using GermanApp.Domain.Interfaces;

namespace GermanApp.Application.Services;

/// <summary>
/// Domain service for managing lesson unlocking business logic.
/// This is part of the Application layer.
/// </summary>
public class LessonUnlockService
{
    private readonly ILessonRepository _lessonRepository;
    private readonly IUserProgressRepository _userProgressRepository;

    public LessonUnlockService(
        ILessonRepository lessonRepository,
        IUserProgressRepository userProgressRepository)
    {
        _lessonRepository = lessonRepository;
        _userProgressRepository = userProgressRepository;
    }

    /// <summary>
    /// Checks if the next lesson in a level is unlocked for the user.
    /// A lesson is unlocked if:
    /// - It's the first lesson in the level, OR
    /// - The previous lesson in the level has been completed
    /// </summary>
    /// <param name="userId">The user ID</param>
    /// <param name="currentLessonId">The current lesson ID</param>
    /// <param name="cancellationToken">Cancellation token</param>
    /// <returns>True if the next lesson is unlocked, false otherwise</returns>
    public virtual async Task<bool> IsNextLessonUnlockedAsync(
        int userId,
        int currentLessonId,
        CancellationToken cancellationToken = default)
    {
        var currentLesson = await _lessonRepository.GetByIdAsync(currentLessonId, cancellationToken);
        if (currentLesson == null)
            return false;

        // If this is the first lesson in the level, the next one is always unlocked
        var firstLesson = await _lessonRepository.GetFirstLessonInLevelAsync(
            currentLesson.LevelId,
            cancellationToken);
        
        if (firstLesson?.Id == currentLessonId)
        {
            var nextLesson = await _lessonRepository.GetNextLessonAsync(currentLessonId, cancellationToken);
            return nextLesson != null;
        }

        // Check if the user has completed the current lesson
        var hasCompleted = await _userProgressRepository.HasUserCompletedLessonAsync(
            userId,
            currentLessonId,
            cancellationToken);
        
        return hasCompleted;
    }

    /// <summary>
    /// Gets the next lesson ID that should be unlocked for a user after completing a lesson.
    /// </summary>
    /// <param name="userId">The user ID</param>
    /// <param name="completedLessonId">The ID of the lesson that was just completed</param>
    /// <param name="cancellationToken">Cancellation token</param>
    /// <returns>The ID of the next lesson, or null if no more lessons in the level</returns>
    public virtual async Task<int?> GetNextLessonIdAsync(
        int userId,
        int completedLessonId,
        CancellationToken cancellationToken = default)
    {
        var currentLesson = await _lessonRepository.GetByIdAsync(completedLessonId, cancellationToken);
        if (currentLesson == null)
            return null;

        var nextLesson = await _lessonRepository.GetNextLessonAsync(completedLessonId, cancellationToken);
        return nextLesson?.Id;
    }

    /// <summary>
    /// Gets all lessons that are accessible to a user (completed lessons + next unlocked lesson).
    /// </summary>
    /// <param name="userId">The user ID</param>
    /// <param name="cancellationToken">Cancellation token</param>
    /// <returns>List of accessible lesson IDs</returns>
    public virtual async Task<IReadOnlyList<int>> GetAccessibleLessonIdsAsync(
        int userId,
        CancellationToken cancellationToken = default)
    {
        var lessons = await _lessonRepository.GetAccessibleLessonsAsync(userId, cancellationToken);
        return lessons.Select(l => l.Id).ToList();
    }

    /// <summary>
    /// Checks if a specific lesson is accessible to a user.
    /// </summary>
    /// <param name="userId">The user ID</param>
    /// <param name="lessonId">The lesson ID to check</param>
    /// <param name="cancellationToken">Cancellation token</param>
    /// <returns>True if the lesson is accessible, false otherwise</returns>
    public virtual async Task<bool> IsLessonAccessibleAsync(
        int userId,
        int lessonId,
        CancellationToken cancellationToken = default)
    {
        var accessibleLessons = await GetAccessibleLessonIdsAsync(userId, cancellationToken);
        return accessibleLessons.Contains(lessonId);
    }
}