/// Badaga has no fixed Latin spelling ("Madhuvae" / "Madhuve", "Hethai" / "Hettai" / "Hethe").
/// Both the query and the searchable text are folded to a common key before matching.
/// Tamil-script text passes through unchanged (only lower-cased Latin is folded).
String foldForSearch(String input) {
  var s = input.toLowerCase().replaceAll(RegExp(r"[^a-z0-9஀-௿ಀ-೿ ]"), ' ');

  const digraphs = {
    'ae': 'e',
    'ee': 'i',
    'oo': 'u',
    'aa': 'a',
    'th': 't',
    'dh': 'd',
    'bh': 'b',
    'ph': 'p',
    'kh': 'k',
    'gh': 'g',
    'sh': 's',
    'zh': 'l',
    'w': 'v',
  };
  digraphs.forEach((from, to) => s = s.replaceAll(from, to));

  // Word-final "ai" and "e" are written interchangeably (Hethai / Hethe).
  s = s.replaceAllMapped(RegExp(r'ai\b'), (_) => 'e');

  // Collapse doubled letters: "hettai" -> "hetai", "madhuvve" -> "madhuve".
  s = s.replaceAllMapped(RegExp(r'([a-z])\1+'), (m) => m[1]!);

  return s.replaceAll(RegExp(r'\s+'), ' ').trim();
}

/// True when every word of [query] appears in at least one of [fields].
bool matchesSearch(String query, Iterable<String?> fields) {
  final words = foldForSearch(query).split(' ').where((w) => w.isNotEmpty).toList();
  if (words.isEmpty) return false;
  final haystack = fields.whereType<String>().map(foldForSearch).join(' ');
  return words.every(haystack.contains);
}