step3/api/QuestionsApi/Models/ChoicePoolSelector.cs

namespace QuestionsApi.Models;

public static class ChoicePoolSelector
{
    private const int PresentedChoiceCount = 5;
    private const double MaximumRateMultiplier = 5.0;

    public static List<string> SelectChoiceIds(
        Question question,
        IReadOnlyDictionary<string, int>? liveSelectionPercentages = null,
        Random? random = null)
    {
        random ??= Random.Shared;
        var rates = liveSelectionPercentages is { Count: > 0 }
            ? liveSelectionPercentages
            : question.ChoiceSelectionPercentages;
        var correct = question.Choices.FirstOrDefault(choice => choice.Id == question.CorrectChoiceId);
        var remaining = question.Choices
            .Where(choice => choice.Id != question.CorrectChoiceId)
            .ToList();
        var selected = new List<QuestionChoice>();

        while (selected.Count < PresentedChoiceCount - 1 && remaining.Count > 0)
        {
            var totalWeight = remaining.Sum(choice => Weight(choice.Id, rates));
            var threshold = random.NextDouble() * totalWeight;
            var cumulative = 0.0;
            var selectedIndex = remaining.Count - 1;
            for (var index = 0; index < remaining.Count; index++)
            {
                cumulative += Weight(remaining[index].Id, rates);
                if (threshold < cumulative)
                {
                    selectedIndex = index;
                    break;
                }
            }
            selected.Add(remaining[selectedIndex]);
            remaining.RemoveAt(selectedIndex);
        }

        if (correct is not null)
            selected.Add(correct);
        else
            selected = question.Choices.Take(PresentedChoiceCount).ToList();

        Shuffle(selected, random);
        return selected.Select(choice => choice.Id).ToList();
    }

    private static double Weight(string choiceId, IReadOnlyDictionary<string, int>? rates)
    {
        var rate = Math.Clamp(rates?.GetValueOrDefault(choiceId, 0) ?? 0, 0, 100);
        return 1.0 + MaximumRateMultiplier * rate / 100.0;
    }

    private static void Shuffle<T>(IList<T> items, Random random)
    {
        for (var index = items.Count - 1; index > 0; index--)
        {
            var replacement = random.Next(index + 1);
            (items[index], items[replacement]) = (items[replacement], items[index]);
        }
    }
}