence which get EXT-X-START
tag from playlist.
process the temp varible after start_seq_no compute from
live_start_index or EXT-X-START.

> +            if (c->live_start_index < 0)
> +                return pls->start_seq_no + FFMAX(pls->n_segments +
> +                                                c->live_start_index, 0);
> +            else
> +                return pls->start_seq_no + FFMIN(c->live_start_index,
> +                                                pls->n_segments - 1);
> +        } else if (c->live_start_index == INT_MIN && !pls->time_offset_flag) {
> +            return pls->start_seq_no + FFMAX(pls->n_segments - 3, 0);
> +        } else {
> +            /* If playlist indicate a TIME-OFFSET and user not use live_start_index,
> +             * need to recalculate seq_no */
> +            int i;
> +            int64_t start_timestamp;
> +            int64_t playlist_duration = 0;
> +            int64_t cur_timestamp = c->cur_timestamp == AV_NOPTS_VALUE ? 0 :
> +                                    c->cur_timestamp;
> +
> +            for (i = 0; i < pls->n_segments; i++) {
> +                playlist_duration += pls->segments[i]->duration;
> +            }
> +            /* If the absolute value of TIME-OFFSET exceeds
> +             * the duration of the playlist, it indicates either the end of the
> +             * playlist (if positive) or the beginning of the playlist (if
> +             * negative). */
> +            if (pls->start_time_offset >=0 &&
> +                pls->start_time_offset > playlist_duration)
> +                start_timestamp = cur_timestamp + playlist_duration;
> +            else if (pls->start_time_offset >= 0 &&
> +                        pls->start_time_offset <= playlist_duration)
> +                start_timestamp = cur_timestamp + pls->start_time_offset;
> +            else if (pls->start_time_offset < 0 &&
> +                        pls->start_time_offset < -playlist_duration)
> +                start_timestamp = cur_timestamp;
> +            else if (pls->start_time_offset < 0 &&
> +                        pls->start_time_offset > -playlist_duration)
> +                start_timestamp = cur_timestamp + playlist_duration +
> +                                    pls->start_time_offset;
> +            else
> +                start_timestamp = cur_timestamp;
> +
> +            find_timestamp_in_playlist(c, pls, start_timestamp, &seq_no, NULL);
> +            av_log(c, AV_LOG_DEBUG, "start_timestamp: %" PRId64
> +                                    "cur_timestamp:%" PRId64
> +                                    "cur_seq_no:%" PRId64 "\n",
> +                                    start_timestamp, cur_timestamp, seq_no);
> +            return seq_no;
> +        }
>      }
>
>      /* Otherwise just start on the first segment. */
> @@ -2475,7 +2534,7 @@ static int hls_probe(const AVProbeData *p)
>  #define FLAGS AV_OPT_FLAG_DECODING_PARAM
>  static const AVOption hls_options[] = {
>      {"live_start_index", "segment index to start live streams at (negative values are from the end)",
> -        OFFSET(live_start_index), AV_OPT_TYPE_INT, {.i64 = -3}, INT_MIN, INT_MAX, FLAGS},
> +        OFFSET(live_start_index), AV_OPT_TYPE_INT, {.i64 = INT_MIN}, INT_MIN, INT_MAX, FLAGS},
This is unnecessary， because the default -3 is ok and be using by
users from many years ago.

>      {"allowed_extensions", "List of file extensions that hls is allowed to access",
>          OFFSET(allowed_extensions), AV_OPT_TYPE_STRING,
>          {.str = "3gp,aac,avi,ac3,eac3,flac,mkv,m3u8,m4a,m4s,m4v,mpg,mov,mp2,mp3,mp4,mpeg,mpegts,ogg,ogv,oga,ts,vob,wav"},
> --
> 2.24.1 (Apple Git-126)
>
>
> Thanks Steven