Cleanup poll_event's try_decode hack.

src/ebml.rs

@@ -219,9 +219,45 @@ pub struct Ebml<Source> {
 }
 
 pub trait FromEbml<'a>: Sized {
+    /// Indicates if this tag's contents should be treated as a blob,
+    /// or if the tag header should be reported as an event and with further
+    /// parsing descending into its content.
+    ///
+    /// Unknown-size tags can *only* be parsed if unwrapped, and will error otherwise.
     fn should_unwrap(element_id: u64) -> bool;
+
+    /// Given an element's ID and its binary payload, if any, construct a suitable
+    /// instance of this type to represent the event. The instance may contain
+    /// references into the given buffer.
     fn decode(element_id: u64, bytes: &'a[u8]) -> Result<Self, Error>;
 
+    /// Check if enough space exists in the given buffer for decode_element() to
+    /// be successful; parsing errors will be returned eagerly.
+    fn check_space(bytes: &[u8]) -> Result<Option<usize>, Error> {
+        match decode_tag(bytes) {
+            Ok(None) => Ok(None),
+            Err(err) => Err(err),
+            Ok(Some((element_id, payload_size_tag, tag_size))) => {
+                let should_unwrap = Self::should_unwrap(element_id);
+
+                let payload_size = match (should_unwrap, payload_size_tag) {
+                    (true, _) => 0,
+                    (false, Varint::Unknown) => return Err(Error::UnknownElementLength),
+                    (false, Varint::Value(size)) => size as usize
+                };
+
+                let element_size = tag_size + payload_size;
+                if element_size > bytes.len() {
+                    // need to read more still
+                    Ok(None)
+                } else {
+                    Ok(Some(element_size))
+                }
+            }
+        }
+    }
+
+    /// Attempt to construct an instance of this type from the given byte slice
     fn decode_element(bytes: &'a[u8]) -> Result<Option<(Self, usize)>, Error> {
         match decode_tag(bytes) {
             Ok(None) => Ok(None),

src/webm_stream.rs

@@ -26,40 +26,34 @@ impl<I: AsRef<[u8]>, S: Stream<Item = I>> WebmBuffer<S> {
         }
     }
 
-    pub fn try_decode(&mut self) -> Result<Async<Option<WebmElement>>, ParsingError<S::Error>> {
-        match WebmElement::decode_element(&self.buffer) {
-            Err(err) => {
-                //println!("EBML error: {:?}", err);
-                return Err(ParsingError::EbmlError(err))
-            },
-            Ok(None) => {
-                //println!("Need refill");
-                // need to refill buffer
-                return Ok(Async::NotReady);
-            },
-            Ok(Some((element, element_size))) => {
-                //println!("Parsed element: {:?}", element);
-                self.last_read = element_size;
-                return Ok(Async::Ready(Some(element)))
-            }
-        };
-    }
-
-    pub fn can_decode(&mut self) -> bool {
-        match self.try_decode() {
-            Ok(Async::NotReady) => false,
-            _ => true
-        }
-    }
-
     pub fn poll_event<'a>(&'a mut self) -> Result<Async<Option<WebmElement<'a>>>, ParsingError<S::Error>> {
         // release buffer from previous event
         self.buffer.advance(self.last_read);
         self.last_read = 0;
 
         loop {
-            if self.can_decode() {
-                return self.try_decode()
+            match WebmElement::check_space(&self.buffer) {
+                Err(err) => {
+                    return Err(ParsingError::EbmlError(err))
+                },
+                Ok(None) => {
+                    // need to refill buffer, below
+                },
+                Ok(Some(_)) => {
+                    return match WebmElement::decode_element(&self.buffer) {
+                        Err(err) => {
+                            Err(ParsingError::EbmlError(err))
+                        },
+                        Ok(None) => {
+                            // buffer should have the data already
+                            panic!("Buffer was supposed to have enough data to parse element, somehow did not.")
+                        },
+                        Ok(Some((element, element_size))) => {
+                            self.last_read = element_size;
+                            return Ok(Async::Ready(Some(element)))
+                        }
+                    }
+                }
             }
 
             match self.stream.poll() {