using LehrerApp.Core.Interfaces;
using LehrerApp.Core.Models;

namespace LehrerApp.Core.Services;

public sealed record GroupRolloverRequest(
    string Name,
    string TargetSchoolYear,
    int GradeLevel,
    IReadOnlyCollection<Guid> MembershipIds,
    bool CopyGradingScheme,
    bool ArchiveSource);

/// <summary>Erzeugt eine fachlich saubere Folgegruppe ohne Leistungs- oder Planungsdaten.</summary>
public sealed class GroupRolloverService(
    IGroupRepository groups,
    IGroupMembershipRepository memberships,
    IGradingSchemeRepository gradingSchemes,
    SchoolYearService schoolYears)
{
    public LearningGroup RollOver(LearningGroup source, GroupRolloverRequest request)
    {
        var targetSchoolYear = request.TargetSchoolYear.Trim();
        var targetStart = schoolYears.SchoolYearStart(targetSchoolYear);
        if (targetSchoolYear != schoolYears.FormatSchoolYear(targetStart.Year))
            throw new InvalidOperationException("Das Zielschuljahr muss das Format JJJJ/JJ haben, z.B. 2027/28.");
        var sourceStart = schoolYears.SchoolYearStart(source.SchoolYear);
        if (targetStart <= sourceStart)
            throw new InvalidOperationException("Das Zielschuljahr muss nach dem bisherigen Schuljahr liegen.");
        if (request.GradeLevel is < 1 or > 13)
            throw new InvalidOperationException("Die Klassenstufe muss zwischen 1 und 13 liegen.");
        if (string.IsNullOrWhiteSpace(request.Name))
            throw new InvalidOperationException("Bitte einen Gruppennamen angeben.");

        var duplicate = groups.GetBySchoolYear(targetSchoolYear, includeInactive: true)
            .Any(g => g.Name.Equals(request.Name.Trim(), StringComparison.OrdinalIgnoreCase)
                      && g.SubjectId == source.SubjectId && g.Type == source.Type);
        if (duplicate)
            throw new InvalidOperationException(
                "Im Zielschuljahr existiert bereits eine gleichnamige Gruppe mit demselben Fach und Typ.");

        var sourceMemberships = memberships.GetByGroup(source.Id)
            .ToDictionary(m => m.Id);
        var selected = request.MembershipIds.Distinct()
            .Select(id => sourceMemberships.GetValueOrDefault(id)
                ?? throw new InvalidOperationException("Eine ausgewählte Mitgliedschaft gehört nicht mehr zur Ausgangsgruppe."))
            .ToList();
        if (selected.Count == 0)
            throw new InvalidOperationException("Bitte mindestens einen Schüler auswählen.");

        var target = new LearningGroup
        {
            Name = request.Name.Trim(),
            Type = source.Type,
            SubjectId = source.SubjectId,
            SchoolYear = targetSchoolYear,
            GradeLevel = request.GradeLevel,
            GradingSystem = source.GradingSystem,
            HoursPerWeek = source.HoursPerWeek,
            IsActive = true,
            IsOwnClass = source.IsOwnClass,
            IsDifferentiated = source.IsDifferentiated,
            PreviousGroupId = source.Id,
            // WebUntisLessonId bewusst nicht übernommen: WebUntis vergibt sie pro Schuljahr neu,
            // eine übernommene alte lsid würde in der Folgegruppe stumm falsche Fehlzeiten liefern.
        };

        var sourceWasActive = source.IsActive;
        try
        {
            groups.Save(target);
            foreach (var membership in selected)
            {
                memberships.Save(new GroupMembership
                {
                    StudentId = membership.StudentId,
                    GroupId = target.Id,
                    AddedOn = DateOnly.FromDateTime(DateTime.Today),
                    Period = MembershipPeriod.FullYear,
                    JoinedAt = targetStart,
                    LeftAt = null,
                    Niveau = membership.Niveau,
                });
            }

            if (request.CopyGradingScheme && gradingSchemes.GetByGroup(source.Id) is { } scheme)
            {
                gradingSchemes.Save(new GradingScheme
                {
                    GroupId = target.Id,
                    ExamsPercent = scheme.ExamsPercent,
                    ParticipationPercent = scheme.ParticipationPercent,
                    OtherPercent = scheme.OtherPercent,
                });
            }

            if (request.ArchiveSource && source.IsActive)
            {
                source.IsActive = false;
                groups.Save(source);
            }
            return target;
        }
        catch
        {
            source.IsActive = sourceWasActive;
            groups.Delete(target.Id);
            throw;
        }
    }
}