/**
 * Convert from a Base64URL-encoded string to an Array Buffer. Best used when converting a
 * credential ID from a JSON string to an ArrayBuffer, like in allowCredentials or
 * excludeCredentials
 *
 * Helper method to compliment `bufferToBase64URLString`
 */
export function toBuffer(base64URLString: string): Uint8Array {
  // Convert from Base64URL to Base64
  const base64 = toBase64(base64URLString);

  // Convert to a binary string
  const binary = atob(base64);

  // Convert binary string to buffer
  const buffer = new ArrayBuffer(binary.length);
  const bytes = new Uint8Array(buffer);

  for (let i = 0; i < binary.length; i++) {
    bytes[i] = binary.charCodeAt(i);
  }

  return bytes;
}

/**
 * Convert the given array buffer into a Base64URL-encoded string. Ideal for converting various
 * credential response ArrayBuffers to string for sending back to the server as JSON.
 *
 * Helper method to compliment `base64URLStringToBuffer`
 */
export function fromBuffer(buffer: Uint8Array): string {
  let str = '';

  for (const charCode of buffer) {
    str += String.fromCharCode(charCode);
  }

  const base64String = btoa(str);

  return base64String.replace(/\+/g, '-').replace(/\//g, '_').replace(/=/g, '');
}

/**
 * Convert a base64url string into base64
 */
export function toBase64(base64URLString: string): string {
  const base64 = base64URLString.replace(/-/g, '+').replace(/_/g, '/');
  /**
   * Pad with '=' until it's a multiple of four
   * (4 - (85 % 4 = 1) = 3) % 4 = 3 padding
   * (4 - (86 % 4 = 2) = 2) % 4 = 2 padding
   * (4 - (87 % 4 = 3) = 1) % 4 = 1 padding
   * (4 - (88 % 4 = 0) = 4) % 4 = 0 padding
   */
  const padLength = (4 - (base64.length % 4)) % 4;
  const padded = base64.padEnd(base64.length + padLength, '=');

  return padded;
}

export default {
  toBuffer,
  fromBuffer,
  toBase64,
};