import 'dart:math';

/// One spelling challenge: a displayed word and whether it is spelled correctly.
class SpellingChallenge {
  const SpellingChallenge({
    required this.displayedWord,
    required this.isCorrectlySpelled,
    required this.canonicalWord,
  });

  final String displayedWord;
  final bool isCorrectlySpelled;
  final String canonicalWord;
}

/// Curated commonly misspelled words for spelling quizzes.
abstract final class SpellingWordList {
  static const List<({String correct, String misspelled})> pairs =
      <({String correct, String misspelled})>[
    (correct: 'accommodate', misspelled: 'acommodate'),
    (correct: 'separate', misspelled: 'seperate'),
    (correct: 'definitely', misspelled: 'definately'),
    (correct: 'occurrence', misspelled: 'occurence'),
    (correct: 'embarrass', misspelled: 'embarass'),
    (correct: 'maintenance', misspelled: 'maintainance'),
    (correct: 'recommend', misspelled: 'reccomend'),
    (correct: 'privilege', misspelled: 'priviledge'),
    (correct: 'necessary', misspelled: 'neccessary'),
    (correct: 'rhythm', misspelled: 'rythm'),
  ];

  /// Known misspelling from the curated list, if any.
  static String? misspellingFor(String canonicalWord) {
    final String normalized = canonicalWord.trim().toLowerCase();
    for (final ({String correct, String misspelled}) pair in pairs) {
      if (pair.correct == normalized) {
        return pair.misspelled;
      }
    }
    return null;
  }

  /// Fallback typo when [misspellingFor] has no curated entry.
  static String generatedMisspelling(String canonicalWord) {
    return misspellingFor(canonicalWord) ?? _naiveMisspelling(canonicalWord);
  }

  static String _naiveMisspelling(String word) {
    final String lower = word.trim().toLowerCase();
    if (lower.length <= 2) {
      return '${lower}x';
    }
    final int dropAt = lower.length ~/ 2;
    return lower.substring(0, dropAt) + lower.substring(dropAt + 1);
  }

  static SpellingChallenge randomChallenge([Random? random]) {
    final Random rng = random ?? Random();
    final ({String correct, String misspelled}) pair =
        pairs[rng.nextInt(pairs.length)];
    final bool showCorrect = rng.nextBool();
    return SpellingChallenge(
      displayedWord: showCorrect ? pair.correct : pair.misspelled,
      isCorrectlySpelled: showCorrect,
      canonicalWord: pair.correct,
    );
  }
}