class WritingCheckResult {
  const WritingCheckResult({required this.complete, required this.message});

  final bool complete;
  final String message;
}

/// A deliberately small offline checker for teaching tasks. It checks only
/// the essential A0 information, accepts variable names/places, and never
/// claims to grade pronunciation or nuanced grammar.
class WritingFeedback {
  const WritingFeedback._();

  static String _normalize(String input) => input
      .toLowerCase()
      .replaceAll('’', "'")
      .replaceAll('‘', "'")
      .replaceAll(RegExp(r'\s+'), ' ')
      .trim();

  static WritingCheckResult check(
    String lessonId,
    String input, {
    String? segmentId,
  }) {
    final text = _normalize(input);
    final words = RegExp(
      r"[a-z]+(?:'[a-z]+)?",
    ).allMatches(text).map((match) => match.group(0)!).toSet();
    bool has(String word) => words.contains(_normalize(word));

    bool hasPhrase(String phrase) => text.contains(_normalize(phrase));

    bool hasAny(Iterable<String> choices) => choices.any(has);
    final hasIntroduction =
        hasPhrase("i'm") || hasPhrase('i am') || hasPhrase('my name is');
    final hasItIs = hasPhrase("it's") || hasPhrase('it is');

    final complete = switch (segmentId) {
      'a0-04-a' => hasAll(words, ['zero', 'one', 'two', 'three']),
      'a0-04-b' => hasAll(words, ['six', 'seven', 'eight']),
      'a0-04-c' =>
        RegExp(
              r'\b(one|two|three|four|five|six|seven|eight|nine|zero)\b',
            ).allMatches(text).length >=
            3,
      'a0-08-a' => hasItIs && hasAny(['monday', 'tuesday', 'wednesday']),
      'a0-08-b' =>
        hasItIs && hasAny(['thursday', 'friday', 'saturday', 'sunday']),
      'a0-08-c' => hasItIs && (has('oclock') || hasPhrase("o'clock")),
      _ => switch (lessonId) {
        'a0-01' => hasAny(['hello', 'hi']) && hasIntroduction,
        'a0-02' =>
          (hasPhrase('my name') || hasIntroduction) &&
              RegExp(r'[a-z](?:[ -]?[a-z]){2,}').hasMatch(text),
        'a0-03' => hasIntroduction && hasAny(['good', 'okay', 'tired']),
        'a0-04' =>
          RegExp(
                r'\b(zero|one|two|three|four|five|six|seven|eight|nine|ten)\b',
              ).allMatches(text).length >=
              3,
        'a0-05' => hasItIs && hasAny(['book', 'pen', 'bag', 'key']),
        'a0-06' => hasIntroduction && has('from') && words.length >= 3,
        'a0-07' =>
          hasPhrase('this is my') &&
              hasAny(['mother', 'father', 'sister', 'brother', 'friend']),
        'a0-08' =>
          hasItIs &&
              (hasAny([
                    'monday',
                    'tuesday',
                    'wednesday',
                    'thursday',
                    'friday',
                    'saturday',
                    'sunday',
                  ]) ||
                  has('clock')),
        'a0-09' => hasIntroduction && has('like') && words.length >= 3,
        'a0-10' => hasIntroduction && has('from') && has('like'),
        _ => words.length >= 2,
      },
    };

    if (complete) {
      return const WritingCheckResult(
        complete: true,
        message: '这句已经表达完整。继续在对话里用一次吧。',
      );
    }
    return WritingCheckResult(
      complete: false,
      message: _hintFor(segmentId ?? lessonId),
    );
  }

  static bool hasAll(Set<String> words, Iterable<String> required) =>
      required.every(words.contains);

  static String _hintFor(String lessonId) => switch (lessonId) {
    'a0-04-a' => '请写出 zero、one、two、three 四个数字。',
    'a0-04-b' => '请写出 six、seven、eight 三个数字。',
    'a0-04-c' => '用英文一个一个写出三个数字，例如 one-three-nine。',
    'a0-08-a' => '用 It is / It’s 加 Monday、Tuesday 或 Wednesday。',
    'a0-08-b' => '用 It is / It’s 加 Thursday、Friday、Saturday 或 Sunday。',
    'a0-08-c' => '用 It is / It’s 加数字和 o’clock，例如 It’s three o’clock。',
    'a0-01' => '试着同时写一句问候和姓名，例如：Hello. I’m …',
    'a0-02' => '写出姓名并拼写一遍，例如 My name is Shen. S-H-E-N.',
    'a0-03' => '用 I’m / I am 加上你的状态，例如 good 或 tired。',
    'a0-04' => '用英文写出三个数字，例如 one-three-nine。',
    'a0-05' => '用 It’s / It is 加一个物品，例如 a pen。',
    'a0-06' => '用 I’m from … 写成一个完整句子。',
    'a0-07' => '用 This is my … 介绍一位家人或朋友。',
    'a0-08' => '用 It’s … 写一个星期或整点时间。',
    'a0-09' => '用 I like … 写出一种喜好。',
    'a0-10' => '分别写姓名、来自哪里和一项喜好三部分。',
    _ => '再补充一个完整英文句子。',
  };
}