port algorithm for normalizing known instances to five visible slots

assets/instance_buttons.js

@@ -1,5 +1,7 @@
 (function instance_buttons(){
 
+    const SLOTS = 5;
+
     const STORAGE_KEY = 'forget_known_instances';
 
     const container = document.querySelector('#mastodon_instance_buttons');
@@ -20,19 +22,73 @@
                 known = await resp.json();
             }
             else {
-                known = [];
+                known = [{
+                    "instance": "mastodon.social",
+                    "hits": 0
+                }];
             }
-            localStorage.setItem(STORAGE_KEY, JSON.stringify(known));
+            save(known)
             fetch('/api/known_instances', {method: 'DELETE'})
         }
 
         return known;
     }
 
+    function normalize(known){
+        /*
+
+        move instances with the most hits to the top SLOTS slots,
+        making sure not to reorder anything that is already there
+
+        */
+        let head = known.slice(0, SLOTS);
+        let tail = known.slice(SLOTS);
+
+        if(tail.length == 0){
+            return known;
+        }
+
+        for(let i = 0; i < SLOTS; i++){
+            let head_min = head.reduce((acc, cur) => acc.hits < cur.hits ? acc : cur);
+            let tail_max = tail.reduce((acc, cur) => acc.hits > cur.hits ? acc : cur);
+            if(head_min.hits < tail_max.hits){
+                // swappy
+                let i = head.indexOf(head_min);
+                let j = tail.indexOf(tail_max);
+                let buf = head[i];
+                head[i] = tail[j];
+                tail[j] = buf;
+            }
+        }
+
+        return head.concat(tail)
+    }
+
+    function save(known){
+        localStorage.setItem(STORAGE_KEY, JSON.stringify(known));
+    }
+
     async function replace_buttons(){
         let known = await get_known();
 
-        let instances = known.concat(top_instances).slice(0, 5);
+        known = normalize(known);
+        save(known);
+
+        let filtered_top_instances = []
+        for(let instance of top_instances){
+            let found = false;
+            for(let k of known){
+                if(k['instance'] == instance){
+                    found = true;
+                    break;
+                }
+            }
+            if(!found){
+                filtered_top_instances.push(instance)
+            }
+        }
+
+        let instances = known.concat(filtered_top_instances).slice(0, SLOTS);
 
         let html = '';