From 7f7163f75a93d0be1997e77be48ba905156d74bd Mon Sep 17 00:00:00 2001
From: Timon Engelke <timon.engelke@online.de>
Date: Tue, 30 Mar 2021 17:19:31 +0200
Subject: [PATCH] improve logic of image cache

---
 .../static/annotations/js/annotations.js            | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/imagetagger/imagetagger/annotations/static/annotations/js/annotations.js b/imagetagger/imagetagger/annotations/static/annotations/js/annotations.js
index 74e205dd..c68707d4 100644
--- a/imagetagger/imagetagger/annotations/static/annotations/js/annotations.js
+++ b/imagetagger/imagetagger/annotations/static/annotations/js/annotations.js
@@ -987,11 +987,14 @@ function calculateImageScale() {
   async function preloadImages() {
     let keepImages = [];
     let cacheLoadings = [];
-    for (let imageId = gImageId - PRELOAD_BACKWARD;
-         imageId <= gImageId + PRELOAD_FORWARD;
-         imageId++) {
-      keepImages.push(imageId);
-      cacheLoadings.push(loadImageToCache(imageId));
+    let currentImageIndex = gImageList.indexOf(gImageId);
+    for (let index = currentImageIndex - PRELOAD_BACKWARD;
+         index <= currentImageIndex + PRELOAD_FORWARD;
+         index++) {
+      if (gImageList[index] !== undefined) {
+        keepImages.push(gImageList[index]);
+        cacheLoadings.push(loadImageToCache(gImageList[index]));
+      }
     }
     await Promise.all(cacheLoadings);
     pruneImageCache(keepImages);