record last key in meta

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

mini-lsm/src/table/builder.rs

@@ -12,6 +12,7 @@ use crate::lsm_storage::BlockCache;
 pub struct SsTableBuilder {
     builder: BlockBuilder,
     first_key: Vec<u8>,
+    last_key: Vec<u8>,
     data: Vec<u8>,
     pub(super) meta: Vec<BlockMeta>,
     block_size: usize,
@@ -24,6 +25,7 @@ impl SsTableBuilder {
             data: Vec::new(),
             meta: Vec::new(),
             first_key: Vec::new(),
+            last_key: Vec::new(),
             block_size,
             builder: BlockBuilder::new(block_size),
         }
@@ -32,18 +34,25 @@ impl SsTableBuilder {
     /// Adds a key-value pair to SSTable
     pub fn add(&mut self, key: &[u8], value: &[u8]) {
         if self.first_key.is_empty() {
-            self.first_key = key.to_vec();
+            self.first_key.clear();
+            self.first_key.extend(key);
         }
 
         if self.builder.add(key, value) {
+            self.last_key.clear();
+            self.last_key.extend(key);
             return;
         }
+
         // create a new block builder and append block data
         self.finish_block();
 
         // add the key-value pair to the next block
         assert!(self.builder.add(key, value));
-        self.first_key = key.to_vec();
+        self.first_key.clear();
+        self.first_key.extend(key);
+        self.last_key.clear();
+        self.last_key.extend(key);
     }
 
     /// Get the estimated size of the SSTable.
@@ -57,6 +66,7 @@ impl SsTableBuilder {
         self.meta.push(BlockMeta {
             offset: self.data.len(),
             first_key: std::mem::take(&mut self.first_key).into(),
+            last_key: std::mem::take(&mut self.last_key).into(),
         });
         self.data.extend(encoded_block);
     }
@@ -78,6 +88,8 @@ impl SsTableBuilder {
         Ok(SsTable {
             id,
             file,
+            first_key: self.meta.first().unwrap().first_key.clone(),
+            last_key: self.meta.last().unwrap().last_key.clone(),
             block_metas: self.meta,
             block_meta_offset: meta_offset,
             block_cache,