embedExactSourceMetadata function

String embedExactSourceMetadata(String rendered, String originalSource)

Appends an invisible exact-source trailer to rendered.

The source is encoded from the exact Dart UTF-16 code-unit sequence rather than from a normalized form, so case, NFC/NFD choice, combining-mark order, punctuation, aliases, and even unpaired UTF-16 code units are retained.

Implementation

String embedExactSourceMetadata(String rendered, String originalSource) {
  final bytes = _stringToUtf16Le(originalSource);
  final encoded = base64Url.encode(bytes).replaceAll('=', '');
  final sourceChecksum = _fnv1a32(bytes).toRadixString(16).padLeft(8, '0');
  final renderedChecksum =
      _fnv1a32(_stringToUtf16Le(rendered)).toRadixString(16).padLeft(8, '0');
  final payload =
      '$_exactSourceMagic$encoded:$sourceChecksum:$renderedChecksum';

  final taggedPayload = <int>[_exactSourceStartTag];
  for (final unit in payload.codeUnits) {
    if (unit < 0x20 || unit > 0x7E) {
      throw StateError('Exact-source payload unexpectedly contains non-ASCII.');
    }
    taggedPayload.add(0xE0000 + unit);
  }
  taggedPayload.add(_exactSourceEndTag);

  return rendered + String.fromCharCodes(taggedPayload);
}