First commit

src/raw_chunk_reader.rs

@@ -0,0 +1,73 @@
+use std::cmp::min;
+
+use std::io::SeekFrom;
+use std::io::SeekFrom::{Start, Current, End};
+use std::io::{Seek,Read,Error,ErrorKind};
+
+// I'm not sure this hasn't already been written somewhere in
+// std but I'm just doing this here as an exercise.
+#[derive(Debug)]
+pub struct RawChunkReader<'a, R: Read + Seek> {
+    reader: &'a mut R,
+    start: u64,
+    length: u64,
+    position: u64
+}
+
+impl<'a,R: Read + Seek> RawChunkReader<'a, R> {
+    pub fn new(reader: &'a mut R, start: u64, length: u64) -> Self {
+        return Self {
+            reader: reader, 
+            start: start, 
+            length: length, 
+            position: 0
+        }
+    }
+
+    pub fn length(&self) -> u64 {
+        self.length
+    } 
+}
+
+impl<'a, R:Read + Seek> Read for RawChunkReader<'_, R> {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize, Error> { 
+        if self.position >= self.length {
+            Err(Error::new(ErrorKind::UnexpectedEof, "RawChunkReader encountered end-of-file"))
+        } else {
+            self.reader.seek(Start(self.start + self.position))?;
+            let to_read = min(self.length - self.position, buf.len() as u64);
+            self.reader.take(to_read).read(buf)?;
+            self.position += to_read;
+            Ok(to_read as usize)
+        }
+    }
+}
+
+impl<'a, R:Read + Seek> Seek for RawChunkReader<'_, R> {
+    fn seek(&mut self, seek: SeekFrom) -> Result<u64, std::io::Error> { 
+        match seek {
+            Start(s) => {
+                self.position = s;
+                Ok(self.position)
+            },
+            Current(s) => {
+                let new_position = s + self.position as i64;
+                if new_position < 0 {
+                    Err( Error::new(ErrorKind::Other, "Attempted seek before beginning of chunk") )
+                } else {
+                    self.position = new_position as u64;
+                    Ok(self.position)
+                }
+            },
+            End(s) => {
+                let new_position = s + self.length as i64;
+                if new_position < 0 {
+                    Err( Error::new(ErrorKind::Other, "Attempted seek before beginning of chunk") )
+                } else {
+                    self.position = new_position as u64;
+                    Ok(self.position)
+                }
+            }
+        }
+    }
+}