Allow usage without Seek: RevReader approach

Implemented my proposal to resolve RustAudio#12 .

src/reading.rs

@@ -528,19 +528,167 @@ impl BasePacketReader {
 	}
 }
 
+// TODO replace 'a with GAT as soon as they are stabilized
+// (https://github.com/rust-lang/rust/issues/44265)
+
+/// A trait for [Read] implementations that allow reversible reads to a buffer.
+/// After such a read, the reader's position can be reverted as with [Seek],
+/// however only as long as it is still within the bounds of the previous
+/// reversible read call. As such, this trait is to be viewed as a subset of
+/// [Seek]. The lifetime `'a` is that of the reference passed to the object in
+/// [RevRead::rev_read] in order to support depending [RevRead::Buf] on that
+/// lifetime.
+///
+/// Usually, you will not need to implement this yourself. Instead, you can
+/// rely either on the blanket implementation provided for all type that
+/// implement [Read] and [Seek] or use the [RevReader] to wrap a type that only
+/// implements [Read].
+pub trait RevRead<'a>: Read {
+
+	/// The type of buffer returned by this reversible reader.
+	type Buf: AsRef<[u8]>;
+
+	/// Executes a reversible read hat returns a buffer with data read from the
+	/// current position in the reader. The buffer must be larger than 28 bytes
+	/// in size, but it does not have to be completely filled with data from
+	/// the reader. The second return value indicates the actual numbe of bytes
+	/// read from the reader.
+	///
+	/// # Arguments
+	///
+	/// * `amount`: An upper bound on the number of bytes that should be read
+	/// from the reader. This does not have to be an upper bound on the size of
+	/// the returned buffer.
+	///
+	/// # Errors
+	///
+	/// If reading fails according to [Read::read].
+	fn rev_read(&'a mut self, amount: usize) -> io::Result<(Self::Buf, usize)>;
+
+	/// Reverts the previous execution of [RevRead::rev_read] by some amount,
+	/// i.e. the next read will yield previous data of that size again as if
+	/// [Seek::seek] was called with [SeekFrom::Current] of `-amount`. It is a
+	/// prerequisite to this function that the previous operation was
+	/// [RevRead::rev_read] and that that returned at least one byte of data,
+	/// otherwise reverting may or may not work.
+	///
+	/// # Arguments
+	///
+	/// * `amount`: The number of bytes by which to revert the previous read.
+	///
+	/// # Errors
+	///
+	/// If reverting by the given amount is unsupported given the current
+	/// situation.
+	fn revert(&mut self, amount: usize) -> io::Result<()>;
+}
+
+// The array's size is freely choosable, but must be > 27.
+const REV_READ_BUF_SIZE: usize = 1024;
+
+impl<'a, R: Read + Seek> RevRead<'a> for R {
+	type Buf = [u8; REV_READ_BUF_SIZE];
+
+	fn rev_read(&'a mut self, amount: usize) -> io::Result<(Self::Buf, usize)> {
+		let amount = amount.min(REV_READ_BUF_SIZE);
+		let mut buf = [0; REV_READ_BUF_SIZE];
+		let count = self.read(&mut buf[..amount])?;
+
+		Ok((buf, count))
+	}
+
+	fn revert(&mut self, amount: usize) -> io::Result<()> {
+		self.seek(SeekFrom::Current(-(amount as i64)))?;
+
+		Ok(())
+	}
+}
+
+/// A reversible reader that wraps around a type implementing [Read] and offers
+/// a [RevRead] implementation by storing read data in a local buffer. Use this
+/// if you want to use the [PacketReader] with a type that does not implement
+/// [Seek].
+pub struct RevReader<R> {
+	/// The underlying reader.
+	read: R,
+	/// The buffer used to store the result of a [RevRead::rev_read] call.
+	buf: Box<[u8; REV_READ_BUF_SIZE]>,
+	/// The number of bytes in the buffer that are valid.
+	buf_len: usize,
+	/// The index of the next byte that should be read from the buffer.
+	buf_idx: usize
+}
+
+impl<R> RevReader<R> {
+
+	/// Creates a new reversible reader that wraps around the given `read`.
+	/// Note in order for the [RevRead] implementation to hold, `read` must
+	/// implement [Read].
+	pub fn new(read: R) -> RevReader<R> {
+		RevReader {
+			read,
+			buf: Box::new([0; REV_READ_BUF_SIZE]),
+			buf_len: 0,
+			buf_idx: 0
+		}
+	}
+}
+
+impl<R: Read> Read for RevReader<R> {
+	fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+		if self.buf_idx < self.buf_len {
+			// We are still within reverted amount => copy from internal buffer
+			let read_len = (self.buf_len - self.buf_idx).min(buf.len());
+			let read_data = &self.buf[self.buf_idx..(self.buf_idx + read_len)];
+
+			(&mut buf[..read_len]).copy_from_slice(read_data);
+			self.buf_idx += read_len;
+			Ok(read_len)
+		}
+		else {
+			// Invalidate the buffer
+			self.buf_len = 0;
+			self.read.read(buf)
+		}
+	}
+}
+
+impl<'a, R: Read> RevRead<'a> for RevReader<R> {
+	type Buf = &'a [u8];
+
+	fn rev_read(&'a mut self, amount: usize) -> io::Result<(Self::Buf, usize)> {
+		let amount = amount.min(self.buf.len());
+		let buf = &mut self.buf[..amount];
+		let count = self.read.read(buf)?;
+
+		self.buf_len = count;
+		self.buf_idx = count;
+		Ok((&*self.buf, count))
+	}
+
+	fn revert(&mut self, amount: usize) -> io::Result<()> {
+		if amount > self.buf_idx || self.buf_len == 0 {
+			return Err(io::Error::new(ErrorKind::Other, "invalid revert"));
+		}
+
+		self.buf_idx -= amount;
+		Ok(())
+	}
+}
+
 #[derive(Clone, Copy)]
 enum UntilPageHeaderReaderMode {
 	Searching,
 	FoundWithNeeded(u8),
-	SeekNeeded(i32),
+	RevertNeeded(usize),
 	Found,
 }
 
 enum UntilPageHeaderResult {
 	Eof,
 	Found,
 	ReadNeeded,
-	SeekNeeded,
+	RevertNeeded,
 }
 
 struct UntilPageHeaderReader {
@@ -583,27 +731,28 @@ impl UntilPageHeaderReader {
 	/// return Ok(true) if the full header has been read and can be extracted with
 	///
 	/// or return Ok(false) if the
-	pub fn do_read<R :Read>(&mut self, mut rdr :R)
-			-> Result<UntilPageHeaderResult, OggReadError> {
+	pub fn do_read<R>(&mut self, rdr :&mut R)
+		-> Result<UntilPageHeaderResult, OggReadError>
+	where
+		for<'a> R: RevRead<'a>
+	{
 		use self::UntilPageHeaderReaderMode::*;
 		use self::UntilPageHeaderResult as Res;
-		// The array's size is freely choosable, but must be > 27,
-		// and must well fit into an i32 (needs to be stored in SeekNeeded)
-		let mut buf :[u8; 1024] = [0; 1024];
-
-		let rd_len = tri!(rdr.read(if self.read_amount < 27 {
+		let rd_amount = if self.read_amount < 27 {
 			// This is an optimisation for the most likely case:
 			// the next page directly follows the current read position.
 			// Then it would be a waste to read more than the needed amount.
-			&mut buf[0 .. 27 - self.read_amount]
+			27 - self.read_amount
 		} else {
 			match self.mode {
-				Searching => &mut buf,
-				FoundWithNeeded(amount) => &mut buf[0 .. amount as usize],
-				SeekNeeded(_) => return Ok(Res::SeekNeeded),
+				Searching => usize::MAX,
+				FoundWithNeeded(amount) => amount as usize,
+				RevertNeeded(_) => return Ok(Res::RevertNeeded),
 				Found => return Ok(Res::Found),
 			}
-		}));
+		};
+		let (buf, rd_len) = tri!(rdr.rev_read(rd_amount));
+		let buf = buf.as_ref();
 
 		if rd_len == 0 {
 			// Reached EOF. This means we're in one of these cases:
@@ -678,18 +827,21 @@ impl UntilPageHeaderReader {
 			// Seek back so that we are at the position
 			// right after the header.
 
-			self.mode = SeekNeeded(needed as i32 - fnd_buf.len() as i32);
-			return Ok(Res::SeekNeeded);
+			self.mode = RevertNeeded(fnd_buf.len() - needed);
+			return Ok(Res::RevertNeeded);
 		}
 	}
-	pub fn do_seek<S :Seek>(&mut self, mut skr :S)
-			-> Result<UntilPageHeaderResult, OggReadError> {
+	pub fn do_revert<R>(&mut self, rdr :&mut R)
+		-> Result<UntilPageHeaderResult, OggReadError>
+	where
+		for<'a> R: RevRead<'a>
+	{
 		use self::UntilPageHeaderReaderMode::*;
 		use self::UntilPageHeaderResult as Res;
 		match self.mode {
 			Searching | FoundWithNeeded(_) => Ok(Res::ReadNeeded),
-			SeekNeeded(offs) => {
-				tri!(skr.seek(SeekFrom::Current(offs as i64)));
+			RevertNeeded(amount) => {
+				tri!(rdr.revert(amount));
 				self.mode = Found;
 				Ok(Res::Found)
 			},
@@ -715,15 +867,21 @@ consistent when it encounters the `WouldBlock` error kind.
 If you desire async functionality, consider enabling the `async` feature
 and look into the async module.
 */
-pub struct PacketReader<T :io::Read + io::Seek> {
+pub struct PacketReader<T>
+where
+	for<'a> T: RevRead<'a>
+{
 	rdr :T,
 
 	base_pck_rdr :BasePacketReader,
 
 	read_some_pg :bool
 }
 
-impl<T :io::Read + io::Seek> PacketReader<T> {
+impl<T> PacketReader<T>
+where
+	for<'a> T: RevRead<'a>
+{
 	/// Constructs a new `PacketReader` with a given `Read`.
 	pub fn new(rdr :T) -> PacketReader<T> {
 		PacketReader { rdr, base_pck_rdr : BasePacketReader::new(), read_some_pg : false }
@@ -785,7 +943,7 @@ impl<T :io::Read + io::Seek> PacketReader<T> {
 					break
 				},
 				ReadNeeded => tri!(r.do_read(&mut self.rdr)),
-				SeekNeeded => tri!(r.do_seek(&mut self.rdr))
+				RevertNeeded => tri!(r.do_revert(&mut self.rdr))
 			}
 		}
 		Ok(Some(r.into_header()))
@@ -815,6 +973,17 @@ impl<T :io::Read + io::Seek> PacketReader<T> {
 		Ok(Some(tri!(pg_prs.parse_packet_data(packet_data))))
 	}
 
+	/// Resets the internal state by deleting all
+	/// unread packets.
+	pub fn delete_unread_packets(&mut self) {
+		self.base_pck_rdr.update_after_seek();
+	}
+}
+
+impl<T> PacketReader<T>
+where
+	for<'a> T: RevRead<'a> + io::Seek
+{
 	/// Seeks the underlying reader
 	///
 	/// Seeks the reader that this PacketReader bases on by the specified
@@ -1015,11 +1184,6 @@ impl<T :io::Read + io::Seek> PacketReader<T> {
 			}
 		}
 	}
-	/// Resets the internal state by deleting all
-	/// unread packets.
-	pub fn delete_unread_packets(&mut self) {
-		self.base_pck_rdr.update_after_seek();
-	}
 }
 
 // util function