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mp3: skip initial xing/info frame [bug #239]

If a stream starts with a xing/info frame, this should be skipped.
Otherwise a frame's worth of silence will be inserted at the start
of the output.

--- a/src/mp3.c

+++ b/src/mp3.c

@@ -363,6 +363,24 @@

     return rc;

 }

+static sox_bool sox_mp3_vbrtag(sox_format_t *ft)

+{

+    priv_t *p = ft->priv;

+    struct mad_bitptr *anc = &p->Stream.anc_ptr;

+

+    if (p->Frame.header.layer != MAD_LAYER_III)

+        return sox_false;

+

+    if (p->Stream.anc_bitlen < 32)

+        return sox_false;

+

+    if (!memcmp(anc->byte, "Xing", 4) ||

+        !memcmp(anc->byte, "Info", 4))

+        return sox_true;

+

+    return sox_false;

+}

+

 static int startread(sox_format_t * ft)

 {

   priv_t *p = (priv_t *) ft->priv;

@@ -457,17 +475,19 @@

           return SOX_EOF;

   }

-  p->FrameCount=1;

-

-  p->mad_timer_add(&p->Timer,p->Frame.header.duration);

-  p->mad_synth_frame(&p->Synth,&p->Frame);

   ft->signal.precision = MP3_MAD_PRECISION;

-  ft->signal.rate=p->Synth.pcm.samplerate;

+  ft->signal.rate=p->Frame.header.samplerate;

   if (ignore_length)

     ft->signal.length = SOX_UNSPEC;

   else {

     ft->signal.length *= ft->signal.channels;  /* Keep separate from line above! */

   }

+

+  if (!sox_mp3_vbrtag(ft))

+      p->Stream.next_frame = p->Stream.this_frame;

+

+  p->mad_frame_init(&p->Frame);

+  sox_mp3_input(ft);

   p->cursamp = 0;