fix: speed up templateHelper's getAncestor() by 500x

Backported from #2150, authored by @mrdoob.

Author: hegemonic

lib/jsdoc/util/templateHelper.js

@@ -805,6 +805,26 @@ exports.getSignatureReturns = ({yields, returns}, cssClass) => {
     return returnTypes;
 };
 
+// Cache for memberof index to speed up ancestor lookups
+let memberofIndex = null;
+
+function buildMemberofIndex(data) {
+    if (memberofIndex) {
+        return memberofIndex;
+    }
+
+    memberofIndex = new Map();
+    const allDoclets = data().get();
+
+    for (const doclet of allDoclets) {
+        if (doclet.longname) {
+            memberofIndex.set(doclet.longname, doclet);
+        }
+    }
+
+    return memberofIndex;
+}
+
 /**
  * Retrieve an ordered list of doclets for a symbol's ancestors.
  *
@@ -818,9 +838,13 @@ exports.getAncestors = (data, doclet) => {
     let doc = doclet;
     let previousDoc;
 
+    // Build index once for all lookups
+    const index = buildMemberofIndex(data);
+
     while (doc) {
         previousDoc = doc;
-        doc = find(data, {longname: doc.memberof})[0];
+        // Use index instead of database query
+        doc = index.get(doc.memberof);
 
         // prevent infinite loop that can be caused by duplicated module definitions
         if (previousDoc === doc) {