[FFmpeg-devel] [PATCH] avformat/utils: Change compute_chapters_end() from O(n²) to O(n log n)

[Michael Niedermayer]

Fixes: Timeout (49sec -> 9sec)
Fixes: 27427/clusterfuzz-testcase-minimized-ffmpeg_dem_FFMETADATA_fuzzer-5140589838073856

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
---
 libavformat/utils.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/libavformat/utils.c b/libavformat/utils.c
index 503e583ad0..9fac3fc2aa 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -3191,15 +3191,58 @@ enum AVCodecID av_codec_get_id(const AVCodecTag *const *tags, unsigned int tag)
     return AV_CODEC_ID_NONE;
 }
 
+static int chapter_start_cmp(const void *p1, const void *p2)
+{
+    AVChapter *ch1 = *(AVChapter**)p1;
+    AVChapter *ch2 = *(AVChapter**)p2;
+    int delta = av_compare_ts(ch1->start, ch1->time_base, ch2->start, ch2->time_base);
+    if (delta)
+        return delta;
+    return (ch1 > ch2) - (ch1 < ch2);
+}
+
 static void compute_chapters_end(AVFormatContext *s)
 {
     unsigned int i, j;
     int64_t max_time = 0;
+    int computations = 0;
 
     if (s->duration > 0 && s->start_time < INT64_MAX - s->duration)
         max_time = s->duration +
                        ((s->start_time == AV_NOPTS_VALUE) ? 0 : s->start_time);
 
+    for (i = 0; i < s->nb_chapters; i++)
+        if (s->chapters[i]->end == AV_NOPTS_VALUE)
+            computations ++;
+
+    if (computations > 5) {
+        AVChapter **timetable = av_malloc(s->nb_chapters * sizeof(*timetable));
+        if (timetable) {
+            for (i = 0; i < s->nb_chapters; i++)
+                timetable[i] = s->chapters[i];
+            qsort(timetable, s->nb_chapters, sizeof(*timetable), chapter_start_cmp);
+
+            for (i = 0; i < s->nb_chapters; i++)
+                if (timetable[i]->end == AV_NOPTS_VALUE) {
+                    AVChapter *ch = timetable[i];
+                    int64_t end = max_time ? av_rescale_q(max_time, AV_TIME_BASE_Q,
+                                                        ch->time_base)
+                                        : INT64_MAX;
+
+                    if (i + 1 < s->nb_chapters) {
+                        AVChapter *ch1     = timetable[i + 1];
+                        int64_t next_start = av_rescale_q(ch1->start, ch1->time_base,
+                                                        ch->time_base);
+                        if (next_start > ch->start && next_start < end)
+                            end = next_start;
+                    }
+                    ch->end = (end == INT64_MAX || end < ch->start) ? ch->start : end;
+                }
+            av_free(timetable);
+            return;
+        }
+    }
+
     for (i = 0; i < s->nb_chapters; i++)
         if (s->chapters[i]->end == AV_NOPTS_VALUE) {
             AVChapter *ch = s->chapters[i];
-- 
2.17.1