mp3enc: Fix Xing tag identification string for CBR files

Fixes the Xing tag identification string to be "Info" for MP3 files with
constant bitrate. The previous "Xing" caused some decoders to recognize the
file as VBR.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mp3enc.c

@@ -94,6 +94,8 @@ typedef struct MP3Context {
     uint32_t seen;
     uint32_t pos;
     uint64_t bag[VBR_NUM_BAGS];
+    int initial_bitrate;
+    int has_variable_bitrate;
 
     /* index of the audio stream */
     int audio_stream_idx;
@@ -238,6 +240,16 @@ static void mp3_fix_xing(AVFormatContext *s)
     int i;
 
     avio_flush(s->pb);
+
+    /* replace "Xing" identification string with "Info" for CBR files. */
+    if (!mp3->has_variable_bitrate) {
+        int64_t tag_offset = mp3->frames_offset
+            - 4   // frames/size/toc flags
+            - 4;  // xing tag
+        avio_seek(s->pb, tag_offset, SEEK_SET);
+        avio_wb32(s->pb, MKBETAG('I', 'n', 'f', 'o'));
+    }
+
     avio_seek(s->pb, mp3->frames_offset, SEEK_SET);
     avio_wb32(s->pb, mp3->frames);
     avio_wb32(s->pb, mp3->size);
@@ -260,12 +272,21 @@ static int mp3_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
         return ff_raw_write_packet(s, pkt);
     else {
         MP3Context  *mp3 = s->priv_data;
-#ifdef FILTER_VBR_HEADERS
         MPADecodeHeader c;
+#ifdef FILTER_VBR_HEADERS
         int base;
+#endif
+
+        avpriv_mpegaudio_decode_header(&c, AV_RB32(pkt->data));
 
-        ff_mpegaudio_decode_header(&c, AV_RB32(pkt->data));
+        if (!mp3->initial_bitrate)
+            mp3->initial_bitrate = c.bit_rate;
+        if (!mp3->has_variable_bitrate) {
+            if ((c.bit_rate == 0) || (mp3->initial_bitrate != c.bit_rate))
+                mp3->has_variable_bitrate = 1;
+        }
 
+#ifdef FILTER_VBR_HEADERS
         /* filter out XING and INFO headers. */
         base = 4 + xing_offtbl[c.lsf == 1][c.nb_channels == 1];