class Router {
  #routes = [];
  #outlet = null;
  #currentPath = null;
  #pageCache = new Map(); // path -> { element }
  #scrollCache = new Map(); // path -> scrollY (persists after element eviction)

  // Only cache these route patterns (timeline, profiles, notifications, explore)
  #cacheablePatterns = [
    /^\/$/,                    // timeline
    /^\/profile\/[^/]+$/,      // profile (not followers/following/gallery)
    /^\/notifications$/,       // notifications
    /^\/explore$/              // explore/search
  ];

  register(path, componentTag) {
    this.#routes.push({ path, componentTag });
    return this;
  }

  connect(outlet) {
    this.#outlet = outlet;
    // Skip View Transitions for popstate - browser gestures provide their own
    window.addEventListener('popstate', () => this.#navigate());
    this.#navigate();
    return this;
  }

  push(path) {
    if (location.pathname === path) return;

    const navigate = () => {
      history.pushState(null, '', path);
      this.#navigate();
      window.dispatchEvent(new CustomEvent('grain:navigate'));
    };

    if (document.startViewTransition) {
      document.startViewTransition(navigate);
    } else {
      navigate();
    }
  }

  replace(path) {
    if (location.pathname === path) return;

    const navigate = () => {
      history.replaceState(null, '', path);
      this.#navigate();
      window.dispatchEvent(new CustomEvent('grain:navigate'));
    };

    if (document.startViewTransition) {
      document.startViewTransition(navigate);
    } else {
      navigate();
    }
  }

  #matchRoute(pathname) {
    for (const route of this.#routes) {
      const params = this.#extractParams(route.path, pathname);
      if (params !== null) {
        return { componentTag: route.componentTag, params };
      }
    }
    return null;
  }

  #extractParams(pattern, pathname) {
    if (pattern === '*') return {};

    const patternParts = pattern.split('/');
    const pathParts = pathname.split('/');

    if (patternParts.length !== pathParts.length) return null;

    const params = {};
    for (let i = 0; i < patternParts.length; i++) {
      if (patternParts[i].startsWith(':')) {
        params[patternParts[i].slice(1)] = decodeURIComponent(pathParts[i]);
      } else if (patternParts[i] !== pathParts[i]) {
        return null;
      }
    }
    return params;
  }

  #isCacheable(pathname) {
    return this.#cacheablePatterns.some(pattern => pattern.test(pathname));
  }

  #navigate() {
    const pathname = location.pathname;

    // Save scroll position of current page before switching
    if (this.#currentPath && this.#outlet) {
      this.#scrollCache.set(this.#currentPath, this.#outlet.scrollTop);
    }

    // Skip if same path
    if (this.#currentPath === pathname) return;

    // Deactivate/remove current page
    if (this.#currentPath) {
      const current = this.#pageCache.get(this.#currentPath);
      if (current) {
        if (this.#isCacheable(this.#currentPath)) {
          // Hide cacheable pages
          current.element.style.display = 'none';
          current.element.dispatchEvent(new CustomEvent('grain:deactivated'));
        } else {
          // Remove non-cacheable pages from DOM
          current.element.remove();
          this.#pageCache.delete(this.#currentPath);
        }
      }
    }

    this.#currentPath = pathname;

    // Check if we have a cached page element
    if (this.#pageCache.has(pathname)) {
      const cached = this.#pageCache.get(pathname);
      cached.element.style.display = '';
      cached.element.dispatchEvent(new CustomEvent('grain:activated'));
      // Restore scroll position after paint
      requestAnimationFrame(() => {
        if (this.#outlet) {
          this.#outlet.scrollTop = this.#scrollCache.get(pathname) || 0;
        }
      });
      return;
    }

    // Create new page
    const match = this.#matchRoute(pathname);
    if (!match || !this.#outlet) return;

    const el = document.createElement(match.componentTag);
    Object.assign(el, match.params);

    // Cache if cacheable, otherwise just track for deactivation
    this.#pageCache.set(pathname, { element: el });
    this.#outlet.appendChild(el);

    // Restore saved scroll position, or start at top for new pages
    requestAnimationFrame(() => {
      if (this.#outlet) {
        this.#outlet.scrollTop = this.#scrollCache.get(pathname) || 0;
      }
    });
  }
}

export const router = new Router();