encodeBase64WithCharset static method

String encodeBase64WithCharset({ required String base64Input, required String charset, })

Implementation

static String encodeBase64WithCharset(
    {required String base64Input, required String charset}) {
  if (base64Input.isEmpty) return '';
  List<int> bytes = base64.decode(base64Input);
  int n = charset.length;
  if (n < 2)
    throw Exception('Charset must have at least 2 unique characters');
  BigInt number = BigInt.zero;
  for (int byte in bytes) {
    number = (number << 8) | BigInt.from(byte & 0xFF);
  }

  int bitLength = bytes.length * 8;
  if (bitLength == 0) return '';

  double log2N = log(n) / log(2);
  int outputLength = (bitLength / log2N).ceil().toInt();

  List<int> digits = [];
  BigInt base = BigInt.from(n);

  while (number > BigInt.zero) {
    digits.add((number % base).toInt());
    number = number ~/ base;
  }

  while (digits.length < outputLength) {
    digits.add(0);
  }

  digits = digits.reversed.toList();

  StringBuffer sb = StringBuffer();
  for (int digit in digits) {
    sb.write(charset[digit]);
  }

  return sb.toString();
}