Implement the document merge function for the update method

milli/src/documents/reader.rs

@@ -72,6 +72,19 @@ impl<R> DocumentsBatchCursor<R> {
 }
 
 impl<R: io::Read + io::Seek> DocumentsBatchCursor<R> {
+    /// Returns a single document from the database.
+    pub fn get(
+        &mut self,
+        offset: u32,
+    ) -> Result<Option<KvReader<'_, FieldId>>, DocumentsBatchCursorError> {
+        match self.cursor.move_on_key_equal_to(offset.to_be_bytes())? {
+            Some((key, value)) if key != DOCUMENTS_BATCH_INDEX_KEY => {
+                Ok(Some(KvReader::new(value)))
+            }
+            _otherwise => Ok(None),
+        }
+    }
+
     /// Returns the next document, starting from the first one. Subsequent calls to
     /// `next_document` advance the document reader until all the documents have been read.
     pub fn next_document(