main.dart

import 'dart:math';

class PassphraseSet {
  List<String> choices;
  num index;

  PassphraseSet({required this.choices, required this.index});
  
  @override
  String toString() {
    return 'choices: $choices, index: $index\n';
  }
}

class UniqueIndexService {
  final _previousValues = [];
  List<int> indexesAvailable;
  
  UniqueIndexService({required this.indexesAvailable}) {
    indexesAvailable.shuffle();
  }
  
  int generateUniqueIndex() {
    return indexesAvailable.removeAt(indexesAvailable.length - 1);
  }
  
  clear() {
    _previousValues.clear();
  }
}

const words = ["word1", "word2", "word3", "word4", "word5", "word6", "word7", "word8", "word9", "word10", "word11", "word12", "word13", "word14", "word15", "word16", "word17", "word18", "word19", "word20", "word21", "word22", "word23", "word24"];


void main() {
  
  var uniqueIndexService = UniqueIndexService(indexesAvailable: List.generate(words.length, (index) => index));
  
  // 12 words, 4 sets of 3
  var sets = [
    generatePassphraseSet(uniqueIndexService),
    generatePassphraseSet(uniqueIndexService),
    generatePassphraseSet(uniqueIndexService),
    generatePassphraseSet(uniqueIndexService),
  ];
            
  print(sets);

}

PassphraseSet generatePassphraseSet(UniqueIndexService uniqueIndexService) {
  var itemset = [
      words[uniqueIndexService.generateUniqueIndex()], 
      words[uniqueIndexService.generateUniqueIndex()],
      words[uniqueIndexService.generateUniqueIndex()]];
  
  var rng = Random();
  var index = rng.nextInt(itemset.length);

  var indexOfWordInMainArray = words.indexOf(itemset[index]);
  return PassphraseSet(choices: itemset, index: indexOfWordInMainArray + 1);
}