import 'dart:math' as math;

class SubjectSuggestionEngine {
  const SubjectSuggestionEngine._();

  static List<String> suggest({
    required Iterable<String> existingSubjects,
    required String query,
    int limit = 8,
  }) {
    final statsByKey = <String, _SubjectStats>{};

    for (final raw in existingSubjects) {
      final cleaned = normalizeDisplay(raw);
      if (cleaned.isEmpty) {
        continue;
      }
      final key = normalizeKey(cleaned);
      if (key.isEmpty) {
        continue;
      }
      final stats = statsByKey.putIfAbsent(
        key,
        () => _SubjectStats(display: cleaned),
      );
      stats.count += 1;
      if (_isBetterDisplay(cleaned, stats.display)) {
        stats.display = cleaned;
      }
    }

    if (statsByKey.isEmpty) {
      return const <String>[];
    }

    final cleanedQuery = normalizeDisplay(query);
    final queryKey = normalizeKey(cleanedQuery);

    final scored =
        statsByKey.entries
            .map((entry) {
              final value = entry.value;
              final score = _score(
                candidateKey: entry.key,
                candidateDisplay: value.display,
                count: value.count,
                queryKey: queryKey,
              );
              return _ScoredSubject(subject: value.display, score: score);
            })
            .where((entry) => entry.score > 0)
            .toList()
          ..sort((a, b) {
            final byScore = b.score.compareTo(a.score);
            if (byScore != 0) {
              return byScore;
            }
            return a.subject.toLowerCase().compareTo(b.subject.toLowerCase());
          });

    return scored.take(limit).map((entry) => entry.subject).toList();
  }

  static String normalizeDisplay(String input) {
    final trimmed = input.trim();
    if (trimmed.isEmpty) {
      return '';
    }

    final compactWhitespace = trimmed.replaceAll(RegExp(r'\s+'), ' ');
    final punctuationSpacing = compactWhitespace
        .replaceAll(RegExp(r'\s+([,.;:!?])'), r'$1')
        .replaceAll(RegExp(r'([,.;:!?])(\S)'), r'$1 $2')
        .replaceAll(RegExp(r'\s+'), ' ')
        .trim();

    final words = punctuationSpacing.split(' ');
    final correctedWords = words.map(_correctWord).toList(growable: false);
    final sentence = correctedWords.join(' ').trim();

    if (sentence.isEmpty) {
      return '';
    }

    return sentence[0].toUpperCase() + sentence.substring(1);
  }

  static String normalizeKey(String input) {
    final lowered = input.toLowerCase();
    return lowered
        .replaceAll(RegExp(r'[^a-z0-9\s]'), ' ')
        .replaceAll(RegExp(r'\s+'), ' ')
        .trim();
  }

  static double _score({
    required String candidateKey,
    required String candidateDisplay,
    required int count,
    required String queryKey,
  }) {
    final popularity = math.log(count + 1) * 0.1;

    if (queryKey.isEmpty) {
      return 0.5 + popularity;
    }

    final startsWith = candidateKey.startsWith(queryKey) ? 1.2 : 0.0;
    final contains =
        !candidateKey.startsWith(queryKey) && candidateKey.contains(queryKey)
        ? 0.5
        : 0.0;

    final vectorSimilarity = _cosineSimilarity(
      _tokenVector(candidateKey),
      _tokenVector(queryKey),
    );

    final displayLower = candidateDisplay.toLowerCase();
    final queryLower = queryKey.toLowerCase();
    final editLikeBoost = displayLower.contains(queryLower) ? 0.25 : 0.0;

    return (vectorSimilarity * 2.0) +
        startsWith +
        contains +
        editLikeBoost +
        popularity;
  }

  static Map<String, int> _tokenVector(String input) {
    final tokens = input
        .split(' ')
        .where((token) => token.isNotEmpty)
        .toList(growable: false);
    final vector = <String, int>{};
    for (final token in tokens) {
      vector[token] = (vector[token] ?? 0) + 1;
    }
    return vector;
  }

  static double _cosineSimilarity(Map<String, int> a, Map<String, int> b) {
    if (a.isEmpty || b.isEmpty) {
      return 0;
    }

    var dot = 0.0;
    var normA = 0.0;
    var normB = 0.0;

    for (final entry in a.entries) {
      final av = entry.value.toDouble();
      normA += av * av;
      final bv = b[entry.key]?.toDouble() ?? 0.0;
      dot += av * bv;
    }

    for (final entry in b.entries) {
      final bv = entry.value.toDouble();
      normB += bv * bv;
    }

    final denominator = math.sqrt(normA) * math.sqrt(normB);
    if (denominator == 0) {
      return 0;
    }

    return dot / denominator;
  }

  static String _correctWord(String rawWord) {
    if (rawWord.isEmpty) {
      return rawWord;
    }

    final punctuationMatch = RegExp(
      r'^([^a-zA-Z0-9]*)(.*?)([^a-zA-Z0-9]*)$',
    ).firstMatch(rawWord);
    if (punctuationMatch == null) {
      return rawWord;
    }

    final leading = punctuationMatch.group(1) ?? '';
    final core = punctuationMatch.group(2) ?? '';
    final trailing = punctuationMatch.group(3) ?? '';

    if (core.isEmpty) {
      return rawWord;
    }

    final isAcronym = core.length > 1 && core == core.toUpperCase();
    final correctedCore = isAcronym
        ? core
        : core[0].toUpperCase() + core.substring(1).toLowerCase();

    return '$leading$correctedCore$trailing';
  }

  static bool _isBetterDisplay(String candidate, String current) {
    if (candidate == current) {
      return false;
    }

    final candidatePenalty = _displayPenalty(candidate);
    final currentPenalty = _displayPenalty(current);
    if (candidatePenalty != currentPenalty) {
      return candidatePenalty < currentPenalty;
    }

    return candidate.length < current.length;
  }

  static int _displayPenalty(String value) {
    var penalty = 0;
    if (value.contains(RegExp(r'\s{2,}'))) {
      penalty += 2;
    }
    if (value == value.toUpperCase()) {
      penalty += 1;
    }
    return penalty;
  }
}

class _SubjectStats {
  _SubjectStats({required this.display});

  String display;
  int count = 0;
}

class _ScoredSubject {
  _ScoredSubject({required this.subject, required this.score});

  final String subject;
  final double score;
}