/// Self-contained bech32 encode (BIP-173 checksum), enough for NIP-19
/// `npub`/`nsec`. Written inline to avoid a dependency's version quirks; NIP-19
/// is plain bech32 (checksum constant 1), not bech32m.
const _charset = 'qpzry9x8gf2tvdw0s3jn54khce6mua7l';

int _polymod(List<int> values) {
  const gen = [0x3b6a57b2, 0x26508e6d, 0x1ea119fa, 0x3d4233dd, 0x2a1462b3];
  var chk = 1;
  for (final v in values) {
    final top = chk >> 25;
    chk = ((chk & 0x1ffffff) << 5) ^ v;
    for (var i = 0; i < 5; i++) {
      if ((top >> i) & 1 == 1) chk ^= gen[i];
    }
  }
  return chk;
}

List<int> _hrpExpand(String hrp) => [
  ...hrp.codeUnits.map((c) => c >> 5),
  0,
  ...hrp.codeUnits.map((c) => c & 31),
];

List<int> _createChecksum(String hrp, List<int> data) {
  final values = [..._hrpExpand(hrp), ...data, 0, 0, 0, 0, 0, 0];
  final mod = _polymod(values) ^ 1;
  return List.generate(6, (i) => (mod >> (5 * (5 - i))) & 31);
}

/// Converts 8-bit bytes to 5-bit groups (bech32 payload), padding with zeros.
List<int> _convertBits8to5(List<int> data) {
  var acc = 0;
  var bits = 0;
  final out = <int>[];
  for (final value in data) {
    acc = (acc << 8) | value;
    bits += 8;
    while (bits >= 5) {
      bits -= 5;
      out.add((acc >> bits) & 31);
    }
  }
  if (bits > 0) out.add((acc << (5 - bits)) & 31);
  return out;
}

/// bech32-encodes [data] (raw bytes) under [hrp] (e.g. `npub`).
String bech32Encode(String hrp, List<int> data) {
  final five = _convertBits8to5(data);
  final checksum = _createChecksum(hrp, five);
  final combined = [...five, ...checksum];
  return '${hrp}1${combined.map((v) => _charset[v]).join()}';
}