fix: ensure WAL is atomic for each write batch (#84)

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

mini-lsm-mvcc/src/mem_table.rs

@@ -122,16 +122,25 @@ impl MemTable {
     ///
     /// In week 1, day 1, simply put the key-value pair into the skipmap.
     /// In week 2, day 6, also flush the data to WAL.
+    /// In week 3, day 5, modify the function to use the batch API.
     pub fn put(&self, key: KeySlice, value: &[u8]) -> Result<()> {
-        let estimated_size = key.raw_len() + value.len();
-        self.map.insert(
-            key.to_key_vec().into_key_bytes(),
-            Bytes::copy_from_slice(value),
-        );
+        self.put_batch(&[(key, value)])
+    }
+
+    /// Implement this in week 3, day 5.
+    pub fn put_batch(&self, data: &[(KeySlice, &[u8])]) -> Result<()> {
+        let mut estimated_size = 0;
+        for (key, value) in data {
+            estimated_size += key.raw_len() + value.len();
+            self.map.insert(
+                key.to_key_vec().into_key_bytes(),
+                Bytes::copy_from_slice(value),
+            );
+        }
         self.approximate_size
             .fetch_add(estimated_size, std::sync::atomic::Ordering::Relaxed);
         if let Some(ref wal) = self.wal {
-            wal.put(key, value)?;
+            wal.put_batch(data)?;
         }
         Ok(())
     }