From ca851a3a082f3678c5f0e7e8dc24480a5f483ec6 Mon Sep 17 00:00:00 2001
From: =?utf8?q?Andreas=20=C3=96man?= <andreas@lonelycoder.com>
Date: Tue, 23 Oct 2007 13:35:20 +0000
Subject: [PATCH] add support for reading duration from Xing-tag in mp3 files

Originally committed as revision 10845 to svn://svn.ffmpeg.org/ffmpeg/trunk
---
 libavformat/mp3.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/libavformat/mp3.c b/libavformat/mp3.c
index 6173be225..b2bc89587 100644
--- a/libavformat/mp3.c
+++ b/libavformat/mp3.c
@@ -21,6 +21,7 @@
 #include "avformat.h"
 #include "mpegaudio.h"
 #include "avstring.h"
+#include "mpegaudiodecheader.h"
 
 #define ID3v2_HEADER_SIZE 10
 #define ID3v1_TAG_SIZE 128
@@ -424,12 +425,40 @@ static int mp3_read_probe(AVProbeData *p)
     else                   return 0;
 }
 
+/**
+ * Try to extract a xing tag from the stream and if found, decode it
+ */
+static void mp3_parse_xing(AVFormatContext *s, AVStream *st)
+{
+    uint32_t v, frames, spf;
+    const offset_t offtbl[2][2] = {{32, 17}, {17,9}};
+    MPADecodeContext c;
+
+    ff_mpegaudio_decode_header(&c, get_be32(&s->pb));
+    url_fseek(&s->pb, offtbl[c.lsf == 1][c.nb_channels == 1], SEEK_CUR);
+    v = get_be32(&s->pb);
+    if(c.layer != 3 ||
+       (v != MKBETAG('X', 'i', 'n', 'g') &&
+        v != MKBETAG('I', 'n', 'f', 'o')))
+        return;
+
+    v = get_be32(&s->pb);
+    if(v & 0x1) {
+        frames = get_be32(&s->pb);   /* Total number of frames in file */
+        spf    = c.lsf ? 576 : 1152; /* Samples per frame, layer 3 */
+
+        st->duration = av_rescale_q(frames, (AVRational){spf, c.sample_rate},
+                                    st->time_base);
+    }
+}
+
 static int mp3_read_header(AVFormatContext *s,
                            AVFormatParameters *ap)
 {
     AVStream *st;
     uint8_t buf[ID3v1_TAG_SIZE];
     int len, ret, filesize;
+    offset_t off;
 
     st = av_new_stream(s, 0);
     if (!st)
@@ -469,6 +498,10 @@ static int mp3_read_header(AVFormatContext *s,
         url_fseek(&s->pb, 0, SEEK_SET);
     }
 
+    off = url_ftell(&s->pb);
+    mp3_parse_xing(s, st);
+    url_fseek(&s->pb, off, SEEK_SET);
+
     /* the parameters will be extracted from the compressed bitstream */
     return 0;
 }
-- 
2.11.0