import 'dart:math';

import 'spelling_item_record.dart';
import 'spelling_items_db.dart';
import 'spelling_progress_policy.dart';

/// Picks a shared spelling item for a user using familiarity-weighted randomness.
class SpellingItemSelector {
  SpellingItemSelector({
    SpellingItemsDb? itemsDb,
    Random? random,
  })  : _itemsDb = itemsDb,
        _random = random ?? Random();

  final SpellingItemsDb? _itemsDb;
  final Random _random;

  Future<SpellingItemRecord?> pickForUser(String firebaseUid) async {
    final SpellingItemsDb? itemsDb = _itemsDb;
    if (itemsDb == null) {
      throw StateError('SpellingItemSelector requires itemsDb for pickForUser');
    }
    final List<SpellingItemRecord> items =
        await itemsDb.listActiveWithProgress(firebaseUid);
    if (items.isEmpty) {
      return null;
    }
    return _weightedPick(items);
  }

  /// Visible for unit tests.
  SpellingItemRecord? pickFromCandidates(List<SpellingItemRecord> items) {
    if (items.isEmpty) {
      return null;
    }
    return _weightedPick(items);
  }

  SpellingItemRecord _weightedPick(List<SpellingItemRecord> items) {
    final List<double> weights = items
        .map(
          (SpellingItemRecord item) =>
              SpellingProgressPolicy.weightForScore(item.familiarityScore),
        )
        .toList(growable: false);
    final double total = weights.fold<double>(0, (double a, double b) => a + b);
    var roll = _random.nextDouble() * total;
    for (var i = 0; i < items.length; i++) {
      roll -= weights[i];
      if (roll < 0) {
        return items[i];
      }
    }
    return items.last;
  }
}