From 1e45fb3facf5cfc8c67828b37b0bbaa994c2ab03 Mon Sep 17 00:00:00 2001
From: Li Kai <wolfleekay@gmail.com>
Date: Thu, 23 Jun 2022 00:55:38 +0800
Subject: Re: [FFmpeg-devel] [PATCH] avformat/hls:use EXT-X-START instead of live_start_index if it's in playlist

Signed-off-by: Li Kai <wolfleekay@gmail.com>
---
 libavformat/hls.c | 55 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 53 insertions(+), 2 deletions(-)

diff --git a/libavformat/hls.c b/libavformat/hls.c
index 8204f55df3..bfb20f6492 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;
@@ -1722,9 +1737,45 @@ 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);
+            seq_no = 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);
+            seq_no = pls->start_seq_no + FFMIN(c->live_start_index,
+                                            pls->n_segments - 1);
+
+        /* If #EXT-X-START in playlist, need to recalculate */
+        if (pls->time_offset_flag) {
+            int64_t start_timestamp;
+            int64_t playlist_duration = 0;
+            int64_t cur_timestamp = c->cur_timestamp == AV_NOPTS_VALUE ? 0 :
+                                    c->cur_timestamp;
+
+            for (int 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);
+        }
+        return seq_no;
     }
 
     /* Otherwise just start on the first segment. */
-- 
2.25.1

Thanks a lot!