decoder/mad: properly calculate ID3 size without libid3tag

Without libid3tag, we were trying to skip the ID3 frame (since
0.15.2).  Its length however was not calculated at all, we were just
dropping everything from the current input buffer.  This lead to the
first few seconds of the file being skipped.  This patch attempts to
calculate the ID3v2 frame size with the formula from:

 http://www.id3.org/id3v2.4.0-structure 3.1 and 6.2

NEWS

@@ -1,4 +1,6 @@
 ver 0.15.10 (2010/??/??)
+* decoders:
+  - mad: properly calculate ID3 size without libid3tag
 
 
 ver 0.15.9 (2010/03/21)

src/decoder/mad_plugin.c

@@ -425,7 +425,27 @@ static void mp3_parse_id3(struct mp3_data *data, size_t tagsize,
 	/* This code is enabled when libid3tag is disabled.  Instead
 	   of parsing the ID3 frame, it just skips it. */
 
-	mad_stream_skip(&data->stream, tagsize);
+	size_t count = data->stream.bufend - data->stream.this_frame;
+
+	if (tagsize <= count) {
+		mad_stream_skip(&data->stream, tagsize);
+	} else {
+		mad_stream_skip(&data->stream, count);
+
+		while (count < tagsize) {
+			size_t len = tagsize - count;
+			if (len > sizeof(buffer))
+				len = sizeof(buffer);
+
+			char ignored[1024];
+			len = decoder_read(data->decoder, data->input_stream,
+					   ignored, len);
+			if (len == 0)
+				break;
+			else
+				count += len;
+		}
+	}
 #endif
 }
 
@@ -433,16 +453,16 @@ static void mp3_parse_id3(struct mp3_data *data, size_t tagsize,
 /**
  * This function emulates libid3tag when it is disabled.  Instead of
  * doing a real analyzation of the frame, it just checks whether the
- * frame begins with the string "ID3".  If so, it returns the full
- * length.
+ * frame begins with the string "ID3".  If so, it returns the length
+ * of the ID3 frame.
  */
 static signed long
 id3_tag_query(const void *p0, size_t length)
 {
 	const char *p = p0;
 
-	return length > 3 && memcmp(p, "ID3", 3) == 0
-		? length
+	return length >= 10 && memcmp(p, "ID3", 3) == 0
+		? (p[8] << 7) + p[9] + 10
 		: 0;
 }
 #endif /* !HAVE_ID3TAG */