add RawDocument abstraction to access bytes in doc store

src/indexer/merger.rs

@@ -918,16 +918,16 @@ impl IndexMerger {
         if let Some(doc_id_mapping) = doc_id_mapping {
             for (old_doc_id, reader_with_ordinal) in doc_id_mapping {
                 let store_reader = reader_with_ordinal.reader.get_store_reader()?;
-                let (block, start_pos, end_pos) = store_reader.get_raw(*old_doc_id)?;
-                store_writer.store_bytes(&block[start_pos..end_pos])?;
+                let raw_doc = store_reader.get_raw(*old_doc_id)?;
+                store_writer.store_bytes(raw_doc.get_bytes())?;
             }
         } else {
             for reader in &self.readers {
                 let store_reader = reader.get_store_reader()?;
                 if reader.num_deleted_docs() > 0 {
                     for doc_id in reader.doc_ids_alive() {
-                        let (block, start_pos, end_pos) = store_reader.get_raw(doc_id)?;
-                        store_writer.store_bytes(&block[start_pos..end_pos])?;
+                        let raw_doc = store_reader.get_raw(doc_id)?;
+                        store_writer.store_bytes(raw_doc.get_bytes())?;
                     }
                 } else {
                     store_writer.stack(&store_reader)?;

src/store/reader.rs

@@ -86,23 +86,25 @@ impl StoreReader {
     /// Reads a given document.
     ///
     /// Calling `.get(doc)` is relatively costly as it requires
-    /// decompressing a compressed block.
+    /// decompressing a compressed block. The store utilizes a LRU cache,
+    /// so accessing docs from the same compressed block should be faster.
     ///
     /// It should not be called to score documents
     /// for instance.
     pub fn get(&self, doc_id: DocId) -> crate::Result<Document> {
-        let (block, start_pos, end_pos) = self.get_raw(doc_id)?;
-        let mut cursor = &block[start_pos..end_pos];
+        let raw_doc = self.get_raw(doc_id)?;
+        let mut cursor = raw_doc.get_bytes();
         Ok(Document::deserialize(&mut cursor)?)
     }
 
-    /// Reads raw bytes of a given document. Returns the block of a document and its start and end
+    /// Reads raw bytes of a given document. Returns `RawDocument`, which contains the block of a document and its start and end
     /// position within the block.
     ///
     /// Calling `.get(doc)` is relatively costly as it requires
-    /// decompressing a compressed block.
+    /// decompressing a compressed block. The store utilizes a LRU cache,
+    /// so accessing docs from the same compressed block should be faster.
     ///
-    pub fn get_raw(&self, doc_id: DocId) -> crate::Result<(Arc<Vec<u8>>, usize, usize)> {
+    pub fn get_raw(&self, doc_id: DocId) -> crate::Result<RawDocument> {
         let checkpoint = self.block_checkpoint(doc_id).ok_or_else(|| {
             crate::TantivyError::InvalidArgument(format!("Failed to lookup Doc #{}.", doc_id))
         })?;
@@ -117,7 +119,11 @@ impl StoreReader {
         let doc_length = VInt::deserialize(&mut cursor)?.val() as usize;
         let start_pos = cursor_len_before - cursor.len();
         let end_pos = cursor_len_before - cursor.len() + doc_length;
-        Ok((block, start_pos, end_pos))
+        Ok(RawDocument {
+            block,
+            start_pos,
+            end_pos,
+        })
     }
 
     /// Summarize total space usage of this store reader.
@@ -126,6 +132,23 @@ impl StoreReader {
     }
 }
 
+/// Get the bytes of a serialized `Document` in a decompressed block.
+pub struct RawDocument {
+    /// the block of data containing multiple documents
+    block: Arc<Vec<u8>>,
+    /// start position of the document in the block
+    start_pos: usize,
+    /// end position of the document in the block
+    end_pos: usize,
+}
+
+impl RawDocument {
+    /// Get the bytes of a serialized `Document` in a decompressed block.
+    pub fn get_bytes(&self) -> &[u8] {
+        &self.block[self.start_pos..self.end_pos]
+    }
+}
+
 fn split_file(data: FileSlice) -> io::Result<(FileSlice, FileSlice)> {
     let (data, footer_len_bytes) = data.split_from_end(size_of::<u64>());
     let serialized_offset: OwnedBytes = footer_len_bytes.read_bytes()?;