rewrite fs to init in `new`

src/fs.rs

@@ -1,30 +1,61 @@
 use error::Error;
-use buffer::Buffer;
+use buffer::{Buffer, BufferSlice};
 use sys::superblock::Superblock;
 
+struct Struct<T> {
+    pub inner: T,
+    pub offset: usize,
+}
+
+impl<T> From<(T, usize)> for Struct<T> {
+    #[inline]
+    fn from((inner, offset): (T, usize)) -> Struct<T> {
+        Struct { inner, offset }
+    }
+}
+
 /// Safe wrapper for raw sys structs
 pub struct Ext2<B: Buffer<u8>> {
     buffer: B,
-    superblock: Option<(Superblock, usize)>,
+    superblock: Struct<Superblock>,
 }
 
 impl<B: Buffer<u8>> Ext2<B>
 where
     Error: From<B::Error>,
 {
-    pub fn new(buffer: B) -> Ext2<B> {
-        Ext2 {
-            buffer,
-            superblock: None,
-        }
+    pub fn new(buffer: B) -> Result<Ext2<B>, Error> {
+        let superblock = Superblock::find(&buffer)?.into();
+        Ok(Ext2 { buffer, superblock })
     }
 
-    pub fn init(&mut self) -> Result<(), Error> {
-        let superblock = Superblock::find(&self.buffer);
-        match superblock {
-            Ok(sb) => Ok(self.superblock = Some(sb)),
-            Err(err) => Err(err),
-        }
+    pub fn update(&mut self) -> Result<(), Error> {
+        let slice = BufferSlice::from_cast(
+            &self.superblock.inner,
+            self.superblock.offset,
+        );
+        let commit = slice.commit();
+        self.buffer.commit(commit).map_err(|err| Error::from(err))
+    }
+
+    fn superblock(&self) -> &Superblock {
+        &self.superblock.inner
+    }
+
+    fn superblock_mut(&mut self) -> &mut Superblock {
+        &mut self.superblock.inner
+    }
+
+    pub fn total_block_count(&self) -> usize {
+        self.superblock().blocks_count as _
+    }
+
+    pub fn free_block_count(&self) -> usize {
+        self.superblock().free_blocks_count as _
+    }
+
+    pub fn block_size(&self) -> usize {
+        self.superblock().block_size()
     }
 }
 
@@ -46,7 +77,7 @@ mod tests {
     #[test]
     fn file() {
         let file = RefCell::new(File::open("ext2.bin").unwrap());
-        let mut fs = Ext2::new(file);
-        assert_eq!(Ok(()), fs.init());
+        let fs = Ext2::new(file);
+        assert_eq!(Ok(()), fs.map(|_| ()));
     }
 }