/**
 * Generates a pseudo random number. Although it is not cryptographically secure
 * and uniformly distributed, it is not a concern for the intended use-case,
 * which is to generate distinct numbers.
 *
 * Credit: https://stackoverflow.com/a/19303725/10254049
 */
export class SimpleRNG {
  static withSeed = (seed: number) => new SimpleRNG(seed);

  private seed: number;

  private constructor(seed: number) {
    this.seed = seed;
  }

  nextInt(): number {
    const x = Math.sin(this.seed++) * 10000;
    return x - Math.floor(x);
  }
}

// getRandomValues rejects requests larger than this
const MAX_RANDOM_BYTES_PER_CALL = 65536;

const getWebCrypto = (): Crypto => {
  const webCrypto = (globalThis as { crypto?: Crypto }).crypto;
  if (!webCrypto || typeof webCrypto.getRandomValues !== 'function') {
    throw new Error(
      'Web Crypto API is unavailable, so cryptographically secure random bytes ' +
        'cannot be generated. A `crypto.getRandomValues` polyfill is required ' +
        'to encrypt documents in this environment.',
    );
  }
  return webCrypto;
};

/**
 * Generates cryptographically secure random bytes using the platform's Web
 * Crypto API (`crypto.getRandomValues`), which is available in modern browsers,
 * Node (>= 18), Deno, and Bun.
 *
 * Throws when `globalThis.crypto.getRandomValues` is missing — Node < 18, or
 * React Native without a polyfill such as `react-native-get-random-values`.
 * Only document encryption needs this; the rest of `pdf-lib` is unaffected.
 */
export const getRandomBytes = (byteCount: number): Uint8Array => {
  const webCrypto = getWebCrypto();
  const bytes = new Uint8Array(byteCount);
  for (
    let offset = 0;
    offset < byteCount;
    offset += MAX_RANDOM_BYTES_PER_CALL
  ) {
    const end = Math.min(offset + MAX_RANDOM_BYTES_PER_CALL, byteCount);
    webCrypto.getRandomValues(bytes.subarray(offset, end));
  }
  return bytes;
};