DeutschLernen/GermanApp/Application/Services/LessonService.cs

using GermanApp.Application.DTOs;
using GermanApp.Domain.Entities;
using GermanApp.Domain.Interfaces;

namespace GermanApp.Application.Services;

/// <summary>
/// Application service for managing lessons.
/// This is part of the Application layer.
/// </summary>
public class LessonService
{
    private readonly ILessonRepository _lessonRepository;
    private readonly ILevelRepository _levelRepository;

    public LessonService(ILessonRepository lessonRepository, ILevelRepository levelRepository)
    {
        _lessonRepository = lessonRepository;
        _levelRepository = levelRepository;
    }

    /// <summary>
    /// Gets all lessons.
    /// </summary>
    public async Task<IReadOnlyList<LessonDto>> GetAllLessonsAsync(CancellationToken cancellationToken = default)
    {
        var lessons = await _lessonRepository.GetAllAsync(cancellationToken);
        return lessons.Select(l => l.ToDto()).ToList();
    }

    /// <summary>
    /// Gets all lessons ordered by level and lesson order.
    /// </summary>
    public async Task<IReadOnlyList<LessonDto>> GetAllOrderedLessonsAsync(CancellationToken cancellationToken = default)
    {
        var lessons = await _lessonRepository.GetAllOrderedAsync(cancellationToken);
        return lessons.Select(l => l.ToDto()).ToList();
    }

    /// <summary>
    /// Gets a lesson by its ID.
    /// </summary>
    public async Task<LessonDto?> GetLessonByIdAsync(int id, CancellationToken cancellationToken = default)
    {
        var lesson = await _lessonRepository.GetByIdAsync(id, cancellationToken);
        return lesson?.ToDto();
    }

    /// <summary>
    /// Gets lessons by level ID.
    /// </summary>
    public async Task<IReadOnlyList<LessonDto>> GetLessonsByLevelAsync(int levelId, CancellationToken cancellationToken = default)
    {
        var lessons = await _lessonRepository.GetByLevelAsync(levelId, cancellationToken);
        return lessons.Select(l => l.ToDto()).ToList();
    }

    /// <summary>
    /// Gets beginner lessons (A1 and A2 - levels 1 and 2).
    /// </summary>
    public async Task<IReadOnlyList<LessonDto>> GetBeginnerLessonsAsync(CancellationToken cancellationToken = default)
    {
        var level1Lessons = await _lessonRepository.GetByLevelAsync(1, cancellationToken);
        var level2Lessons = await _lessonRepository.GetByLevelAsync(2, cancellationToken);
        var allLessons = level1Lessons.Concat(level2Lessons)
            .OrderBy(l => l.LevelId)
            .ThenBy(l => l.Order)
            .ToList();
        return allLessons.Select(l => l.ToDto()).ToList();
    }

    /// <summary>
    /// Gets advanced lessons (B2 and C1 - levels 4 and 5).
    /// </summary>
    public async Task<IReadOnlyList<LessonDto>> GetAdvancedLessonsAsync(CancellationToken cancellationToken = default)
    {
        var level4Lessons = await _lessonRepository.GetByLevelAsync(4, cancellationToken);
        var level5Lessons = await _lessonRepository.GetByLevelAsync(5, cancellationToken);
        var allLessons = level4Lessons.Concat(level5Lessons)
            .OrderBy(l => l.LevelId)
            .ThenBy(l => l.Order)
            .ToList();
        return allLessons.Select(l => l.ToDto()).ToList();
    }

    /// <summary>
    /// Creates a new lesson.
    /// </summary>
    public async Task<LessonDto> CreateLessonAsync(CreateLessonDto dto, CancellationToken cancellationToken = default)
    {
        // Validate that the level exists
        var level = await _levelRepository.GetByIdAsync(dto.LevelId, cancellationToken);
        if (level == null)
            throw new ArgumentException("Level does not exist");

        var lesson = dto.ToEntity();
        var createdLesson = await _lessonRepository.AddAsync(lesson, cancellationToken);
        return createdLesson.ToDto();
    }

    /// <summary>
    /// Updates an existing lesson.
    /// </summary>
    public async Task<LessonDto?> UpdateLessonAsync(int id, UpdateLessonDto dto, CancellationToken cancellationToken = default)
    {
        var lesson = await _lessonRepository.GetByIdAsync(id, cancellationToken);
        if (lesson == null)
            return null;

        // Validate that the level exists
        var level = await _levelRepository.GetByIdAsync(dto.LevelId, cancellationToken);
        if (level == null)
            throw new ArgumentException("Level does not exist");

        lesson.UpdateFromDto(dto);
        await _lessonRepository.UpdateAsync(lesson, cancellationToken);
        return lesson.ToDto();
    }

    /// <summary>
    /// Deletes a lesson by its ID.
    /// </summary>
    public async Task<bool> DeleteLessonAsync(int id, CancellationToken cancellationToken = default)
    {
        var lesson = await _lessonRepository.GetByIdAsync(id, cancellationToken);
        if (lesson == null)
            return false;

        await _lessonRepository.DeleteAsync(lesson, cancellationToken);
        return true;
    }

    /// <summary>
    /// Gets the first lesson in a level.
    /// </summary>
    public async Task<LessonDto?> GetFirstLessonInLevelAsync(int levelId, CancellationToken cancellationToken = default)
    {
        var lesson = await _lessonRepository.GetFirstLessonInLevelAsync(levelId, cancellationToken);
        return lesson?.ToDto();
    }

    /// <summary>
    /// Gets the next lesson after the specified one.
    /// </summary>
    public async Task<LessonDto?> GetNextLessonAsync(int currentLessonId, CancellationToken cancellationToken = default)
    {
        var lesson = await _lessonRepository.GetNextLessonAsync(currentLessonId, cancellationToken);
        return lesson?.ToDto();
    }

    /// <summary>
    /// Gets lessons that the user can access (based on completion of previous lessons).
    /// </summary>
    public async Task<IReadOnlyList<LessonDto>> GetAccessibleLessonsAsync(int userId, CancellationToken cancellationToken = default)
    {
        var lessons = await _lessonRepository.GetAccessibleLessonsAsync(userId, cancellationToken);
        return lessons.Select(l => l.ToDto()).ToList();
    }
}