Fix chain-test1.ogg decoding mismatch

Prior to this commit, the individual
streams that make up chain-test1.ogg
all had no difference to libvorbis
in isolation, but when their concatenation
was played back, a decoding mismatch
occured for every packet of the second
stream.
The issue was that the last packet of
the first stream should have been shorter
than the one lewton outputted.
E.g. if you put code like

if n == 2022 {
	dec_data.truncate(1044);
}

right before the let mut diffs = 0;
in the `cmp_output` function,
you'd get a match because that'd
have leveled out the mismatch.

The vorbis spec says that the absgp
of each page marks the last sample
inside the last packet that ends
it. It also says that if the absgp
of the last page of the entire
stream is before the actually
decoded length of the last packet,
truncation is neccessary.

This truncation was the cause for
the mismatch as vorbis implemented
and we didn't.
This commit implements the truncation
and thus fixes the mismatch for
chain-test1.ogg.

Addresses parts of #26.

dev/cmp/tests/vals.rs

@@ -118,8 +118,7 @@ fn test_xiph_vals_2() {
 	println!();
 
 	cmp_output!("bimS-silence.ogg", 0);
-	// TODO fix these
-	//cmp_output!("chain-test1.ogg", 1);
+	cmp_output!("chain-test1.ogg", 0);
 	cmp_output!("chain-test2.ogg", 0);
 	cmp_output!("chain-test3.ogg", 1);
 	cmp_output!("highrate-test.ogg", 0);

src/inside_ogg.rs

@@ -64,7 +64,7 @@ pub struct OggStreamReader<T: Read + Seek> {
 	pub comment_hdr :CommentHeader,
 	pub setup_hdr :SetupHeader,
 
-	absgp_of_last_read :Option<u64>,
+	cur_absgp :Option<u64>,
 }
 
 impl<T: Read + Seek> OggStreamReader<T> {
@@ -96,7 +96,7 @@ impl<T: Read + Seek> OggStreamReader<T> {
 			comment_hdr,
 			setup_hdr,
 			stream_serial,
-			absgp_of_last_read : None,
+			cur_absgp : None,
 		});
 	}
 	pub fn into_inner(self) -> PacketReader<T> {
@@ -126,7 +126,7 @@ impl<T: Read + Seek> OggStreamReader<T> {
 				self.comment_hdr = comment_hdr;
 				self.setup_hdr = setup_hdr;
 				self.stream_serial = pck.stream_serial();
-				self.absgp_of_last_read = None;
+				self.cur_absgp = None;
 
 				// Now, read the first audio packet to prime the pwr
 				// and discard the packet.
@@ -136,7 +136,7 @@ impl<T: Read + Seek> OggStreamReader<T> {
 				};
 				let _decoded_pck = try!(read_audio_packet(&self.ident_hdr,
 					&self.setup_hdr, &pck.data, &mut self.pwr));
-				self.absgp_of_last_read = Some(pck.absgp_page());
+				self.cur_absgp = Some(pck.absgp_page());
 
 				return Ok(try!(self.rdr.read_packet()));
 			} else {
@@ -161,9 +161,30 @@ impl<T: Read + Seek> OggStreamReader<T> {
 			Some(p) => p,
 			None => return Ok(None),
 		};
-		let decoded_pck = try!(read_audio_packet(&self.ident_hdr,
+		let mut decoded_pck = try!(read_audio_packet(&self.ident_hdr,
 			&self.setup_hdr, &pck.data, &mut self.pwr));
-		self.absgp_of_last_read = Some(pck.absgp_page());
+
+		// If this is the last packet in the logical bitstream,
+		// we need to truncate it so that its ending matches
+		// the absgp of the current page.
+		// This is what the spec mandates and also the behaviour
+		// of libvorbis.
+		if let (Some(absgp), true) = (self.cur_absgp, pck.last_in_stream()) {
+			let target_length = pck.absgp_page().saturating_sub(absgp) as usize;
+			for ch in decoded_pck.iter_mut() {
+				if target_length < ch.len() {
+					ch.truncate(target_length);
+				}
+			}
+		}
+		if pck.last_in_page() {
+			self.cur_absgp = Some(pck.absgp_page());
+		} else if let &mut Some(ref mut absgp) = &mut self.cur_absgp {
+			if let Some(v) = decoded_pck.get(0) {
+				*absgp += v.len() as u64;
+			}
+		}
+
 		return Ok(Some(decoded_pck));
 	}
 	/// Reads and decompresses an audio packet from the stream (interleaved).
@@ -210,7 +231,7 @@ impl<T: Read + Seek> OggStreamReader<T> {
 	/// In the case of ogg/vorbis, the absolute granule position is given
 	/// as number of PCM samples, on a per channel basis.
 	pub fn get_last_absgp(&self) -> Option<u64> {
-		self.absgp_of_last_read
+		self.cur_absgp
 	}
 
 	/// Seeks to the specified absolute granule position, with a page granularity.
@@ -223,7 +244,7 @@ impl<T: Read + Seek> OggStreamReader<T> {
 	pub fn seek_absgp_pg(&mut self, absgp :u64) -> Result<(), VorbisError> {
 		try!(self.rdr.seek_absgp(None, absgp));
 		// Reset the internal state after the seek
-		self.absgp_of_last_read = None;
+		self.cur_absgp = None;
 		self.pwr = PreviousWindowRight::new();
 		Ok(())
 	}