From 6c076666b6c579113eb5e57b5d1260b936233c74 Mon Sep 17 00:00:00 2001
From: Ziyang Hu <hu.ziyang@cantab.net>
Date: Wed, 21 Jun 2023 20:10:43 +0800
Subject: [PATCH] fix panic when running KNN on empty index

---
 cozo-core/src/runtime/hnsw.rs | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/cozo-core/src/runtime/hnsw.rs b/cozo-core/src/runtime/hnsw.rs
index 47724072..1b713c57 100644
--- a/cozo-core/src/runtime/hnsw.rs
+++ b/cozo-core/src/runtime/hnsw.rs
@@ -900,10 +900,14 @@ impl<'a> SessionTx<'a> {
         if let Some(ep) = ep_res {
             let ep = ep?;
             let bottom_level = ep[0].get_int().unwrap();
+            let ep_idx = match ep[config.base_handle.metadata.keys.len() + 1].get_int() {
+                Some(x) => x as usize,
+                None => {
+                    // this occurs if the index is empty
+                    return Ok(vec![])
+                },
+            };
             let ep_t_key = ep[1..config.base_handle.metadata.keys.len() + 1].to_vec();
-            let ep_idx = ep[config.base_handle.metadata.keys.len() + 1]
-                .get_int()
-                .unwrap() as usize;
             let ep_subidx = ep[config.base_handle.metadata.keys.len() + 2]
                 .get_int()
                 .unwrap() as i32;