using Core.Entities;
using Core.Models;
using Data;
using Microsoft.EntityFrameworkCore;

namespace WebApp.Services;

/// <summary>
/// Replaces event rankings for students present in a successful ranking import.
/// </summary>
public class StudentEventRankingSaveService : IStudentEventRankingSaveService
{
    private readonly AppDbContext _context;
    private readonly ILogger<StudentEventRankingSaveService> _logger;

    public StudentEventRankingSaveService(AppDbContext context, ILogger<StudentEventRankingSaveService> logger)
    {
        _context = context;
        _logger = logger;
    }

    /// <inheritdoc />
    public async Task<IReadOnlyList<string>> GetStudentsWithExistingRankingsAsync(
        StudentEventRankingParseResult parseResult,
        CancellationToken cancellationToken = default)
    {
        var studentIds = GetStudentIds(parseResult);
        if (studentIds.Count == 0)
            return [];

        return await _context.Students
            .AsNoTracking()
            .Where(s => studentIds.Contains(s.Id) && s.EventRankings.Any())
            .OrderBy(s => s.LastName)
            .ThenBy(s => s.FirstName)
            .Select(s => s.FirstName + " " + s.LastName)
            .ToListAsync(cancellationToken);
    }

    /// <inheritdoc />
    public async Task<StudentEventRankingSaveResult> SaveAsync(
        StudentEventRankingParseResult parseResult,
        CancellationToken cancellationToken = default)
    {
        var studentIds = GetStudentIds(parseResult);
        if (studentIds.Count == 0)
            return new StudentEventRankingSaveResult();

        var students = await _context.Students
            .Include(s => s.EventRankings)
            .Where(s => studentIds.Contains(s.Id))
            .ToListAsync(cancellationToken);

        var eventIds = parseResult.Matches
            .Select(m => m.Ranking.EventDefinition.Id)
            .Where(id => id != 0)
            .Distinct()
            .ToList();

        var events = await _context.Events
            .Where(e => eventIds.Contains(e.Id))
            .ToDictionaryAsync(e => e.Id, cancellationToken);

        var matchesByStudent = parseResult.Matches
            .Where(m => m.Ranking.Student.Id != 0)
            .GroupBy(m => m.Ranking.Student.Id)
            .ToDictionary(g => g.Key, g => g.ToList());

        var rankingsSaved = 0;
        foreach (var student in students)
        {
            if (!matchesByStudent.TryGetValue(student.Id, out var matches))
                continue;

            student.EventRankings.Clear();
            foreach (var match in matches.OrderBy(m => m.Ranking.Rank))
            {
                if (!events.TryGetValue(match.Ranking.EventDefinition.Id, out var eventDefinition))
                {
                    _logger.LogWarning(
                        "Skipping ranking for student {StudentId}: event {EventId} was not found",
                        student.Id,
                        match.Ranking.EventDefinition.Id);
                    continue;
                }

                student.EventRankings.Add(new StudentEventRanking
                {
                    Student = student,
                    EventDefinition = eventDefinition,
                    Rank = match.Ranking.Rank
                });
                rankingsSaved++;
            }
        }

        await _context.SaveChangesAsync(cancellationToken);

        return new StudentEventRankingSaveResult
        {
            StudentsUpdated = students.Count,
            RankingsSaved = rankingsSaved
        };
    }

    private static List<int> GetStudentIds(StudentEventRankingParseResult parseResult) =>
        [.. parseResult.Matches
            .Select(m => m.Ranking.Student.Id)
            .Where(id => id != 0)
            .Distinct()];
}