High Level Designs

// ot-client.ts — Simplified OT client
class OTClient {
  private pendingOps: Operation[] = [];  // ops sent but not yet acked
  private revision: number = 0;          // last known server revision

  constructor(
    private docId: string,
    private ws: WebSocket,
    private editor: Editor
  ) {
    this.ws.onmessage = this.handleServerMessage.bind(this);
  }

  // Called when user types
  applyLocalOp(op: Operation): void {
    // 1. Apply immediately to local editor (optimistic update)
    this.editor.apply(op);

    // 2. Tag with current revision so server knows what state we were at
    op.revision = this.revision;

    // 3. Add to pending queue
    this.pendingOps.push(op);

    // 4. Send to server
    this.ws.send(JSON.stringify({ type: "op", op }));
  }

  // Called when server broadcasts an op from another user
  handleServerMessage(event: MessageEvent): void {
    const msg = JSON.parse(event.data);

    if (msg.type === "op") {
      // Transform the incoming op against our pending ops
      // (because pending ops haven't been acked yet — they happened "concurrently")
      let transformedOp = msg.op;
      for (const pending of this.pendingOps) {
        transformedOp = transform(transformedOp, pending);
      }

      // Apply the transformed op
      this.editor.apply(transformedOp);
      this.revision = msg.revision;
    }

    if (msg.type === "ack") {
      // Server acknowledged our op — remove from pending queue
      this.pendingOps.shift();
      this.revision = msg.revision;
    }
  }
}

// Simple transform function for insert operations
function transform(op1: Operation, op2: Operation): Operation {
  if (op1.type === "insert" && op2.type === "insert") {
    // If op2 inserted before op1's position, shift op1 right
    if (op2.position <= op1.position) {
      return { ...op1, position: op1.position + op2.text.length };
    }
  }
  return op1; // no conflict
}

// video-player.tsx — Core player setup with HLS
import Hls from 'hls.js';
import { useEffect, useRef } from 'react';

interface VideoPlayerProps {
  manifestUrl: string;      // The .m3u8 URL
  thumbnailUrl: string;
  autoplay?: boolean;
}

export function VideoPlayer({ manifestUrl, thumbnailUrl, autoplay }: VideoPlayerProps) {
  const videoRef = useRef<HTMLVideoElement>(null);
  const hlsRef = useRef<Hls | null>(null);

  useEffect(() => {
    const video = videoRef.current;
    if (!video) return;

    if (Hls.isSupported()) {
      // HLS.js handles ABR automatically
      const hls = new Hls({
        // Start with low quality, let ABR ramp up
        startLevel: -1,          // auto
        // Buffer settings
        maxBufferLength: 30,      // buffer 30s ahead
        maxMaxBufferLength: 60,   // absolute max
        // Enable low latency mode for live streams
        lowLatencyMode: false,
      });

      hls.loadSource(manifestUrl);
      hls.attachMedia(video);

      // Monitor quality switches for analytics
      hls.on(Hls.Events.LEVEL_SWITCHED, (event, data) => {
        console.log('Quality switched to level', data.level);
        // Send to analytics: { event: 'quality_change', level: data.level }
      });

      // Handle errors gracefully
      hls.on(Hls.Events.ERROR, (event, data) => {
        if (data.fatal) {
          switch (data.type) {
            case Hls.ErrorTypes.NETWORK_ERROR:
              // Try to recover from network error
              hls.startLoad();
              break;
            case Hls.ErrorTypes.MEDIA_ERROR:
              hls.recoverMediaError();
              break;
            default:
              // Cannot recover
              hls.destroy();
              break;
          }
        }
      });

      hlsRef.current = hls;
    } else if (video.canPlayType('application/vnd.apple.mpegurl')) {
      // Safari has native HLS support
      video.src = manifestUrl;
    }

    return () => {
      hlsRef.current?.destroy();
    };
  }, [manifestUrl]);

  return (
    <div className="video-container">
      <video
        ref={videoRef}
        poster={thumbnailUrl}   // Show thumbnail until video loads
        controls
        autoPlay={autoplay}
        className="w-full aspect-video bg-black"
      />
    </div>
  );
}

// feed-hook.ts — Infinite scroll with IntersectionObserver
import { useEffect, useRef, useCallback, useState } from 'react';

interface FeedItem {
  id: string;
  title: string;
  thumbnailUrl: string;
  channelName: string;
  viewCount: number;
}

interface FeedPage {
  items: FeedItem[];
  nextPageToken: string | null;  // null means no more pages
}

export function useFeed() {
  const [pages, setPages] = useState<FeedPage[]>([]);
  const [isLoading, setIsLoading] = useState(false);
  const pageTokenRef = useRef<string | null>('');  // '' = first page
  const sentinelRef = useRef<HTMLDivElement>(null); // bottom of feed

  const loadNextPage = useCallback(async () => {
    // pageTokenRef.current is null when there are no more pages
    if (isLoading || pageTokenRef.current === null) return;

    setIsLoading(true);
    try {
      const params = new URLSearchParams();
      if (pageTokenRef.current) {
        params.set('pageToken', pageTokenRef.current);
      }

      const res = await fetch('/api/feed?' + params.toString());
      const data: FeedPage = await res.json();

      setPages(prev => [...prev, data]);
      pageTokenRef.current = data.nextPageToken;
    } finally {
      setIsLoading(false);
    }
  }, [isLoading]);

  // IntersectionObserver watches the sentinel div at bottom of feed
  useEffect(() => {
    const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.isIntersecting) loadNextPage();
      },
      { rootMargin: '400px' }  // trigger 400px before bottom — smooth UX
    );

    if (sentinelRef.current) observer.observe(sentinelRef.current);
    return () => observer.disconnect();
  }, [loadNextPage]);

  const allItems = pages.flatMap(page => page.items);

  return { allItems, isLoading, sentinelRef };
}

// virtual-feed.tsx — Virtual list for Twitter feed
import { VariableSizeList } from 'react-window';
import { useRef, useCallback, useState } from 'react';
import AutoSizer from 'react-virtualized-auto-sizer';

interface Tweet {
  id: string;
  text: string;
  author: string;
  imageUrl?: string;    // tweets with images are taller
  likeCount: number;
  timestamp: string;
}

// Tweets have variable heights — text-only = 120px, with image = 400px
function getItemSize(tweet: Tweet): number {
  if (tweet.imageUrl) return 400;
  if (tweet.text.length > 200) return 180;
  return 120;
}

export function VirtualFeed({ tweets }: { tweets: Tweet[] }) {
  const listRef = useRef<VariableSizeList>(null);
  const [newTweetCount, setNewTweetCount] = useState(0);

  // When new tweets arrive via SSE, show banner instead of auto-scrolling
  // (auto-scroll is jarring UX — Twitter's approach)
  const handleNewTweets = useCallback((count: number) => {
    setNewTweetCount(prev => prev + count);
  }, []);

  const scrollToTop = () => {
    listRef.current?.scrollToItem(0, 'start');
    setNewTweetCount(0);
  };

  const Row = ({ index, style }: { index: number; style: React.CSSProperties }) => (
    // style must be applied — it contains the absolute position from react-window
    <div style={style}>
      <TweetCard tweet={tweets[index]} />
    </div>
  );

  return (
    <div className="relative h-screen">
      {/* New tweets banner */}
      {newTweetCount > 0 && (
        <button
          onClick={scrollToTop}
          className="fixed top-16 left-1/2 -translate-x-1/2 z-50
                     bg-blue-500 text-white px-4 py-2 rounded-full"
        >
          Show {newTweetCount} new tweets
        </button>
      )}

      {/* Auto-sizing virtual list — fills available height */}
      <AutoSizer>
        {({ height, width }) => (
          <VariableSizeList
            ref={listRef}
            height={height}
            width={width}
            itemCount={tweets.length}
            itemSize={(index) => getItemSize(tweets[index])}
            overscanCount={3}  // render 3 extra items outside viewport
          >
            {Row}
          </VariableSizeList>
        )}
      </AutoSizer>
    </div>
  );
}

// use-feed-sse.ts — Real-time feed updates via SSE
import { useEffect, useRef, useCallback } from 'react';

interface NewTweetEvent {
  id: string;
  authorId: string;
  preview: string;
}

export function useFeedSSE(
  onNewTweets: (tweets: NewTweetEvent[]) => void
) {
  const esRef = useRef<EventSource | null>(null);
  const reconnectTimeoutRef = useRef<number>(0);

  const connect = useCallback(() => {
    // EventSource is the browser's built-in SSE client
    const es = new EventSource('/api/feed/stream', {
      withCredentials: true,  // send auth cookies
    });

    es.addEventListener('new_tweets', (event) => {
      const tweets: NewTweetEvent[] = JSON.parse(event.data);
      onNewTweets(tweets);
    });

    es.addEventListener('ping', () => {
      // Server sends periodic pings to keep connection alive
      // and detect dead connections
    });

    es.onerror = () => {
      es.close();
      esRef.current = null;
      // Exponential backoff reconnection
      reconnectTimeoutRef.current = Math.min(
        reconnectTimeoutRef.current * 2 || 1000,
        30000  // max 30s
      );
      setTimeout(connect, reconnectTimeoutRef.current);
    };

    es.onopen = () => {
      // Reset backoff on successful connection
      reconnectTimeoutRef.current = 0;
    };

    esRef.current = es;
  }, [onNewTweets]);

  useEffect(() => {
    connect();
    return () => {
      esRef.current?.close();
    };
  }, [connect]);
}

// use-filters.ts — URL-driven filter state
import { useRouter, useSearchParams } from 'next/navigation';
import { useCallback, useMemo } from 'react';

interface Filters {
  brands: string[];
  priceMin: number;
  priceMax: number;
  minRating: number;
  colors: string[];
  sort: 'price_asc' | 'price_desc' | 'rating' | 'newest';
}

export function useFilters() {
  const router = useRouter();
  const searchParams = useSearchParams();

  // Parse filters FROM URL
  const filters = useMemo((): Filters => ({
    brands: searchParams.get('brand')?.split(',').filter(Boolean) ?? [],
    priceMin: Number(searchParams.get('priceMin')) || 0,
    priceMax: Number(searchParams.get('priceMax')) || Infinity,
    minRating: Number(searchParams.get('rating')) || 0,
    colors: searchParams.get('color')?.split(',').filter(Boolean) ?? [],
    sort: (searchParams.get('sort') as Filters['sort']) ?? 'rating',
  }), [searchParams]);

  // Update filters → pushes to URL (triggers re-render + SSR re-fetch)
  const updateFilter = useCallback(<K extends keyof Filters>(
    key: K,
    value: Filters[K]
  ) => {
    const params = new URLSearchParams(searchParams.toString());

    if (key === 'brands' || key === 'colors') {
      const arr = value as string[];
      if (arr.length === 0) params.delete(key === 'brands' ? 'brand' : 'color');
      else params.set(key === 'brands' ? 'brand' : 'color', arr.join(','));
    } else if (key === 'priceMin') {
      params.set('priceMin', String(value));
    } else if (key === 'priceMax') {
      params.set('priceMax', String(value));
    } else if (key === 'minRating') {
      params.set('rating', String(value));
    } else if (key === 'sort') {
      params.set('sort', value as string);
    }

    // Reset to page 1 when filters change
    params.delete('page');

    // Use router.push for browser history (back button works!)
    router.push('?' + params.toString());
  }, [router, searchParams]);

  const clearAllFilters = useCallback(() => {
    router.push('?');  // Clear all params
  }, [router]);

  return { filters, updateFilter, clearAllFilters };
}

// types.ts — API response structure
interface PLPResponse {
  products: Product[];
  totalCount: number;
  facets: Facets;         // filter options with counts
  appliedFilters: Filter[]; // for the breadcrumb strip
}

interface Facets {
  brands: FacetOption[];
  colors: FacetOption[];
  ratings: FacetOption[];
  priceRange: { min: number; max: number; histogram: number[] };
}

interface FacetOption {
  value: string;          // e.g., "Dell"
  count: number;          // e.g., 234 — how many products match
  isSelected: boolean;    // true if currently filtered
}

// server/api/products.ts — SSR data fetching
export async function getProducts(searchParams: URLSearchParams) {
  const query = {
    category: searchParams.get('category'),
    brands: searchParams.get('brand')?.split(','),
    priceMin: Number(searchParams.get('priceMin')) || undefined,
    priceMax: Number(searchParams.get('priceMax')) || undefined,
    sort: searchParams.get('sort') || 'rating',
    page: Number(searchParams.get('page')) || 1,
  };

  // Elasticsearch query with aggregations for facets
  const esQuery = {
    query: buildESQuery(query),
    aggs: {
      brands: { terms: { field: 'brand.keyword', size: 20 } },
      colors: { terms: { field: 'color.keyword', size: 20 } },
      price_range: { stats: { field: 'price' } },
    },
    from: (query.page - 1) * 20,
    size: 20,
  };

  const result = await elasticsearch.search(esQuery);
  return transformESResponse(result);
}

// canvas-renderer.ts — Core canvas drawing
const COL_WIDTH = 100;   // pixels per column
const ROW_HEIGHT = 25;   // pixels per row
const HEADER_WIDTH = 50; // row number column
const HEADER_HEIGHT = 25; // column letter row

interface Viewport {
  startRow: number;
  startCol: number;
  endRow: number;
  endCol: number;
  scrollX: number;
  scrollY: number;
}

class SheetRenderer {
  private ctx: CanvasRenderingContext2D;
  private viewport: Viewport;
  private cellStore: CellStore;

  constructor(canvas: HTMLCanvasElement, cellStore: CellStore) {
    this.ctx = canvas.getContext('2d')!;
    this.cellStore = cellStore;
    this.viewport = { startRow: 0, startCol: 0, endRow: 40, endCol: 26, scrollX: 0, scrollY: 0 };
  }

  render(): void {
    const ctx = this.ctx;
    // Clear the entire canvas
    ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);

    this.drawBackground();
    this.drawCells();      // cell contents and backgrounds
    this.drawGridLines();  // the grid overlay
    this.drawHeaders();    // row numbers + column letters
    this.drawSelection();  // blue selection border
  }

  private drawCells(): void {
    const { startRow, endRow, startCol, endCol } = this.viewport;

    for (let row = startRow; row <= endRow; row++) {
      for (let col = startCol; col <= endCol; col++) {
        const cell = this.cellStore.getCell(row, col);
        if (!cell) continue;

        const x = HEADER_WIDTH + (col - startCol) * COL_WIDTH;
        const y = HEADER_HEIGHT + (row - startRow) * ROW_HEIGHT;

        // Draw cell background
        this.ctx.fillStyle = cell.bgColor ?? '#ffffff';
        this.ctx.fillRect(x, y, COL_WIDTH, ROW_HEIGHT);

        // Draw cell text (computed value, not formula)
        this.ctx.fillStyle = cell.textColor ?? '#000000';
        this.ctx.font = `${cell.bold ? 'bold ' : ''}13px Arial`;
        this.ctx.textAlign = cell.align ?? 'left';

        // Clip text to cell boundaries
        this.ctx.save();
        this.ctx.rect(x + 2, y, COL_WIDTH - 4, ROW_HEIGHT);
        this.ctx.clip();
        this.ctx.fillText(cell.computed ?? '', x + 4, y + 17);
        this.ctx.restore();
      }
    }
  }

  // Hit test: given mouse x,y — which cell is it?
  hitTest(mouseX: number, mouseY: number): { row: number; col: number } | null {
    const col = Math.floor((mouseX - HEADER_WIDTH) / COL_WIDTH) + this.viewport.startCol;
    const row = Math.floor((mouseY - HEADER_HEIGHT) / ROW_HEIGHT) + this.viewport.startRow;
    if (col < 0 || row < 0) return null;
    return { row, col };
  }

  // When user starts typing in a cell — show real <input>
  showCellEditor(row: number, col: number): void {
    const x = HEADER_WIDTH + (col - this.viewport.startCol) * COL_WIDTH;
    const y = HEADER_HEIGHT + (row - this.viewport.startRow) * ROW_HEIGHT;

    const input = document.createElement('input');
    input.style.position = 'absolute';
    input.style.left = x + 'px';
    input.style.top = y + 'px';
    input.style.width = COL_WIDTH + 'px';
    input.style.height = ROW_HEIGHT + 'px';
    input.style.border = '2px solid #1a73e8';
    input.value = this.cellStore.getCell(row, col)?.raw ?? '';

    // On Enter or blur — commit the edit
    const commit = () => {
      this.cellStore.setCell(row, col, input.value);
      input.remove();
      this.render(); // re-draw canvas
    };
    input.addEventListener('keydown', (e) => {
      if (e.key === 'Enter') commit();
      if (e.key === 'Escape') input.remove();
    });
    input.addEventListener('blur', commit);

    document.querySelector('.sheet-container')!.appendChild(input);
    input.focus();
  }
}

// hover-preview.tsx — Netflix-style hover preview
import { useRef, useCallback, useState } from 'react';

interface ContentCard {
  id: string;
  title: string;
  thumbnailUrl: string;
  previewVideoUrl: string;
}

export function ContentCard({ content }: { content: ContentCard }) {
  const [isPreviewActive, setIsPreviewActive] = useState(false);
  const videoRef = useRef<HTMLVideoElement>(null);
  const hoverTimerRef = useRef<number | null>(null);
  const abortRef = useRef<AbortController | null>(null);

  const handleMouseEnter = useCallback(() => {
    // 400ms delay — avoid triggering on quick mouse pass-through
    hoverTimerRef.current = window.setTimeout(async () => {
      try {
        abortRef.current = new AbortController();

        // Fetch the preview URL (could be a signed CDN URL)
        const res = await fetch(
          '/api/preview/' + content.id,
          { signal: abortRef.current.signal }
        );
        const { videoUrl } = await res.json();

        if (videoRef.current) {
          videoRef.current.src = videoUrl;
          videoRef.current.load();
          // Play when enough is buffered
          videoRef.current.addEventListener('canplay', () => {
            videoRef.current?.play();
            setIsPreviewActive(true);
          }, { once: true });
        }
      } catch (err) {
        // AbortError is expected when user moves away — ignore it
        if ((err as Error).name !== 'AbortError') {
          console.error('Preview failed:', err);
        }
      }
    }, 400);
  }, [content.id]);

  const handleMouseLeave = useCallback(() => {
    // Cancel the 400ms timer if user left before it fired
    if (hoverTimerRef.current !== null) {
      clearTimeout(hoverTimerRef.current);
    }
    // Cancel in-flight fetch
    abortRef.current?.abort();

    // Clean up video
    if (videoRef.current) {
      videoRef.current.pause();
      videoRef.current.src = '';
    }
    setIsPreviewActive(false);
  }, []);

  return (
    <div
      className={`content-card ${isPreviewActive ? 'preview-active' : ''}`}
      onMouseEnter={handleMouseEnter}
      onMouseLeave={handleMouseLeave}
    >
      <img src={content.thumbnailUrl} alt={content.title} />

      {/* Hidden video element — shown when preview is active */}
      <video
        ref={videoRef}
        className={`preview-video ${isPreviewActive ? 'visible' : ''}`}
        muted
        loop
        playsInline
      />

      {isPreviewActive && (
        <div className="preview-overlay">
          <h3>{content.title}</h3>
          <div className="action-buttons">
            <button>Play</button>
            <button>Add to List</button>
            <button>More Info</button>
          </div>
        </div>
      )}
    </div>
  );
}

// home-page.tsx — Rendering personalized rows
interface ContentRow {
  id: string;
  title: string;        // "Top Picks for You", "Action Movies", etc.
  algorithm: string;    // for analytics
  items: ContentItem[];
  totalCount: number;
}

interface HomePageData {
  rows: ContentRow[];
  heroContent: ContentItem;
}

// Server-side fetch — personalized per user
async function getHomePageData(userId: string): Promise<HomePageData> {
  // Netflix's Falcor or GraphQL API — fetches all rows in one request
  const response = await fetch('/api/home', {
    headers: { 'X-User-Id': userId },
    // Cache for 5 minutes — balance freshness vs performance
    next: { revalidate: 300 },
  });
  return response.json();
}

// The row component with horizontal scroll
function ContentRow({ row }: { row: ContentRow }) {
  const scrollRef = useRef<HTMLDivElement>(null);

  const scroll = (direction: 'left' | 'right') => {
    const el = scrollRef.current;
    if (!el) return;
    el.scrollBy({
      left: direction === 'right' ? 600 : -600,
      behavior: 'smooth',
    });
  };

  return (
    <section className="content-row">
      <h2>{row.title}</h2>
      <div className="row-container">
        <button className="scroll-btn left" onClick={() => scroll('left')}>
          ←
        </button>
        <div ref={scrollRef} className="cards-container">
          {row.items.map(item => (
            <ContentCard key={item.id} content={item} />
          ))}
        </div>
        <button className="scroll-btn right" onClick={() => scroll('right')}>
          →
        </button>
      </div>
    </section>
  );
}

// chat-socket.ts — Robust WebSocket client
type MessageHandler = (msg: ChatMessage) => void;

class ChatSocket {
  private ws: WebSocket | null = null;
  private reconnectAttempts = 0;
  private maxReconnectAttempts = 10;
  private offlineQueue: ChatMessage[] = [];  // messages sent while disconnected
  private handlers: Map<string, MessageHandler[]> = new Map();
  private pingInterval: number | null = null;

  constructor(private userId: string, private serverUrl: string) {}

  connect(): void {
    this.ws = new WebSocket(this.serverUrl + '?userId=' + this.userId);

    this.ws.onopen = () => {
      console.log('WS connected');
      this.reconnectAttempts = 0;

      // Send any messages that were queued while offline
      this.flushOfflineQueue();

      // Start ping to detect dead connections (proxy timeout prevention)
      this.pingInterval = window.setInterval(() => {
        if (this.ws?.readyState === WebSocket.OPEN) {
          this.ws.send(JSON.stringify({ type: 'ping' }));
        }
      }, 30000);  // every 30 seconds
    };

    this.ws.onmessage = (event) => {
      const msg = JSON.parse(event.data);

      if (msg.type === 'pong') return;  // ignore ping responses

      // Route to registered handlers
      const handlers = this.handlers.get(msg.type) ?? [];
      handlers.forEach(h => h(msg));
    };

    this.ws.onclose = (event) => {
      clearInterval(this.pingInterval!);

      if (event.code === 1000) {
        // Normal closure — don't reconnect
        return;
      }

      this.scheduleReconnect();
    };

    this.ws.onerror = () => {
      // onclose will fire after onerror — handle reconnect there
    };
  }

  private scheduleReconnect(): void {
    if (this.reconnectAttempts >= this.maxReconnectAttempts) {
      // Show "Connection lost" UI
      this.emit('connection_failed', null);
      return;
    }

    // Exponential backoff: 1s, 2s, 4s, 8s... max 30s
    const delay = Math.min(1000 * Math.pow(2, this.reconnectAttempts), 30000);
    this.reconnectAttempts++;

    setTimeout(() => this.connect(), delay);
  }

  send(message: ChatMessage): void {
    if (this.ws?.readyState === WebSocket.OPEN) {
      this.ws.send(JSON.stringify(message));
    } else {
      // Offline — queue the message
      this.offlineQueue.push(message);
      // Persist to IndexedDB for page refresh survival
      this.persistToIndexedDB(message);
    }
  }

  private flushOfflineQueue(): void {
    const queue = [...this.offlineQueue];
    this.offlineQueue = [];

    for (const msg of queue) {
      this.ws!.send(JSON.stringify(msg));
    }
  }

  on(type: string, handler: MessageHandler): () => void {
    if (!this.handlers.has(type)) this.handlers.set(type, []);
    this.handlers.get(type)!.push(handler);
    // Return unsubscribe function
    return () => {
      const list = this.handlers.get(type) ?? [];
      this.handlers.set(type, list.filter(h => h !== handler));
    };
  }

  private emit(type: string, data: any): void {
    const handlers = this.handlers.get(type) ?? [];
    handlers.forEach(h => h(data));
  }

  private async persistToIndexedDB(message: ChatMessage): Promise<void> {
    // Store in IndexedDB so messages survive page refresh
    const db = await openDB('chat-queue', 1);
    await db.put('pending', message);
  }

  disconnect(): void {
    this.ws?.close(1000, 'User logged out');
  }
}

// typing-indicator.ts — Efficient typing status
class TypingIndicator {
  private typingTimer: number | null = null;
  private isCurrentlyTyping = false;

  constructor(
    private chatSocket: ChatSocket,
    private conversationId: string
  ) {}

  // Called on every keystroke in the message input
  onKeyPress(): void {
    if (!this.isCurrentlyTyping) {
      // First keystroke — send typing start
      this.isCurrentlyTyping = true;
      this.chatSocket.send({
        type: 'typing_start',
        conversationId: this.conversationId,
      });
    }

    // Reset the stop timer on every keystroke
    if (this.typingTimer !== null) {
      clearTimeout(this.typingTimer);
    }

    // Stop typing indicator after 3s of inactivity
    this.typingTimer = window.setTimeout(() => {
      this.isCurrentlyTyping = false;
      this.chatSocket.send({
        type: 'typing_stop',
        conversationId: this.conversationId,
      });
    }, 3000);
  }

  // Called when message is sent
  onMessageSent(): void {
    if (this.typingTimer !== null) {
      clearTimeout(this.typingTimer);
    }
    if (this.isCurrentlyTyping) {
      this.isCurrentlyTyping = false;
      this.chatSocket.send({
        type: 'typing_stop',
        conversationId: this.conversationId,
      });
    }
  }
}

// use-autocomplete.ts — Complete autocomplete hook
import { useState, useEffect, useRef, useCallback } from 'react';

interface UseAutocompleteOptions {
  debounceMs?: number;    // default 300
  minChars?: number;      // don't search for 1-char queries
  maxCacheSize?: number;  // LRU cache size
}

export function useAutocomplete(options: UseAutocompleteOptions = {}) {
  const {
    debounceMs = 300,
    minChars = 2,
    maxCacheSize = 100,
  } = options;

  const [query, setQuery] = useState('');
  const [suggestions, setSuggestions] = useState<string[]>([]);
  const [isLoading, setIsLoading] = useState(false);
  const [error, setError] = useState<string | null>(null);
  const [activeIndex, setActiveIndex] = useState(-1);  // for keyboard nav

  // LRU cache: Map preserves insertion order
  const cacheRef = useRef<Map<string, string[]>>(new Map());

  // AbortController ref — cancel in-flight requests
  const abortRef = useRef<AbortController | null>(null);

  // Debounce timer ref
  const timerRef = useRef<number | null>(null);

  const fetchSuggestions = useCallback(async (q: string) => {
    // Check cache first
    if (cacheRef.current.has(q)) {
      setSuggestions(cacheRef.current.get(q)!);
      return;
    }

    // Cancel any previous request
    abortRef.current?.abort();
    abortRef.current = new AbortController();

    setIsLoading(true);
    setError(null);

    try {
      const res = await fetch(
        '/api/search/suggest?q=' + encodeURIComponent(q),
        { signal: abortRef.current.signal }
      );

      if (!res.ok) throw new Error('Search failed');

      const data: string[] = await res.json();

      // Update cache (evict oldest if over limit)
      if (cacheRef.current.size >= maxCacheSize) {
        const firstKey = cacheRef.current.keys().next().value;
        cacheRef.current.delete(firstKey);
      }
      cacheRef.current.set(q, data);

      setSuggestions(data);
    } catch (err) {
      if ((err as Error).name === 'AbortError') {
        // This is expected — a newer query superseded this one
        return;
      }
      setError('Could not load suggestions');
      setSuggestions([]);
    } finally {
      setIsLoading(false);
    }
  }, [maxCacheSize]);

  // Effect: debounce the fetch on query change
  useEffect(() => {
    if (timerRef.current !== null) {
      clearTimeout(timerRef.current);
    }

    if (query.length < minChars) {
      setSuggestions([]);
      setActiveIndex(-1);
      return;
    }

    timerRef.current = window.setTimeout(() => {
      fetchSuggestions(query);
    }, debounceMs);

    return () => {
      if (timerRef.current !== null) clearTimeout(timerRef.current);
    };
  }, [query, fetchSuggestions, debounceMs, minChars]);

  // Keyboard navigation handler
  const handleKeyDown = useCallback((e: React.KeyboardEvent) => {
    switch (e.key) {
      case 'ArrowDown':
        e.preventDefault();  // prevent scroll
        setActiveIndex(i => Math.min(i + 1, suggestions.length - 1));
        break;
      case 'ArrowUp':
        e.preventDefault();
        setActiveIndex(i => Math.max(i - 1, -1));
        break;
      case 'Enter':
        if (activeIndex >= 0) {
          // Select the highlighted suggestion
          setQuery(suggestions[activeIndex]);
          setSuggestions([]);
        }
        break;
      case 'Escape':
        setSuggestions([]);
        setActiveIndex(-1);
        break;
    }
  }, [suggestions, activeIndex]);

  return {
    query,
    setQuery,
    suggestions,
    isLoading,
    error,
    activeIndex,
    handleKeyDown,
  };
}

// search-autocomplete.tsx — Accessible dropdown
import { useId } from 'react';
import { useAutocomplete } from './use-autocomplete';

export function SearchAutocomplete() {
  const listboxId = useId();  // stable ID for ARIA
  const {
    query,
    setQuery,
    suggestions,
    isLoading,
    error,
    activeIndex,
    handleKeyDown,
  } = useAutocomplete({ debounceMs: 300, minChars: 2 });

  const isOpen = suggestions.length > 0 || isLoading || !!error;

  return (
    <div className="autocomplete-container" role="combobox"
         aria-expanded={isOpen}
         aria-haspopup="listbox"
         aria-owns={listboxId}>
      <input
        type="search"
        value={query}
        onChange={e => setQuery(e.target.value)}
        onKeyDown={handleKeyDown}
        placeholder="Search..."
        // ARIA: tell screen readers which item is highlighted
        aria-autocomplete="list"
        aria-controls={listboxId}
        aria-activedescendant={
          activeIndex >= 0 ? 'suggestion-' + activeIndex : undefined
        }
      />

      {isOpen && (
        <ul
          id={listboxId}
          role="listbox"
          className="suggestions-dropdown"
        >
          {isLoading && (
            <li role="status" aria-live="polite">Loading...</li>
          )}
          {error && (
            <li role="alert">{error}</li>
          )}
          {suggestions.map((suggestion, index) => (
            <li
              key={suggestion}
              id={'suggestion-' + index}
              role="option"
              aria-selected={index === activeIndex}
              className={index === activeIndex ? 'highlighted' : ''}
              onMouseDown={(e) => {
                // mousedown instead of click — fires before input blur
                e.preventDefault();
                setQuery(suggestion);
              }}
            >
              {/* Highlight matching portion */}
              <HighlightMatch text={suggestion} query={query} />
            </li>
          ))}
        </ul>
      )}
    </div>
  );
}

// Highlight the matching part of the suggestion
function HighlightMatch({ text, query }: { text: string; query: string }) {
  const idx = text.toLowerCase().indexOf(query.toLowerCase());
  if (idx === -1) return <span>{text}</span>;

  return (
    <span>
      {text.slice(0, idx)}
      <strong>{text.slice(idx, idx + query.length)}</strong>
      {text.slice(idx + query.length)}
    </span>
  );
}