using QuestionsApi.Models;

namespace QuestionsApi.Services;

public static class MasteryCalculator
{
    private static readonly IReadOnlyDictionary<int, double> DifficultyWeights = new Dictionary<int, double>
    {
        [1] = 0.6, [2] = 0.8, [3] = 1.0, [4] = 1.2, [5] = 1.4
    };

    public static List<DashboardSystemPerformance> Calculate(
        IEnumerable<QuestionAttempt> attempts,
        IEnumerable<QuestionReviewAttempt> reviews,
        IEnumerable<ReviewQueueItem> queueItems,
        IReadOnlyDictionary<string, Question> questions,
        DateTime nowUtc)
    {
        var mappedAttempts = attempts.Where(attempt => questions.ContainsKey(attempt.QuestionExternalId)).ToList();
        var answered = mappedAttempts.Where(attempt => !attempt.IsSkip).ToList();
        var maxCycle = mappedAttempts.Count == 0 ? 0 : mappedAttempts.Max(attempt => attempt.BankCycle);
        var reviewList = reviews.OrderBy(review => review.CreatedUtc).ToList();
        var queueList = queueItems.ToList();
        var results = new List<DashboardSystemPerformance>();
        foreach (var system in Step3SystemCatalog.All)
        {
            var systemAttempts = answered.Where(attempt =>
                Step3SystemCatalog.Normalize(questions[attempt.QuestionExternalId]) == system).ToList();
            var ids = systemAttempts.Select(attempt => attempt.QuestionExternalId).Distinct(StringComparer.Ordinal).ToHashSet(StringComparer.Ordinal);
            var systemReviews = reviewList.Where(review => ids.Contains(review.QuestionExternalId)).ToList();
            var pending = queueList.Count(item => item.State == ReviewQueueStates.Pending
                && questions.TryGetValue(item.QuestionExternalId, out var question)
                && Step3SystemCatalog.Normalize(question) == system);
            double earned = 0;
            double possible = 0;
            var advancedSuccesses = new HashSet<string>(StringComparer.Ordinal);
            foreach (var attempt in systemAttempts)
            {
                var question = questions[attempt.QuestionExternalId];
                var weight = DifficultyWeights.GetValueOrDefault(Math.Clamp(question.Difficulty, 1, 5), 1.0)
                    * Math.Pow(0.5, Math.Max(0, maxCycle - attempt.BankCycle));
                var outcome = attempt.IsCorrect ? 1.0 : RecoveryOutcome(attempt, systemAttempts, systemReviews);
                earned += weight * outcome;
                possible += weight;
                if (question.Difficulty >= 4 && outcome >= 0.75) advancedSuccesses.Add(question.ExternalId);
            }
            var score = possible == 0 ? 0 : (int)Math.Round(earned / possible * 100, MidpointRounding.AwayFromZero);
            var state = State(ids.Count, score, advancedSuccesses.Count, pending);
            var lastEvidence = systemAttempts.Select(value => (DateTime?)value.CreatedUtc)
                .Concat(systemReviews.Select(value => (DateTime?)value.CreatedUtc)).Max();
            var refreshDue = state is MasteryStates.Strong or MasteryStates.Mastered
                && lastEvidence.HasValue && lastEvidence.Value < nowUtc.AddDays(-30);
            results.Add(new DashboardSystemPerformance
            {
                System = system,
                Correct = systemAttempts.Count(value => value.IsCorrect),
                Total = systemAttempts.Count,
                Accuracy = DashboardStore.Percentage(systemAttempts.Count(value => value.IsCorrect), systemAttempts.Count),
                MasteryScore = score,
                MasteryState = state,
                DistinctQuestions = ids.Count,
                Difficulty4PlusSuccessful = advancedSuccesses.Count,
                PendingReviews = pending,
                LastEvidenceUtc = lastEvidence,
                RefreshDue = refreshDue,
                RecommendedAction = Action(state, pending, refreshDue)
            });
        }
        return results;
    }

    private static double RecoveryOutcome(
        QuestionAttempt attempt,
        IReadOnlyCollection<QuestionAttempt> attempts,
        IReadOnlyCollection<QuestionReviewAttempt> reviews)
    {
        var nextAttempt = attempts.Where(value => value.QuestionExternalId == attempt.QuestionExternalId
                && value.CreatedUtc > attempt.CreatedUtc)
            .OrderBy(value => value.CreatedUtc).FirstOrDefault();
        var recovery = reviews.Where(value => value.QuestionExternalId == attempt.QuestionExternalId
                && value.IsCorrect && value.CreatedUtc > attempt.CreatedUtc
                && (nextAttempt is null || value.CreatedUtc < nextAttempt.CreatedUtc))
            .OrderBy(value => value.CreatedUtc).FirstOrDefault();
        if (recovery is null) return 0;
        return recovery.CreatedUtc - attempt.CreatedUtc >= TimeSpan.FromHours(24) ? 0.75 : 0.5;
    }

    private static string State(int distinct, int score, int advanced, int pending)
    {
        if (distinct == 0) return MasteryStates.Unseen;
        if (distinct < 5) return MasteryStates.Learning;
        if (distinct >= 25 && score >= 85 && advanced >= 5 && pending == 0) return MasteryStates.Mastered;
        if (distinct >= 15 && score >= 75) return MasteryStates.Strong;
        return MasteryStates.Practicing;
    }

    private static DashboardMasteryAction Action(string state, int pending, bool refreshDue)
    {
        if (pending > 0) return new() { Type = "review", Label = "Review missed concepts", QuestionCount = pending };
        return state switch
        {
            MasteryStates.Unseen or MasteryStates.Learning => new() { Type = "practice", Label = "Start learning", QuestionCount = 10 },
            MasteryStates.Practicing => new() { Type = "practice", Label = "Build mastery", QuestionCount = 10, DifficultyMin = 3, DifficultyMax = 4 },
            MasteryStates.Strong => new() { Type = "practice", Label = "Challenge this system", QuestionCount = 10, DifficultyMin = 4, DifficultyMax = 5 },
            MasteryStates.Mastered when refreshDue => new() { Type = "practice", Label = "Refresh mastery", QuestionCount = 5, DifficultyMin = 4, DifficultyMax = 5 },
            _ => new() { Type = "practice", Label = "Maintain mastery", QuestionCount = 5, DifficultyMin = 4, DifficultyMax = 5 }
        };
    }
}