using Microsoft.EntityFrameworkCore;
using System.Text.Json;
using Tiku.Application.Learning;
using Tiku.Domain.Content;
using Tiku.Domain.Learning;
using Tiku.Domain.QuestionBanks;
using Tiku.Infrastructure.Persistence;

namespace Tiku.Infrastructure.Learning;

public sealed class LearningActivityService(TikuDbContext dbContext) : ILearningActivityService
{
    private const int DefaultLimit = 100;
    private const int MaxLimit = 500;

    public async Task<AnswerRecordItem> SubmitAnswerAsync(
        LearningActor actor,
        SubmitAnswerCommand command,
        CancellationToken cancellationToken = default)
    {
        var question = await dbContext.Questions
            .AsNoTracking()
            .Where(item =>
                item.TenantId == actor.TenantId &&
                item.Id == command.QuestionId &&
                item.Status == QuestionStatus.Published)
            .Select(item => new
            {
                item.Id,
                item.CurrentVersionId
            })
            .SingleOrDefaultAsync(cancellationToken);

        if (question is null)
        {
            throw new LearningResourceNotFoundException("question_not_found", "Question was not found.");
        }

        if (command.PracticeSessionId.HasValue)
        {
            var sessionExists = await dbContext.PracticeSessions.AnyAsync(
                session =>
                    session.TenantId == actor.TenantId &&
                    session.UserId == actor.UserId &&
                    session.Id == command.PracticeSessionId.Value,
                cancellationToken);

            if (!sessionExists)
            {
                throw new LearningResourceNotFoundException("practice_session_not_found", "Practice session was not found.");
            }
        }

        var now = DateTimeOffset.UtcNow;
        var record = new AnswerRecord
        {
            TenantId = actor.TenantId,
            UserId = actor.UserId,
            QuestionId = question.Id,
            QuestionVersionId = question.CurrentVersionId,
            PracticeSessionId = command.PracticeSessionId,
            SelectedOptions = JsonSerializer.SerializeToElement(command.SelectedOptions ?? []),
            AnswerText = command.AnswerText,
            IsCorrect = command.SelfJudgedCorrect,
            AnsweredAt = now,
            CreatedAt = now
        };
        dbContext.AnswerRecords.Add(record);

        if (command.SelfJudgedCorrect == false)
        {
            var wrongQuestion = await dbContext.WrongQuestions.FindAsync(
                [actor.TenantId, actor.UserId, question.Id],
                cancellationToken);

            if (wrongQuestion is null)
            {
                dbContext.WrongQuestions.Add(new WrongQuestion
                {
                    TenantId = actor.TenantId,
                    UserId = actor.UserId,
                    QuestionId = question.Id,
                    WrongCount = 1,
                    LastWrongAt = now
                });
            }
            else
            {
                wrongQuestion.WrongCount++;
                wrongQuestion.LastWrongAt = now;
                wrongQuestion.ResolvedAt = null;
            }
        }

        await dbContext.SaveChangesAsync(cancellationToken);
        return ToItem(record);
    }

    public async Task<LearningList<FavoriteQuestionItem>> GetFavoriteQuestionsAsync(
        LearningActor actor,
        LearningLimitFilter filter,
        CancellationToken cancellationToken = default)
    {
        var items = await dbContext.FavoriteQuestions
            .AsNoTracking()
            .Where(item =>
                item.TenantId == actor.TenantId &&
                item.UserId == actor.UserId)
            .OrderByDescending(item => item.CreatedAt)
            .Take(ResolveLimit(filter.Limit))
            .Select(item => new FavoriteQuestionItem(
                item.QuestionId,
                item.Source,
                item.CreatedAt))
            .ToArrayAsync(cancellationToken);

        return new LearningList<FavoriteQuestionItem>(items);
    }

    public async Task<LearningActionResult> ToggleFavoriteQuestionAsync(
        LearningActor actor,
        QuestionActionCommand command,
        CancellationToken cancellationToken = default)
    {
        await EnsureQuestionExistsAsync(actor.TenantId, command.QuestionId, cancellationToken);

        var favorite = command.Favorite ?? true;
        var item = await dbContext.FavoriteQuestions.FindAsync(
            [actor.TenantId, actor.UserId, command.QuestionId],
            cancellationToken);

        if (favorite)
        {
            if (item is null)
            {
                dbContext.FavoriteQuestions.Add(new FavoriteQuestion
                {
                    TenantId = actor.TenantId,
                    UserId = actor.UserId,
                    QuestionId = command.QuestionId,
                    Source = "api",
                    CreatedAt = DateTimeOffset.UtcNow
                });
            }
        }
        else if (item is not null)
        {
            dbContext.FavoriteQuestions.Remove(item);
        }

        await dbContext.SaveChangesAsync(cancellationToken);
        return new LearningActionResult(true, favorite);
    }

    public async Task<LearningList<WrongQuestionItem>> GetWrongQuestionsAsync(
        LearningActor actor,
        LearningLimitFilter filter,
        CancellationToken cancellationToken = default)
    {
        var query = dbContext.WrongQuestions
            .AsNoTracking()
            .Where(item =>
                item.TenantId == actor.TenantId &&
                item.UserId == actor.UserId);

        if (!string.Equals(filter.Status, "all", StringComparison.OrdinalIgnoreCase))
        {
            query = query.Where(item => item.ResolvedAt == null);
        }

        var items = await query
            .OrderByDescending(item => item.LastWrongAt)
            .Take(ResolveLimit(filter.Limit))
            .Select(item => new WrongQuestionItem(
                item.QuestionId,
                item.WrongCount,
                item.LastWrongAt,
                item.ResolvedAt))
            .ToArrayAsync(cancellationToken);

        return new LearningList<WrongQuestionItem>(items);
    }

    public async Task<LearningActionResult> ResolveWrongQuestionAsync(
        LearningActor actor,
        QuestionActionCommand command,
        CancellationToken cancellationToken = default)
    {
        var item = await dbContext.WrongQuestions.FindAsync(
            [actor.TenantId, actor.UserId, command.QuestionId],
            cancellationToken);

        if (item is null)
        {
            throw new LearningResourceNotFoundException("wrong_question_not_found", "Wrong question was not found.");
        }

        item.ResolvedAt = DateTimeOffset.UtcNow;
        await dbContext.SaveChangesAsync(cancellationToken);
        return new LearningActionResult(true);
    }

    public async Task<LearningList<WordProgressItem>> GetWordProgressAsync(
        LearningActor actor,
        LearningLimitFilter filter,
        CancellationToken cancellationToken = default)
    {
        var query = dbContext.UserWordProgress
            .AsNoTracking()
            .Where(item =>
                item.TenantId == actor.TenantId &&
                item.UserId == actor.UserId);

        if (filter.UnitId.HasValue)
        {
            query = query.Where(item => dbContext.VocabularyWords.Any(word =>
                word.TenantId == actor.TenantId &&
                word.Id == item.WordId &&
                word.UnitId == filter.UnitId.Value));
        }

        if (TryParseWordProgressStatus(filter.Status, out var status))
        {
            query = query.Where(item => item.Status == status);
        }

        var items = await query
            .OrderBy(item => item.NextReviewAt == null)
            .ThenBy(item => item.NextReviewAt)
            .ThenByDescending(item => item.UpdatedAt)
            .Take(ResolveLimit(filter.Limit))
            .Select(item => new WordProgressItem(
                item.WordId,
                item.Status,
                item.CorrectCount,
                item.WrongCount,
                item.LastReviewAt,
                item.NextReviewAt,
                item.ReviewCount,
                item.CorrectStreak,
                item.LastResult,
                item.DueLevel,
                item.Metadata))
            .ToArrayAsync(cancellationToken);

        return new LearningList<WordProgressItem>(items);
    }

    public async Task<WordProgressItem> UpdateWordProgressAsync(
        LearningActor actor,
        WordProgressCommand command,
        CancellationToken cancellationToken = default)
    {
        await EnsureWordExistsAsync(actor.TenantId, command.WordId, cancellationToken);

        var now = DateTimeOffset.UtcNow;
        var item = await dbContext.UserWordProgress
            .SingleOrDefaultAsync(
                progress =>
                    progress.TenantId == actor.TenantId &&
                    progress.UserId == actor.UserId &&
                    progress.WordId == command.WordId,
                cancellationToken);

        if (item is null)
        {
            item = new UserWordProgress
            {
                TenantId = actor.TenantId,
                UserId = actor.UserId,
                WordId = command.WordId
            };
            dbContext.UserWordProgress.Add(item);
        }

        if (TryParseWordProgressStatus(command.Status, out var status))
        {
            item.Status = status;
        }
        else if (string.IsNullOrWhiteSpace(command.Status))
        {
            item.Status = WordProgressStatus.Learning;
        }
        else
        {
            throw new LearningValidationException("invalid_word_status", "Word progress status is invalid.");
        }

        var correctDelta = command.CorrectDelta ?? 0;
        var wrongDelta = command.WrongDelta ?? 0;
        item.CorrectCount += correctDelta;
        item.WrongCount += wrongDelta;
        item.ReviewCount += correctDelta + wrongDelta;
        item.CorrectStreak = wrongDelta > 0
            ? 0
            : item.CorrectStreak + correctDelta;
        item.LastResult = wrongDelta > 0
            ? WordReviewResult.Wrong
            : correctDelta > 0
                ? WordReviewResult.Correct
                : item.LastResult;
        item.LastReviewAt = correctDelta + wrongDelta > 0 ? now : item.LastReviewAt;
        item.NextReviewAt = command.NextReviewAt ?? item.NextReviewAt;
        item.DueLevel = item.Status == WordProgressStatus.Mastered
            ? WordDueLevel.Mastered
            : item.WrongCount > 0 && item.CorrectStreak == 0
                ? WordDueLevel.Again
                : item.DueLevel;

        await dbContext.SaveChangesAsync(cancellationToken);
        return ToItem(item);
    }

    public async Task<LearningList<FavoriteWordItem>> GetFavoriteWordsAsync(
        LearningActor actor,
        LearningLimitFilter filter,
        CancellationToken cancellationToken = default)
    {
        var query = dbContext.UserWordFavorites
            .AsNoTracking()
            .Where(item =>
                item.TenantId == actor.TenantId &&
                item.UserId == actor.UserId);

        if (filter.UnitId.HasValue)
        {
            query = query.Where(item => dbContext.VocabularyWords.Any(word =>
                word.TenantId == actor.TenantId &&
                word.Id == item.WordId &&
                word.UnitId == filter.UnitId.Value));
        }

        var items = await query
            .OrderByDescending(item => item.FavoritedAt ?? item.CreatedAt)
            .Take(ResolveLimit(filter.Limit))
            .Select(item => new FavoriteWordItem(
                item.WordId,
                item.Note,
                item.FavoritedAt))
            .ToArrayAsync(cancellationToken);

        return new LearningList<FavoriteWordItem>(items);
    }

    public async Task<LearningActionResult> ToggleFavoriteWordAsync(
        LearningActor actor,
        FavoriteWordCommand command,
        CancellationToken cancellationToken = default)
    {
        await EnsureWordExistsAsync(actor.TenantId, command.WordId, cancellationToken);

        var favorite = command.Favorite ?? true;
        var item = await dbContext.UserWordFavorites
            .SingleOrDefaultAsync(
                favoriteWord =>
                    favoriteWord.TenantId == actor.TenantId &&
                    favoriteWord.UserId == actor.UserId &&
                    favoriteWord.WordId == command.WordId,
                cancellationToken);

        if (favorite)
        {
            if (item is null)
            {
                item = new UserWordFavorite
                {
                    TenantId = actor.TenantId,
                    UserId = actor.UserId,
                    WordId = command.WordId
                };
                dbContext.UserWordFavorites.Add(item);
            }

            item.Note = command.Note ?? item.Note;
            item.FavoritedAt ??= DateTimeOffset.UtcNow;
        }
        else if (item is not null)
        {
            dbContext.UserWordFavorites.Remove(item);
        }

        await dbContext.SaveChangesAsync(cancellationToken);
        return new LearningActionResult(true, favorite);
    }

    private async Task EnsureQuestionExistsAsync(
        Guid tenantId,
        Guid questionId,
        CancellationToken cancellationToken)
    {
        var exists = await dbContext.Questions.AnyAsync(
            question =>
                question.TenantId == tenantId &&
                question.Id == questionId &&
                question.Status == QuestionStatus.Published,
            cancellationToken);

        if (!exists)
        {
            throw new LearningResourceNotFoundException("question_not_found", "Question was not found.");
        }
    }

    private async Task EnsureWordExistsAsync(
        Guid tenantId,
        Guid wordId,
        CancellationToken cancellationToken)
    {
        var exists = await dbContext.VocabularyWords.AnyAsync(
            word =>
                word.TenantId == tenantId &&
                word.Id == wordId &&
                word.IsActive,
            cancellationToken);

        if (!exists)
        {
            throw new LearningResourceNotFoundException("word_not_found", "Word was not found.");
        }
    }

    private static AnswerRecordItem ToItem(AnswerRecord record)
    {
        return new AnswerRecordItem(
            record.Id,
            record.QuestionId!.Value,
            record.QuestionVersionId,
            record.PracticeSessionId,
            record.SelectedOptions,
            record.AnswerText,
            record.IsCorrect,
            record.AnsweredAt);
    }

    private static WordProgressItem ToItem(UserWordProgress item)
    {
        return new WordProgressItem(
            item.WordId,
            item.Status,
            item.CorrectCount,
            item.WrongCount,
            item.LastReviewAt,
            item.NextReviewAt,
            item.ReviewCount,
            item.CorrectStreak,
            item.LastResult,
            item.DueLevel,
            item.Metadata);
    }

    private static bool TryParseWordProgressStatus(string? value, out WordProgressStatus status)
    {
        return Enum.TryParse(NormalizeEnumValue(value), ignoreCase: true, out status);
    }

    private static int ResolveLimit(int? limit)
    {
        return Math.Clamp(limit ?? DefaultLimit, 1, MaxLimit);
    }

    private static string? NormalizeEnumValue(string? value)
    {
        return string.IsNullOrWhiteSpace(value)
            ? null
            : value.Replace("_", string.Empty, StringComparison.Ordinal)
                .Replace("-", string.Empty, StringComparison.Ordinal);
    }
}

public class LearningException(string code, string message) : Exception(message)
{
    public string Code { get; } = code;
}

public sealed class LearningResourceNotFoundException(string code, string message) : LearningException(code, message);

public sealed class LearningValidationException(string code, string message) : LearningException(code, message);