[FFmpeg-devel] [PATCH] avformat/flvenc: allow to write qualifying metadata as number

[Gyan Doshi]

The FLV format can store metadata as numbers which are used and handled
by many streaming platforms.

Now, metadata values can be sent as AMF Number type if
1) tag key starts with "num_"
2) value is scanned and can be represented as a double.

Written tag will have "num_" prefix removed if written as AMF Number.
---
 doc/muxers.texi      |  2 ++
 libavformat/flvenc.c | 33 ++++++++++++++++++++++++++++-----
 2 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/doc/muxers.texi b/doc/muxers.texi
index ed5341be39..c9bafeec19 100644
--- a/doc/muxers.texi
+++ b/doc/muxers.texi
@@ -509,6 +509,8 @@ ffmpeg -re -i ... -c:v libx264 -c:a aac -f fifo -fifo_format flv -map 0:v -map 0
 @section flv
 
 Adobe Flash Video Format muxer.
+This muxer will store user metadata whose keys start with @code{num_} and whose value is identified as a
+pure value of type double as an AMF Number. All other user metadata is stored as a String.
 
 This muxer accepts the following options:
 
diff --git a/libavformat/flvenc.c b/libavformat/flvenc.c
index 81d9b6100d..25d09ef304 100644
--- a/libavformat/flvenc.c
+++ b/libavformat/flvenc.c
@@ -23,6 +23,7 @@
 #include "libavutil/dict.h"
 #include "libavutil/intfloat.h"
 #include "libavutil/avassert.h"
+#include "libavutil/eval.h"
 #include "libavutil/mathematics.h"
 #include "libavcodec/codec_desc.h"
 #include "libavcodec/mpeg4audio.h"
@@ -275,8 +276,10 @@ static void write_metadata(AVFormatContext *s, unsigned int ts)
     AVIOContext *pb = s->pb;
     FLVContext *flv = s->priv_data;
     int write_duration_filesize = !(flv->flags & FLV_NO_DURATION_FILESIZE);
-    int metadata_count = 0;
+    int metadata_count = 0, is_num;
     int64_t metadata_count_pos;
+    double numvalue;
+    char *key, *tail;
     const AVDictionaryEntry *tag = NULL;
 
     /* write meta_tag */
@@ -378,10 +381,30 @@ static void write_metadata(AVFormatContext *s, unsigned int ts)
             av_log(s, AV_LOG_DEBUG, "Ignoring metadata for %s\n", tag->key);
             continue;
         }
-        put_amf_string(pb, tag->key);
-        avio_w8(pb, AMF_DATA_TYPE_STRING);
-        put_amf_string(pb, tag->value);
-        metadata_count++;
+
+        is_num = !strncmp(tag->key, "num_", 4) && *(tag->key + 4);
+        key = is_num ? (tag->key + 4) : tag->key;
+
+        if (is_num) {
+            numvalue = av_strtod(tag->value, &tail);
+            if (*tail || numvalue == HUGE_VAL) {
+                av_log(s, AV_LOG_ERROR, "Value %s for key %s cannot be stored as a double-typed number. Will be written as a string\n", tag->value, key);
+                key = tag->key;
+                is_num = 0;
+            }
+        }
+
+        av_log(s, AV_LOG_DEBUG, "Writing tag %s with value %s count: %d\n", key, tag->value, metadata_count);
+
+        put_amf_string(pb, key);
+        if (is_num) {
+            put_amf_double(pb, numvalue);
+        } else {
+            avio_w8(pb, AMF_DATA_TYPE_STRING);
+            put_amf_string(pb, tag->value);
+        }
+
+            metadata_count++;
     }
 
     if (write_duration_filesize) {
-- 
2.39.1