/**
 * UUID v4 generator.
 *
 * Uses crypto.getRandomValues when available (React Native / modern browsers)
 * and falls back to Math.random otherwise.
 */
export function uuid(): string {
  if (typeof crypto !== 'undefined' && typeof crypto.getRandomValues === 'function') {
    // Native crypto path
    const bytes = new Uint8Array(16);
    crypto.getRandomValues(bytes);
    // Set version 4 bits
    bytes[6] = (bytes[6] & 0x0f) | 0x40;
    // Set variant bits (RFC 4122)
    bytes[8] = (bytes[8] & 0x3f) | 0x80;
    return formatBytes(bytes);
  }

  // Fallback: Math.random based
  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 formatBytes(bytes: Uint8Array): string {
  const hex: string[] = [];
  for (let i = 0; i < bytes.length; i++) {
    hex.push(bytes[i].toString(16).padStart(2, '0'));
  }
  return [
    hex.slice(0, 4).join(''),
    hex.slice(4, 6).join(''),
    hex.slice(6, 8).join(''),
    hex.slice(8, 10).join(''),
    hex.slice(10, 16).join(''),
  ].join('-');
}