more comments, sync check

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-starter/src/block/iterator.rs

@@ -5,57 +5,60 @@ use std::sync::Arc;
 
 use super::Block;
 
-pub struct BlockIterator {}
+/// Iterates on a block.
+pub struct BlockIterator {
+    block: Arc<Block>,
+    key: Vec<u8>,
+    value: Vec<u8>,
+    idx: usize,
+}
 
 impl BlockIterator {
     fn new(block: Arc<Block>) -> Self {
-        unimplemented!()
+        Self {
+            block,
+            key: Vec::new(),
+            value: Vec::new(),
+            idx: 0,
+        }
     }
 
+    /// Creates a block iterator and seek to the first entry.
     pub fn create_and_seek_to_first(block: Arc<Block>) -> Self {
         unimplemented!()
     }
 
+    /// Creates a block iterator and seek to the first key that >= `key`.
     pub fn create_and_seek_to_key(block: Arc<Block>, key: &[u8]) -> Self {
         unimplemented!()
     }
 
+    /// Returns the key of the current entry.
     pub fn key(&self) -> &[u8] {
         unimplemented!()
     }
 
+    /// Returns the value of the current entry.
     pub fn value(&self) -> &[u8] {
         unimplemented!()
     }
 
+    /// Returns true if the iterator is valid.
     pub fn is_valid(&self) -> bool {
         unimplemented!()
     }
 
+    /// Seeks to the first key in the block.
     pub fn seek_to_first(&mut self) {
         unimplemented!()
     }
 
-    pub fn seek_to_last(&mut self) {
-        unimplemented!()
-    }
-
-    pub fn len(&self) -> usize {
-        unimplemented!()
-    }
-
-    pub fn is_empty(&self) -> bool {
-        unimplemented!()
-    }
-
-    pub fn seek_to(&mut self, idx: usize) {
-        unimplemented!()
-    }
-
+    /// Move to the next key in the block.
     pub fn next(&mut self) {
         unimplemented!()
     }
 
+    /// Seek to the first key that >= `key`.
     pub fn seek_to_key(&mut self, key: &[u8]) {
         unimplemented!()
     }