perf(web): fast rules search with memoized sections and highlighting

Memoize Markdown rendering (React.memo) so sections parse once on mount.
Search filters by toggling visibility (no re-render), with debounced
input and DOM-based text highlighting for matches in both titles and
content.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/index.css

@@ -671,3 +671,11 @@ details[open] > .rules-summary::before {
 .rules-content p {
   margin: var(--spacing-xs) 0;
 }
+
+mark.search-hl {
+  background: rgba(255, 170, 0, 0.35);
+  color: inherit;
+  border-radius: 2px;
+  padding: 0 1px;
+}
+

web/src/pages/RulesPage.tsx

@@ -1,22 +1,124 @@
-import { useState, useMemo } from 'react';
+import { useState, useMemo, useEffect, useRef, memo } from 'react';
 import Markdown from 'react-markdown';
 import remarkGfm from 'remark-gfm';
 import TopBar from '../components/layout/TopBar';
 import PageContainer from '../components/layout/PageContainer';
 import { battleReferenceSections } from '../rules/battle-reference';
 
+const plugins = [remarkGfm];
+const mdComponents = {
+  table: ({ children }: { children?: React.ReactNode }) => (
+    <div className="table-wrap"><table>{children}</table></div>
+  ),
+};
+
+// Memoised so Markdown only renders once per section, never re-parses on search
+const SectionContent = memo(function SectionContent({ content }: { content: string }) {
+  return (
+    <div className="rules-content">
+      <Markdown remarkPlugins={plugins} components={mdComponents}>
+        {content}
+      </Markdown>
+    </div>
+  );
+});
+
+function HighlightText({ text, query }: { text: string; query: string }) {
+  if (!query) return <>{text}</>;
+  const lower = text.toLowerCase();
+  const q = query.toLowerCase();
+  const parts: React.ReactNode[] = [];
+  let lastIdx = 0;
+  let idx = lower.indexOf(q);
+  let key = 0;
+
+  while (idx !== -1) {
+    if (idx > lastIdx) parts.push(text.slice(lastIdx, idx));
+    parts.push(<mark key={key++} className="search-hl">{text.slice(idx, idx + q.length)}</mark>);
+    lastIdx = idx + q.length;
+    idx = lower.indexOf(q, lastIdx);
+  }
+
+  if (lastIdx < text.length) parts.push(text.slice(lastIdx));
+  return <>{parts}</>;
+}
+
 export default function RulesPage() {
   const [openSections, setOpenSections] = useState<Set<number>>(new Set());
   const [search, setSearch] = useState('');
+  const [debouncedSearch, setDebouncedSearch] = useState('');
+  const sectionsRef = useRef<HTMLDivElement>(null);
 
-  const filtered = useMemo(() => {
-    if (!search.trim()) return battleReferenceSections.map((s, i) => ({ ...s, originalIndex: i }));
-    const q = search.toLowerCase();
-    return battleReferenceSections
-      .map((s, i) => ({ ...s, originalIndex: i }))
-      .filter((s) => s.title.toLowerCase().includes(q) || s.content.toLowerCase().includes(q));
+  useEffect(() => {
+    const timer = setTimeout(() => setDebouncedSearch(search), 250);
+    return () => clearTimeout(timer);
   }, [search]);
 
+  // Which sections match (null = show all)
+  const matchSet = useMemo(() => {
+    const q = debouncedSearch.trim().toLowerCase();
+    if (!q) return null;
+    const set = new Set<number>();
+    battleReferenceSections.forEach((s, i) => {
+      if (s.title.toLowerCase().includes(q) || s.content.toLowerCase().includes(q))
+        set.add(i);
+    });
+    return set;
+  }, [debouncedSearch]);
+
+  // Auto-expand matching sections when searching
+  useEffect(() => {
+    if (matchSet) setOpenSections(new Set(matchSet));
+  }, [matchSet]);
+
+  // Highlight matching text inside rendered markdown (DOM-based, no React re-render)
+  useEffect(() => {
+    const container = sectionsRef.current;
+    if (!container) return;
+
+    // Clear old highlights
+    container.querySelectorAll('mark.search-hl').forEach((mark) => {
+      const parent = mark.parentNode;
+      if (parent) {
+        parent.replaceChild(document.createTextNode(mark.textContent || ''), mark);
+        parent.normalize();
+      }
+    });
+
+    const q = debouncedSearch.trim().toLowerCase();
+    if (!q) return;
+
+    // Walk text nodes inside visible .rules-content and wrap matches
+    container.querySelectorAll('.rules-content').forEach((content) => {
+      const walker = document.createTreeWalker(content, NodeFilter.SHOW_TEXT);
+      const textNodes: Text[] = [];
+      while (walker.nextNode()) textNodes.push(walker.currentNode as Text);
+
+      textNodes.forEach((node) => {
+        const text = node.textContent || '';
+        const lower = text.toLowerCase();
+        if (!lower.includes(q)) return;
+
+        const frag = document.createDocumentFragment();
+        let lastIdx = 0;
+        let idx = lower.indexOf(q);
+
+        while (idx !== -1) {
+          if (idx > lastIdx) frag.appendChild(document.createTextNode(text.slice(lastIdx, idx)));
+          const mark = document.createElement('mark');
+          mark.className = 'search-hl';
+          mark.textContent = text.slice(idx, idx + q.length);
+          frag.appendChild(mark);
+          lastIdx = idx + q.length;
+          idx = lower.indexOf(q, lastIdx);
+        }
+
+        if (lastIdx < text.length) frag.appendChild(document.createTextNode(text.slice(lastIdx)));
+        node.parentNode?.replaceChild(frag, node);
+      });
+    });
+  }, [debouncedSearch, openSections]);
+
   const toggle = (index: number) => {
     setOpenSections((prev) => {
       const next = new Set(prev);
@@ -30,13 +132,15 @@ export default function RulesPage() {
   };
 
   const expandAll = () => {
-    setOpenSections(new Set(filtered.map((s) => s.originalIndex)));
+    setOpenSections(new Set(battleReferenceSections.map((_, i) => i)));
   };
 
   const collapseAll = () => {
     setOpenSections(new Set());
   };
 
+  const matchCount = matchSet?.size ?? battleReferenceSections.length;
+
   return (
     <>
       <TopBar title="Ship Reference" />
@@ -56,30 +160,28 @@ export default function RulesPage() {
           <button className="btn-secondary btn-sm" onClick={collapseAll}>
             Collapse All
           </button>
-          {search.trim() && (
-            <span className="rules-count">{filtered.length} result{filtered.length !== 1 ? 's' : ''}</span>
+          {debouncedSearch.trim() && (
+            <span className="rules-count">
+              {matchCount} result{matchCount !== 1 ? 's' : ''}
+            </span>
           )}
         </div>
 
-        <div className="rules-sections">
-          {filtered.map((section) => (
-            <details key={section.originalIndex} className="rules-section" open={openSections.has(section.originalIndex)}>
-              <summary className="rules-summary" onClick={(e) => { e.preventDefault(); toggle(section.originalIndex); }}>
-                {section.title}
+        <div className="rules-sections" ref={sectionsRef}>
+          {battleReferenceSections.map((section, i) => (
+            <details
+              key={i}
+              className="rules-section"
+              open={openSections.has(i)}
+              style={matchSet !== null && !matchSet.has(i) ? { display: 'none' } : undefined}
+            >
+              <summary className="rules-summary" onClick={(e) => { e.preventDefault(); toggle(i); }}>
+                <HighlightText text={section.title} query={debouncedSearch} />
               </summary>
-              <div className="rules-content">
-                <Markdown
-                  remarkPlugins={[remarkGfm]}
-                  components={{
-                    table: ({ children }) => (
-                      <div className="table-wrap"><table>{children}</table></div>
-                    ),
-                  }}
-                >{section.content}</Markdown>
-              </div>
+              <SectionContent content={section.content} />
             </details>
           ))}
-          {filtered.length === 0 && (
+          {matchSet !== null && matchSet.size === 0 && (
             <p className="empty-text" style={{ padding: 'var(--spacing-md) 0' }}>
               No sections match "{search}".
             </p>