const FOCUSABLE_SELECTOR = [
  'a[href]',
  'button:not([disabled])',
  'input:not([disabled])',
  'select:not([disabled])',
  'textarea:not([disabled])',
  '[tabindex]:not([tabindex="-1"])',
].join(',');

export function getFocusable(container: HTMLElement): HTMLElement[] {
  return Array.from(container.querySelectorAll<HTMLElement>(FOCUSABLE_SELECTOR));
}

/**
 * Confines Tab/Shift+Tab navigation within a container. Layout-agnostic so it
 * also works under jsdom (no visibility checks).
 */
export class FocusTrap {
  private readonly container: HTMLElement;
  private active = false;

  constructor(container: HTMLElement) {
    this.container = container;
  }

  private readonly onKeydown = (event: KeyboardEvent): void => {
    if (event.key !== 'Tab') {
      return;
    }
    const focusable = getFocusable(this.container);
    if (focusable.length === 0) {
      event.preventDefault();
      return;
    }
    const first = focusable[0]!;
    const last = focusable[focusable.length - 1]!;
    const current = document.activeElement;

    if (event.shiftKey && current === first) {
      event.preventDefault();
      last.focus();
    } else if (!event.shiftKey && current === last) {
      event.preventDefault();
      first.focus();
    }
  };

  activate(): void {
    if (this.active) {
      return;
    }
    this.active = true;
    document.addEventListener('keydown', this.onKeydown, true);
  }

  deactivate(): void {
    if (!this.active) {
      return;
    }
    this.active = false;
    document.removeEventListener('keydown', this.onKeydown, true);
  }
}