// Lightweight CDP tracker for browsers (React / Next.js / vanilla).
// Zero dependencies. Copy this file into your app and import the `cdp` object.
//
//   import { cdp } from '@/lib/cdp';
//
//   // Once, at app startup (e.g. layout.tsx / _app.tsx):
//   cdp.init({
//     writeKey: process.env.NEXT_PUBLIC_CDP_WRITE_KEY!,
//     endpoint: process.env.NEXT_PUBLIC_CDP_ENDPOINT ?? 'http://localhost:3049',
//   });
//
//   cdp.identify('user_42', { plan: 'pro' });
//   cdp.track('Button Clicked', { id: 'cta-hero' });
//   cdp.page();  // pulls path/url from window.location
//
// Payload shape matches Segment's Track/Identify/Page API, so the same code
// works with Segment if you ever swap endpoints.

type Json = Record<string, unknown>;
type EventType = 'track' | 'identify' | 'page' | 'group' | 'alias' | 'screen';

interface CDPConfig {
  writeKey: string;
  /** Base URL of the ingest service. e.g. http://localhost:3049 */
  endpoint: string;
  /** localStorage key for the anonymous id. Default: "cdp_anon". */
  anonymousIdKey?: string;
  /** localStorage key for the resolved user id. Default: "cdp_uid". */
  userIdKey?: string;
  /** Auto-fire `page` on every history change. Default: false. */
  autoPage?: boolean;
}

interface CommonPayload {
  type: EventType;
  messageId: string;
  anonymousId: string;
  userId?: string;
  sentAt: string;
  context: Json;
}

const DEFAULTS = {
  anonymousIdKey: 'cdp_anon',
  userIdKey: 'cdp_uid',
  autoPage: false,
};

class CDPClient {
  private cfg: Required<CDPConfig> | null = null;
  private authHeader = '';

  init(config: CDPConfig) {
    this.cfg = { ...DEFAULTS, ...config };
    // Segment-style basic auth: base64(writeKey + ":").
    this.authHeader = 'Basic ' + b64(`${config.writeKey}:`);
    this.ensureAnonymousId();
    if (this.cfg.autoPage && typeof window !== 'undefined') {
      this.installAutoPage();
    }
  }

  identify(userId: string, traits: Json = {}) {
    this.setUserId(userId);
    return this.send('identify', { userId, traits });
  }

  track(event: string, properties: Json = {}) {
    return this.send('track', { event, properties });
  }

  page(name?: string, properties: Json = {}) {
    const loc = typeof window !== 'undefined' ? window.location : null;
    const merged: Json = {
      ...(loc ? { url: loc.href, path: loc.pathname, referrer: document.referrer } : {}),
      ...properties,
    };
    return this.send('page', { name, properties: merged });
  }

  group(groupId: string, traits: Json = {}) {
    return this.send('group', { groupId, traits });
  }

  alias(userId: string, previousId: string) {
    return this.send('alias', { userId, previousId });
  }

  /** Forget the user id (e.g. on logout). Anonymous id is preserved. */
  reset() {
    if (typeof localStorage === 'undefined' || !this.cfg) return;
    localStorage.removeItem(this.cfg.userIdKey);
  }

  // ---------------------------------------------------------------------------
  // internals
  // ---------------------------------------------------------------------------

  private send(type: EventType, body: Json) {
    if (!this.cfg) {
      console.warn('[cdp] send() before init(); call cdp.init() at startup');
      return Promise.resolve();
    }
    const payload: CommonPayload & Json = {
      type,
      messageId: uuidv4(),
      anonymousId: this.getAnonymousId(),
      userId: this.getUserId() ?? undefined,
      sentAt: new Date().toISOString(),
      context: this.buildContext(),
      ...body,
    };
    const url = `${this.cfg.endpoint}/v1/${type}`;
    const blob = JSON.stringify(payload);

    // Prefer sendBeacon for unload safety (page-close, route-change).
    if (typeof navigator !== 'undefined' && 'sendBeacon' in navigator) {
      // sendBeacon can't set Authorization; fall back to fetch with keepalive
      // when an auth header is required. We always use fetch.
    }
    return fetch(url, {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        Authorization: this.authHeader,
      },
      body: blob,
      keepalive: true,
    }).catch((err) => {
      console.warn('[cdp] send failed', err);
    });
  }

  private buildContext(): Json {
    if (typeof window === 'undefined') return { library_name: 'cdp-web', library_version: '0.1.0' };
    return {
      library_name: 'cdp-web',
      library_version: '0.1.0',
      user_agent: navigator.userAgent,
      locale: navigator.language,
      screen_width: window.screen?.width,
      screen_height: window.screen?.height,
    };
  }

  private ensureAnonymousId() {
    if (typeof localStorage === 'undefined' || !this.cfg) return;
    if (!localStorage.getItem(this.cfg.anonymousIdKey)) {
      localStorage.setItem(this.cfg.anonymousIdKey, uuidv4());
    }
  }

  private getAnonymousId(): string {
    if (!this.cfg || typeof localStorage === 'undefined') return uuidv4();
    let id = localStorage.getItem(this.cfg.anonymousIdKey);
    if (!id) {
      id = uuidv4();
      localStorage.setItem(this.cfg.anonymousIdKey, id);
    }
    return id;
  }

  private setUserId(id: string) {
    if (!this.cfg || typeof localStorage === 'undefined') return;
    localStorage.setItem(this.cfg.userIdKey, id);
  }

  private getUserId(): string | null {
    if (!this.cfg || typeof localStorage === 'undefined') return null;
    return localStorage.getItem(this.cfg.userIdKey);
  }

  private installAutoPage() {
    let lastPath = location.pathname;
    const fire = () => {
      if (location.pathname !== lastPath) {
        lastPath = location.pathname;
        this.page();
      }
    };
    // Patch pushState/replaceState so SPA route changes fire `page`.
    const origPush = history.pushState;
    const origReplace = history.replaceState;
    history.pushState = function (...args) {
      const r = origPush.apply(this, args);
      fire();
      return r;
    };
    history.replaceState = function (...args) {
      const r = origReplace.apply(this, args);
      fire();
      return r;
    };
    window.addEventListener('popstate', fire);
    // First page load.
    this.page();
  }
}

// ---------------------------------------------------------------------------
// tiny helpers (no deps)
// ---------------------------------------------------------------------------

function uuidv4(): string {
  // crypto.randomUUID is available in all evergreen browsers and Node 19+.
  if (typeof crypto !== 'undefined' && 'randomUUID' in crypto) {
    return (crypto as Crypto).randomUUID();
  }
  // RFC 4122-ish fallback.
  return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, (c) => {
    const r = (Math.random() * 16) | 0;
    const v = c === 'x' ? r : (r & 0x3) | 0x8;
    return v.toString(16);
  });
}

function b64(input: string): string {
  if (typeof btoa !== 'undefined') return btoa(input);
  // Node SSR
  // @ts-expect-error: Buffer exists in Node.
  return Buffer.from(input, 'utf-8').toString('base64');
}

export const cdp = new CDPClient();
export type { CDPConfig };