perf: reduce duplicate rendering in localSearch (#3170)

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

src/client/theme-default/components/VPLocalSearchBox.vue

@@ -31,6 +31,7 @@ import {
 import type { ModalTranslations } from '../../../../types/local-search'
 import { pathToFile } from '../../app/utils'
 import { useData } from '../composables/data'
+import { LRUCache } from '../support/lru'
 import { createTranslate } from '../support/translation'
 
 const emit = defineEmits<{
@@ -142,6 +143,8 @@ const mark = computedAsync(async () => {
   return markRaw(new Mark(resultsEl.value))
 }, null)
 
+const cache = new LRUCache<string, Map<string, string>>(16) // 16 files
+
 debouncedWatch(
   () => [searchIndex.value, filterText.value, showDetailedList.value] as const,
   async ([index, filterTextValue, showDetailedListValue], old, onCleanup) => {
@@ -163,13 +166,12 @@ debouncedWatch(
       ? await Promise.all(results.value.map((r) => fetchExcerpt(r.id)))
       : []
     if (canceled) return
-    const c = new Map<string, Map<string, string>>()
     for (const { id, mod } of mods) {
       const mapId = id.slice(0, id.indexOf('#'))
-      let map = c.get(mapId)
+      let map = cache.get(mapId)
       if (map) continue
       map = new Map()
-      c.set(mapId, map)
+      cache.set(mapId, map)
       const comp = mod.default ?? mod
       if (comp?.render || comp?.setup) {
         const app = createApp(comp)
@@ -209,7 +211,7 @@ debouncedWatch(
 
     results.value = results.value.map((r) => {
       const [id, anchor] = r.id.split('#')
-      const map = c.get(id)
+      const map = cache.get(id)
       const text = map?.get(anchor) ?? ''
       for (const term in r.match) {
         terms.add(term)

src/client/theme-default/support/lru.ts

@@ -0,0 +1,33 @@
+// adapted from https://stackoverflow.com/a/46432113/11613622
+
+export class LRUCache<K, V> {
+  private max: number
+  private cache: Map<K, V>
+
+  constructor(max: number = 10) {
+    this.max = max
+    this.cache = new Map<K, V>()
+  }
+
+  get(key: K): V | undefined {
+    let item = this.cache.get(key)
+    if (item !== undefined) {
+      // refresh key
+      this.cache.delete(key)
+      this.cache.set(key, item)
+    }
+    return item
+  }
+
+  set(key: K, val: V): void {
+    // refresh key
+    if (this.cache.has(key)) this.cache.delete(key)
+    // evict oldest
+    else if (this.cache.size === this.max) this.cache.delete(this.first()!)
+    this.cache.set(key, val)
+  }
+
+  first(): K | undefined {
+    return this.cache.keys().next().value
+  }
+}