From bd161c61fa665fb4e91485e2107946f73affcdb8 Mon Sep 17 00:00:00 2001
From: Josh Coalson <jcoalson@users.sourceforce.net>
Date: Wed, 7 Jan 2009 06:54:05 +0000
Subject: [PATCH] Improve decoder's ability to distinguish between a FLAC sync
 code and an MPEG one (SF #2491433
 https://sourceforge.net/tracker2/?func=detail&aid=2491433&group_id=13478&atid=113478

---
 doc/html/changelog.html      |  1 +
 doc/html/format.html         | 17 ++++++++++-------
 src/libFLAC/stream_decoder.c |  4 ++--
 3 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/doc/html/changelog.html b/doc/html/changelog.html
index 9595085..db1229d 100644
--- a/doc/html/changelog.html
+++ b/doc/html/changelog.html
@@ -121,6 +121,7 @@
 				<ul>
 					<li>libFLAC encoder was defaulting to level 0 compression instead of 5 (<a href="https://sourceforge.net/tracker2/?func=detail&aid=1816825&group_id=13478&atid=113478">SF #1816825</a>).</li>
 					<li>Fix bug in bitreader handling of read callback returning a short count (<a href="https://sourceforge.net/tracker2/?func=detail&aid=2490454&group_id=13478&atid=113478">SF #2490454</a>).</li>
+					<li>Improve decoder's ability to distinguish between a FLAC sync code and an MPEG one (<a href="https://sourceforge.net/tracker2/?func=detail&aid=2491433&group_id=13478&atid=113478">SF #2491433</a>).</li>
 				</ul>
 			</li>
 			<li>
diff --git a/doc/html/format.html b/doc/html/format.html
index 98b30b0..a9294db 100644
--- a/doc/html/format.html
+++ b/doc/html/format.html
@@ -1121,7 +1121,7 @@
 				&lt;1&gt;
 			</td>
 			<td>
-				Reserved:<br />
+				Reserved: <a href="#frame_header_notes">[1]</a><br />
 				<ul>
 				<li>
 					<tt>0</tt> : mandatory value
@@ -1137,7 +1137,7 @@
 				&lt;1&gt;
 			</td>
 			<td>
-				Blocking strategy:<br />
+				Blocking strategy: <a href="#frame_header_notes">[2]</a> <a href="#frame_header_notes">[3]</a><br />
 				<ul>
 				<li>
 					<tt>0</tt> : fixed-blocksize stream; frame header encodes the frame number
@@ -1325,9 +1325,9 @@
 			</td>
 			<td>
 				if(variable blocksize)<br />
-				&nbsp;&nbsp;&nbsp;&lt;8-56&gt;:"UTF-8" coded sample number (decoded number is 36 bits)<br />
+				&nbsp;&nbsp;&nbsp;&lt;8-56&gt;:"UTF-8" coded sample number (decoded number is 36 bits) <a href="#frame_header_notes">[4]</a><br />
 				else<br />
-				&nbsp;&nbsp;&nbsp;&lt;8-48&gt;:"UTF-8" coded frame number (decoded number is 31 bits)
+				&nbsp;&nbsp;&nbsp;&lt;8-48&gt;:"UTF-8" coded frame number (decoded number is 31 bits) <a href="#frame_header_notes">[4]</a>
 			</td>
 		</tr>
 		<tr>
@@ -1360,8 +1360,11 @@
 			<td>
 			</td>
 			<td bgcolor="#F4F4CC">
-				<font size="+1">NOTES</font><br />
-				<ul>
+				<a name="frame_header_notes"><font size="+1">NOTES</font></a><br />
+				<ol>
+				<li>
+					This bit must remain reserved for <tt>0</tt> in order for a FLAC frame's initial 15 bits to be distinguishable from the start of an MPEG audio frame (<a href="http://lists.xiph.org/pipermail/flac-dev/2008-December/002607.html">see also</a>).
+				</li>
 				<li>
 					The "blocking strategy" bit must be the same throughout the entire stream.
 				</li>
@@ -1371,7 +1374,7 @@
 				<li>
 					The "UTF-8" coding used for the sample/frame number is the same variable length code used to store compressed UCS-2, extended to handle larger input.
 				</li>
-				</ul>
+				</ol>
 			</td>
 		</tr>
 	</table>
diff --git a/src/libFLAC/stream_decoder.c b/src/libFLAC/stream_decoder.c
index 8756d6f..75f896a 100644
--- a/src/libFLAC/stream_decoder.c
+++ b/src/libFLAC/stream_decoder.c
@@ -1407,7 +1407,7 @@ FLAC__bool find_metadata_(FLAC__StreamDecoder *decoder)
 				decoder->private_->lookahead = (FLAC__byte)x;
 				decoder->private_->cached = true;
 			}
-			else if(x >> 2 == 0x3e) { /* MAGIC NUMBER for the last 6 sync bits */
+			else if(x >> 1 == 0x7c) { /* MAGIC NUMBER for the last 6 sync bits and reserved 7th bit */
 				decoder->private_->header_warmup[1] = (FLAC__byte)x;
 				decoder->protected_->state = FLAC__STREAM_DECODER_READ_FRAME;
 				return true;
@@ -1977,7 +1977,7 @@ FLAC__bool frame_sync_(FLAC__StreamDecoder *decoder)
 				decoder->private_->lookahead = (FLAC__byte)x;
 				decoder->private_->cached = true;
 			}
-			else if(x >> 2 == 0x3e) { /* MAGIC NUMBER for the last 6 sync bits */
+			else if(x >> 1 == 0x7c) { /* MAGIC NUMBER for the last 6 sync bits and reserved 7th bit */
 				decoder->private_->header_warmup[1] = (FLAC__byte)x;
 				decoder->protected_->state = FLAC__STREAM_DECODER_READ_FRAME;
 				return true;
-- 
2.7.4