avformat/matroskaenc: Simplify writing Cues

When the Matroska muxer writes the Cues (the index), it groups index
entries with the same timestamp into the same CuePoint to save space.
But given Matroska's variable-length length fields, it either needs
to have an upper bound of the final size of the CuePoint before writing it
or the CuePoint has to be assembled in a different buffer, so that after
having assembled the CuePoint (when the real size is known), the CuePoint's
header can be written and its data copied after it.

The first of these approaches is the currently used one. This entails
finding out the number of entries in a CuePoint before starting the
CuePoint and therefore means that the list is read at least twice.
Furthermore, a worst-case upper-bound for the length of a single entry
was used, so that sometimes bytes are wasted on length fields.

This commit switches to the second approach. This is no longer more
expensive than the current approach if one only resets the dynamic
buffer used to write the CuePoint's content instead of opening a new
buffer for every CuePoint: Writing the trailer of a file with 540.000
CuePoints improved actually from 219054414 decicycles to 2164379394
decicycles (based upon 50 iterations).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

1 files changed, 33 insertions, 41 deletions

diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index 433a2b5579..a2901e6aa5 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -170,12 +170,8 @@ typedef struct MatroskaMuxContext {
  * offset, 4 bytes for target EBML ID */
 #define MAX_SEEKENTRY_SIZE 21
 
-/** per-cuepoint-track - 5 1-byte EBML IDs, 5 1-byte EBML sizes, 3 8-byte uint max
- * and one 1-byte uint for the track number (this assumes MAX_TRACKS to be <= 255) */
-#define MAX_CUETRACKPOS_SIZE 35
-
-/** per-cuepoint - 1 1-byte EBML ID, 1 1-byte EBML size, 8-byte uint max */
-#define MAX_CUEPOINT_CONTENT_SIZE(num_tracks) 10 + MAX_CUETRACKPOS_SIZE * num_tracks
+/** 4 * (1-byte EBML ID, 1-byte EBML size, 8-byte uint max) */
+#define MAX_CUETRACKPOS_SIZE 40
 
 /** Seek preroll value for opus */
 #define OPUS_SEEK_PREROLL 80000000
@@ -506,58 +502,54 @@ static int mkv_add_cuepoint(MatroskaMuxContext *mkv, int stream, int tracknum, i
 static int64_t mkv_write_cues(AVFormatContext *s, mkv_cues *cues, mkv_track *tracks, int num_tracks)
 {
     MatroskaMuxContext *mkv = s->priv_data;
-    AVIOContext *dyn_cp, *pb = s->pb;
+    AVIOContext *dyn_cp, *pb = s->pb, *cuepoint;
     int64_t currentpos;
-    int i, j, ret;
+    int ret;
 
     currentpos = avio_tell(pb);
     ret = start_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_CUES);
     if (ret < 0)
         return ret;
 
-    for (i = 0; i < cues->num_entries; i++) {
-        ebml_master cuepoint, track_positions;
-        mkv_cuepoint *entry = &cues->entries[i];
-        uint64_t pts = entry->pts;
-        int ctp_nb = 0;
-
-        // Calculate the number of entries, so we know the element size
-        for (j = 0; j < num_tracks; j++)
-            tracks[j].has_cue = 0;
-        for (j = 0; j < cues->num_entries - i && entry[j].pts == pts; j++) {
-            int idx = entry[j].stream_idx;
+    ret = avio_open_dyn_buf(&cuepoint);
+    if (ret < 0) {
+        ffio_free_dyn_buf(&dyn_cp);
+        return ret;
+    }
 
-            av_assert0(idx >= 0 && idx < num_tracks);
-            if (tracks[idx].has_cue && s->streams[idx]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE)
-                continue;
-            tracks[idx].has_cue = 1;
-            ctp_nb ++;
-        }
+    for (mkv_cuepoint *entry = cues->entries, *end = entry + cues->num_entries;
+         entry < end;) {
+        uint64_t pts = entry->pts;
+        uint8_t *buf;
+        int size;
 
-        cuepoint = start_ebml_master(dyn_cp, MATROSKA_ID_POINTENTRY, MAX_CUEPOINT_CONTENT_SIZE(ctp_nb));
-        put_ebml_uint(dyn_cp, MATROSKA_ID_CUETIME, pts);
+        put_ebml_uint(cuepoint, MATROSKA_ID_CUETIME, pts);
 
         // put all the entries from different tracks that have the exact same
         // timestamp into the same CuePoint
-        for (j = 0; j < num_tracks; j++)
+        for (int j = 0; j < num_tracks; j++)
             tracks[j].has_cue = 0;
-        for (j = 0; j < cues->num_entries - i && entry[j].pts == pts; j++) {
-            int idx = entry[j].stream_idx;
+        do {
+            ebml_master track_positions;
+            int idx = entry->stream_idx;
 
+            av_assert0(idx >= 0 && idx < num_tracks);
             if (tracks[idx].has_cue && s->streams[idx]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE)
                 continue;
             tracks[idx].has_cue = 1;
-            track_positions = start_ebml_master(dyn_cp, MATROSKA_ID_CUETRACKPOSITION, MAX_CUETRACKPOS_SIZE);
-            put_ebml_uint(dyn_cp, MATROSKA_ID_CUETRACK           , entry[j].tracknum   );
-            put_ebml_uint(dyn_cp, MATROSKA_ID_CUECLUSTERPOSITION , entry[j].cluster_pos);
-            put_ebml_uint(dyn_cp, MATROSKA_ID_CUERELATIVEPOSITION, entry[j].relative_pos);
-            if (entry[j].duration != -1)
-                put_ebml_uint(dyn_cp, MATROSKA_ID_CUEDURATION    , entry[j].duration);
-            end_ebml_master(dyn_cp, track_positions);
-        }
-        i += j - 1;
-        end_ebml_master(dyn_cp, cuepoint);
-    }
+            track_positions = start_ebml_master(cuepoint, MATROSKA_ID_CUETRACKPOSITION, MAX_CUETRACKPOS_SIZE);
+            put_ebml_uint(cuepoint, MATROSKA_ID_CUETRACK           , entry->tracknum   );
+            put_ebml_uint(cuepoint, MATROSKA_ID_CUECLUSTERPOSITION , entry->cluster_pos);
+            put_ebml_uint(cuepoint, MATROSKA_ID_CUERELATIVEPOSITION, entry->relative_pos);
+            if (entry->duration != -1)
+                put_ebml_uint(cuepoint, MATROSKA_ID_CUEDURATION    , entry->duration);
+            end_ebml_master(cuepoint, track_positions);
+        } while (++entry < end && entry->pts == pts);
+        size = avio_get_dyn_buf(cuepoint, &buf);
+        put_ebml_binary(dyn_cp, MATROSKA_ID_POINTENTRY, buf, size);
+        ffio_reset_dyn_buf(cuepoint);
+    }
+    ffio_free_dyn_buf(&cuepoint);
     end_ebml_master_crc32(pb, &dyn_cp, mkv);
 
     return currentpos;