From 2700aafe66d54c32051573863795305b47345b5c Mon Sep 17 00:00:00 2001
From: Jerome Martinez <jerome@mediaarea.net>
Date: Wed, 3 Sep 2025 21:33:50 +0200
Subject: [PATCH 08/18] matroskaenc: reserve_video_track_space option

The number of block additions is not known in advance, so we need to be able to set more reserved space for when we add a lot more block additions.
It is also possible to reduce the reserved space (or even eliminate it) for those who want to keep the smallest header possible.
---
 libavformat/matroskaenc.c | 82 ++++++++++++++++++++++++++++-----------
 1 file changed, 59 insertions(+), 23 deletions(-)

diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index 7b43451a70..01239bea57 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -80,6 +80,8 @@
  * and so has avio_write(). */
 #define MAX_SUPPORTED_EBML_LENGTH FFMIN(MAX_EBML_LENGTH, INT_MAX)
 
+#define MAX_MATROSKA_BLOCK_ADD_ITU_T_T35 1 /* currently only 1 such element is supported */
+
 #define MODE_MATROSKAv2 0x01
 #define MODE_WEBM       0x02
 
@@ -196,6 +198,7 @@ typedef struct mkv_track {
     int64_t         duration;
     int64_t         duration_offset;
     uint64_t        max_blockaddid;
+    int             itu_t_t35_count;
     int64_t         blockadditionmapping_offset;
     int             codecpriv_offset;
     unsigned        codecpriv_size;     ///< size reserved for CodecPrivate excluding header+length field
@@ -249,6 +252,7 @@ typedef struct MatroskaMuxContext {
     int                 wrote_tags;
 
     int                 reserve_cues_space;
+    int                 reserve_video_track_space;
     int                 cluster_size_limit;
     int64_t             cluster_time_limit;
     int                 write_crc;
@@ -1706,19 +1710,12 @@ static void mkv_write_blockadditionmapping(AVFormatContext *s, const MatroskaMux
     const AVDOVIDecoderConfigurationRecord *dovi;
     const AVPacketSideData *sd;
 
-    if (IS_SEEKABLE(s->pb, mkv) && par->codec_type == AVMEDIA_TYPE_VIDEO) {
+    if (IS_SEEKABLE(s->pb, mkv) && par->codec_type == AVMEDIA_TYPE_VIDEO && mkv->reserve_video_track_space > 1) {
         track->blockadditionmapping_offset = avio_tell(pb);
         // We can't know at this point if there will be a block with BlockAdditions, so
         // we either write the default value here, or a void element. Either of them will
         // be overwritten when finishing the track.
-        if (par->codec_type == AVMEDIA_TYPE_VIDEO) {
-            // Similarly, reserve space for an eventual
-            // HDR10+ ITU T.35 metadata BlockAdditionMapping.
-            put_ebml_void(pb, 4 /* MaxBlockAdditionID */
-                            + 3 /* BlockAdditionMapping */
-                            + 4 /* BlockAddIDValue */
-                            + 4 /* BlockAddIDType */);
-        }
+        put_ebml_void(pb, mkv->reserve_video_track_space);
     }
 
     sd = av_packet_side_data_get(par->coded_side_data, par->nb_coded_side_data,
@@ -2796,6 +2793,31 @@ static void mkv_write_blockadditional(EbmlWriter *writer, const uint8_t *buf,
     ebml_writer_close_master(writer);
 }
 
+static int mkv_simulate_blockadditional_header(AVFormatContext *s, int* remaining_video_track_space, uint64_t additional_type, uint64_t additional_id)
+{
+    int size = 0;
+    size += uint_size(MATROSKA_ID_BLKADDIDTYPE) + 1 + uint_size(additional_type);
+    size += uint_size(MATROSKA_ID_BLKADDIDVALUE) + 1 + uint_size(additional_id);
+    size += uint_size(MATROSKA_ID_TRACKBLKADDMAPPING) + uint_size(size);
+    if (size > *remaining_video_track_space || *remaining_video_track_space - size == 1) { /* min element full size is 2 so 1 byte is not something which can be let alone */
+        av_log(s, AV_LOG_WARNING, "Block addition mapping in track header is not written for type %"PRIu64" id %"PRIu64" due to lack of reserved bytes.\n", additional_type, additional_id);
+        return 0;
+    }
+    *remaining_video_track_space -= size;
+    return 1;
+}
+
+static void mkv_write_blockadditional_header(AVFormatContext *s, uint64_t additional_type, uint64_t additional_id)
+{
+    /* when you modify this function, adapt mkv_simulate_blockadditional_header accordingly */
+    MatroskaMuxContext *mkv = s->priv_data;
+    AVIOContext *writer = mkv->track.bc;
+    ebml_master mapping_master = start_ebml_master(writer, MATROSKA_ID_TRACKBLKADDMAPPING, 8);
+    put_ebml_uint(writer, MATROSKA_ID_BLKADDIDTYPE, additional_type);
+    put_ebml_uint(writer, MATROSKA_ID_BLKADDIDVALUE, additional_id);
+    end_ebml_master(writer, mapping_master);
+}
+
 static int mkv_write_block(void *logctx, MatroskaMuxContext *mkv,
                            AVIOContext *pb, const AVCodecParameters *par,
                            mkv_track *track, const AVPacket *pkt,
@@ -2807,7 +2829,7 @@ static int mkv_write_block(void *logctx, MatroskaMuxContext *mkv,
     size_t side_data_size;
     uint64_t additional_id;
     unsigned track_number = track->track_num;
-    EBML_WRITER(12);
+    EBML_WRITER(11 + MAX_MATROSKA_BLOCK_ADD_ITU_T_T35);
     int ret;
 
     mkv->cur_block.track  = track;
@@ -2882,6 +2904,7 @@ static int mkv_write_block(void *logctx, MatroskaMuxContext *mkv,
                                       MATROSKA_BLOCK_ADD_ID_ITU_T_T35);
             track->max_blockaddid = FFMAX(track->max_blockaddid,
                                           MATROSKA_BLOCK_ADD_ID_ITU_T_T35);
+            track->itu_t_t35_count = 1;
         }
     }
 
@@ -3293,24 +3316,36 @@ after_cues:
 
             for (unsigned i = 0; i < s->nb_streams; i++) {
                 const mkv_track *track = &mkv->tracks[i];
+                int remaining_video_track_space = mkv->reserve_video_track_space;
+                int max_block_add_id_count = 0;
+                int max_block_add_id_size = 3 + uint_size(track->max_blockaddid);
+                int block_type_t35_count = 0;
 
-                if (!track->max_blockaddid || !track->blockadditionmapping_offset)
+                if (!track->max_blockaddid)
                     continue;
 
-                // We reserved a single byte to write this value.
-                av_assert0(track->max_blockaddid <= 0xFF);
+                /* check what is possible to write in the reserved space, in priority order */
+                for (int i = 0; i < track->itu_t_t35_count; i++) {
+                    block_type_t35_count += mkv_simulate_blockadditional_header(s, &remaining_video_track_space, MATROSKA_BLOCK_ADD_ID_TYPE_ITU_T_T35, MATROSKA_BLOCK_ADD_ID_TYPE_ITU_T_T35);
+                }                
+                if (remaining_video_track_space >= max_block_add_id_size && remaining_video_track_space != max_block_add_id_size + 1) { /* min element full size is 2 so 1 byte is not something which can be let alone */
+                    max_block_add_id_count++;
+                    remaining_video_track_space -= max_block_add_id_size;
+                }
 
+                /* write what is possible to write in the reserved space */
+                /* when you modify this part, adapt the check part above accordingly */
+                if (!track->blockadditionmapping_offset)
+                    continue;
                 avio_seek(track_bc, track->blockadditionmapping_offset, SEEK_SET);
-
-                put_ebml_uint(track_bc, MATROSKA_ID_TRACKMAXBLKADDID,
-                              track->max_blockaddid);
-                if (track->max_blockaddid == MATROSKA_BLOCK_ADD_ID_ITU_T_T35) {
-                    ebml_master mapping_master = start_ebml_master(track_bc, MATROSKA_ID_TRACKBLKADDMAPPING, 8);
-                    put_ebml_uint(track_bc, MATROSKA_ID_BLKADDIDTYPE,
-                                  MATROSKA_BLOCK_ADD_ID_TYPE_ITU_T_T35);
-                    put_ebml_uint(track_bc, MATROSKA_ID_BLKADDIDVALUE,
-                                  MATROSKA_BLOCK_ADD_ID_ITU_T_T35);
-                    end_ebml_master(track_bc, mapping_master);
+                if (max_block_add_id_count) {
+                    put_ebml_uint(track_bc, MATROSKA_ID_TRACKMAXBLKADDID, track->max_blockaddid);
+                }
+                for (int i = 0; i < block_type_t35_count; i++) {
+                    mkv_write_blockadditional_header(s, MATROSKA_BLOCK_ADD_ID_TYPE_ITU_T_T35, MATROSKA_BLOCK_ADD_ID_ITU_T_T35);
+                }
+                if (remaining_video_track_space > 1) {
+                    put_ebml_void(track_bc, remaining_video_track_space);
                 }
             }
         }
@@ -3545,6 +3580,7 @@ static const AVCodecTag additional_subtitle_tags[] = {
 #define FLAGS AV_OPT_FLAG_ENCODING_PARAM
 static const AVOption options[] = {
     { "reserve_index_space", "reserve a given amount of space (in bytes) at the beginning of the file for the index (cues)", OFFSET(reserve_cues_space), AV_OPT_TYPE_INT,   { .i64 = 0 },   0, INT_MAX,   FLAGS },
+    { "reserve_video_track_space", "reserve a given amount of space (in bytes) at the beginning of the file for the block additions (HDR, timecodes...)", OFFSET(reserve_video_track_space), AV_OPT_TYPE_INT, { .i64 = 15 }, 0, INT_MAX, FLAGS }, /* the number of block additions is not known in advance, so the default value strikes a balance between the size of the reserved header size and the maximum number of block additions that can be registered at the end of writing */
     { "cues_to_front", "move Cues (the index) to the front by shifting data if necessary", OFFSET(move_cues_to_front), AV_OPT_TYPE_BOOL, { .i64 = 0}, 0, 1, FLAGS },
     { "cluster_size_limit",  "store at most the provided amount of bytes in a cluster",                                     OFFSET(cluster_size_limit), AV_OPT_TYPE_INT  , { .i64 = -1 }, -1, INT_MAX,   FLAGS },
     { "cluster_time_limit",  "store at most the provided number of milliseconds in a cluster",                               OFFSET(cluster_time_limit), AV_OPT_TYPE_INT64, { .i64 = -1 }, -1, INT64_MAX, FLAGS },
-- 
2.52.0

