/// Letters of an A–Z index; names that don't start with a letter go under `#`.
final indexLetters = [...'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.split(''), '#'];

const _accented = 'ÀÁÂÃÄÅÇÈÉÊËÌÍÎÏÑÒÓÔÕÖÙÚÛÜÝŸ';
const _unaccented = 'AAAAAACEEEEIIIINOOOOOUUUUYY';

/// Uppercase without accents, so "Émile" sorts and indexes under E.
String _fold(String name) {
  return [
    for (final char in name.trim().toUpperCase().split(''))
      switch (_accented.indexOf(char)) {
        -1 => char,
        final index => _unaccented[index],
      },
  ].join();
}

String indexLetterOf(String name) {
  final folded = _fold(name);
  if (folded.isEmpty) return '#';

  final first = folded[0];
  return indexLetters.contains(first) ? first : '#';
}

/// A–Z regardless of case and accents, with the `#` names last.
int compareByIndex(String a, String b) {
  final byLetter = indexLetters
      .indexOf(indexLetterOf(a))
      .compareTo(indexLetters.indexOf(indexLetterOf(b)));
  if (byLetter != 0) return byLetter;

  final byName = _fold(a).compareTo(_fold(b));
  return byName != 0 ? byName : a.compareTo(b);
}