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

[wolfleekay@gmail.com]

From: Li Kai <wolfleekay@gmail.com>

Refer to https://datatracker.ietf.org/doc/html/rfc8216#section-4.3.5.2

Signed-off-by: Li Kai <wolfleekay@gmail.com>
---
 libavformat/hls.c | 68 ++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 62 insertions(+), 6 deletions(-)

diff --git a/libavformat/hls.c b/libavformat/hls.c
index 8204f55df3..96923c3eac 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;
@@ -1719,12 +1734,53 @@ static int64_t select_cur_seq_no(HLSContext *c, struct playlist *pls)
              * require us to download a segment to inspect its timestamps. */
             return c->cur_seq_no;
 
-        /* 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 (!pls->time_offset_flag) {
+            /* 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);
+        } else {
+            /* If playlist indicate a TIME-OFFSET, 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. */
-- 
2.25.1

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