Merge pull request #37 from KillingSpark/skipable_frames

add basic support for skippable frames

src/bin/zstd.rs

@@ -2,8 +2,12 @@ extern crate ruzstd;
 use std::fs::File;
 use std::io::Read;
 use std::io::Seek;
+use std::io::SeekFrom;
 use std::io::Write;
 
+use ruzstd::frame::ReadFrameHeaderError;
+use ruzstd::frame_decoder::FrameDecoderError;
+
 struct StateTracker {
     bytes_used: u64,
     frames_used: usize,
@@ -57,7 +61,18 @@ fn main() {
         let mut result = vec![0; batch_size];
 
         while tracker.file_pos < tracker.file_size {
-            frame_dec.reset(&mut f).unwrap();
+            match frame_dec.reset(&mut f) {
+                Err(FrameDecoderError::ReadFrameHeaderError(ReadFrameHeaderError::SkipFrame(
+                    magic_num,
+                    skip_size,
+                ))) => {
+                    eprintln!("Found a skippable frame with magic number: {magic_num} and size: {skip_size}");
+                    tracker.file_pos += skip_size as u64;
+                    f.seek(SeekFrom::Current(skip_size as i64)).unwrap();
+                    continue;
+                }
+                other => other.unwrap(),
+            }
 
             tracker.frames_used += 1;
 

src/frame.rs

@@ -225,6 +225,8 @@ pub enum ReadFrameHeaderError {
     DictionaryIdReadError(#[source] io::Error),
     #[error("Error while reading frame content size: {0}")]
     FrameContentSizeReadError(#[source] io::Error),
+    #[error("SkippableFrame encountered with MagicNumber {0} and length {1} bytes")]
+    SkipFrame(u32, u32),
 }
 
 use std::io::Read;
@@ -235,6 +237,14 @@ pub fn read_frame_header(mut r: impl Read) -> Result<(Frame, u8), ReadFrameHeade
     r.read_exact(&mut buf).map_err(err::MagicNumberReadError)?;
     let magic_num = u32::from_le_bytes(buf);
 
+    // Skippable frames have a magic number in this interval
+    if (0x184D2A50..=0x184D2A5F).contains(&magic_num) {
+        r.read_exact(&mut buf)
+            .map_err(err::FrameDescriptorReadError)?;
+        let skip_size = u32::from_le_bytes(buf);
+        return Err(ReadFrameHeaderError::SkipFrame(magic_num, skip_size));
+    }
+
     let mut bytes_read = 4;
 
     r.read_exact(&mut buf[0..1])

src/tests/mod.rs

@@ -1,3 +1,32 @@
+#[cfg(test)]
+#[test]
+fn skippable_frame() {
+    use crate::frame;
+
+    let mut content = vec![];
+    content.extend_from_slice(&0x184D2A50u32.to_le_bytes());
+    content.extend_from_slice(&300u32.to_le_bytes());
+    assert_eq!(8, content.len());
+    let err = frame::read_frame_header(content.as_slice());
+    assert!(matches!(
+        err,
+        Err(frame::ReadFrameHeaderError::SkipFrame(0x184D2A50u32, 300))
+    ));
+
+    content.clear();
+    content.extend_from_slice(&0x184D2A5Fu32.to_le_bytes());
+    content.extend_from_slice(&0xFFFFFFFFu32.to_le_bytes());
+    assert_eq!(8, content.len());
+    let err = frame::read_frame_header(content.as_slice());
+    assert!(matches!(
+        err,
+        Err(frame::ReadFrameHeaderError::SkipFrame(
+            0x184D2A5Fu32,
+            0xFFFFFFFF
+        ))
+    ));
+}
+
 #[cfg(test)]
 #[test]
 fn test_frame_header_reading() {