using LiteDB;
using LehrerApp.Core.Models;

namespace LehrerApp.Data;

/// Wird nach jedem Save/Delete einer Entität aufgerufen; payload ist die gespeicherte Entität
/// bzw. null bei Delete. Sync-agnostisch – siehe <see cref="LiteDbContext.OnChange"/>.
public delegate void ChangeHandler(string entityType, string entityId, string operation, object? payload);

/// <summary>
/// Zentrale LiteDB-Verbindung. Singleton – eine Datei = ein Nutzer.
/// </summary>
public class LiteDbContext : IDisposable
{
    /// Aktuelle Schema-Version. Migrationsschritte werden versioniert unter
    /// <see cref="RunVersionedMigrations"/> ergänzt, statt bei jedem Start erneut
    /// (idempotent, aber unnötig) über alle Daten zu laufen.
    private const int CurrentSchemaVersion = 4;

    private readonly LiteDatabase _db;

    public LiteDbContext(string databasePath, string? password = null)
    {
        _db = new LiteDatabase(new ConnectionString(databasePath)
        {
            Connection = ConnectionType.Shared,
            Password   = password,
        });
        RunVersionedMigrations();
        EnsureIndexes();
    }

    /// Für Tests: In-Memory-Datenbank ohne Datei auf der Festplatte.
    public LiteDbContext(Stream stream)
    {
        _db = new LiteDatabase(stream);
        RunVersionedMigrations();
        EnsureIndexes();
    }

    public ILiteCollection<Student>       Students      => _db.GetCollection<Student>("students");
    public ILiteCollection<LearningGroup> Groups        => _db.GetCollection<LearningGroup>("groups");
    public ILiteCollection<SeatingPlan>   SeatingPlans  => _db.GetCollection<SeatingPlan>("seating_plans");
    public ILiteCollection<GroupMembership> Memberships => _db.GetCollection<GroupMembership>("group_memberships");
    public ILiteCollection<Exam>          Exams         => _db.GetCollection<Exam>("exams");
    public ILiteCollection<ExamResult>    ExamResults   => _db.GetCollection<ExamResult>("exam_results");
    public ILiteCollection<Grade>         Grades        => _db.GetCollection<Grade>("grades");
    public ILiteCollection<GradingScheme> GradingSchemes => _db.GetCollection<GradingScheme>("grading_schemes");
    public ILiteCollection<ReportGrade>   ReportGrades  => _db.GetCollection<ReportGrade>("report_grades");
    public ILiteCollection<GradingKeyTemplate> GradingKeyTemplates => _db.GetCollection<GradingKeyTemplate>("grading_key_templates");
    public ILiteCollection<Unit>          Units         => _db.GetCollection<Unit>("units");
    public ILiteCollection<Lesson>        Lessons       => _db.GetCollection<Lesson>("lessons");
    public ILiteCollection<Documentation> Documentation => _db.GetCollection<Documentation>("documentation");
    public ILiteStorage<string> Attachments => _db.GetStorage<string>("attachments", "attachments_chunks");
    public ILiteCollection<WorkTask>           Tasks              => _db.GetCollection<WorkTask>("tasks");
    public ILiteCollection<TimeEntry>          TimeEntries        => _db.GetCollection<TimeEntry>("time_entries");
    public ILiteCollection<ParticipationSession> ParticipationSessions => _db.GetCollection<ParticipationSession>("participation_sessions");
    public ILiteCollection<ParticipationEntry>   ParticipationEntries  => _db.GetCollection<ParticipationEntry>("participation");
    public ILiteCollection<ParticipationAspect>  ParticipationAspects  => _db.GetCollection<ParticipationAspect>("participation_aspects");
    public ILiteCollection<ParticipationSection> ParticipationSections => _db.GetCollection<ParticipationSection>("participation_sections");
    public ILiteCollection<Subject>              Subjects              => _db.GetCollection<Subject>("subjects");
    public ILiteCollection<CompetencyDomain>     CompetencyDomains     => _db.GetCollection<CompetencyDomain>("competency_domains");
    public ILiteCollection<ShorthandCode>        ShorthandCodes        => _db.GetCollection<ShorthandCode>("shorthand_codes");
    public ILiteCollection<AlternativeLessonPath> AlternativeLessonPaths => _db.GetCollection<AlternativeLessonPath>("alternative_lesson_paths");
    public ILiteCollection<TimetableSlot>  TimetableSlots  => _db.GetCollection<TimetableSlot>("timetable_slots");
    public ILiteCollection<SchoolHoliday>  SchoolHolidays  => _db.GetCollection<SchoolHoliday>("school_holidays");
    public ILiteCollection<SupervisionDuty> SupervisionDuties => _db.GetCollection<SupervisionDuty>("supervision_duties");
    public ILiteCollection<SubstitutionEntry> SubstitutionEntries => _db.GetCollection<SubstitutionEntry>("substitution_entries");

    public void Checkpoint() => _db.Checkpoint();

    public int SchemaVersion => ReadSchemaVersion();

    /// Wird von den Repositories nach jedem Save/Delete aufgerufen (payload = die gespeicherte
    /// Entität bzw. null bei Delete). Bewusst hier statt in LehrerApp.Sync definiert, damit Data
    /// weiterhin ohne Verweis auf Sync auskommt — die eigentliche Sync-Anbindung setzt diesen
    /// Hook von außen (siehe AppBootstrapper).
    public ChangeHandler? OnChange { get; set; }

    /// Führt dieselbe Kaskade wie <c>GroupRepository.Delete</c> aus. Hier auf dem Context statt
    /// im Repository, damit ein später eingehendes Sync-Ereignis (Baustein 5) dieselbe Kaskade
    /// nachvollziehen kann, ohne über die Repository-Save/Delete-Methoden (und damit erneut über
    /// <see cref="OnChange"/>) zu laufen.
    internal void CascadeDeleteGroup(Guid id)
    {
        ExecuteInTransaction(() =>
        {
            foreach (var membership in Memberships.Find(e => e.GroupId == id).ToList())
                Memberships.Delete(membership.Id);

            foreach (var exam in Exams.Find(e => e.GroupId == id).ToList())
            {
                foreach (var result in ExamResults.Find(r => r.ExamId == exam.Id).ToList())
                    ExamResults.Delete(result.Id);
                Exams.Delete(exam.Id);
            }

            foreach (var grade in Grades.Find(g => g.GroupId == id).ToList())
                Grades.Delete(grade.Id);

            foreach (var reportGrade in ReportGrades.Find(g => g.GroupId == id).ToList())
                ReportGrades.Delete(reportGrade.Id);

            foreach (var scheme in GradingSchemes.Find(s => s.GroupId == id).ToList())
                GradingSchemes.Delete(scheme.Id);

            foreach (var unit in Units.Find(u => u.GroupId == id).ToList())
            {
                foreach (var lesson in Lessons.Find(l => l.UnitId == unit.Id).ToList())
                    Lessons.Delete(lesson.Id);
                Units.Delete(unit.Id);
            }

            foreach (var lesson in Lessons.Find(l => l.GroupId == id).ToList())
                Lessons.Delete(lesson.Id);

            foreach (var session in ParticipationSessions.Find(s => s.GroupId == id).ToList())
            {
                foreach (var entry in ParticipationEntries.Find(e => e.SessionId == session.Id).ToList())
                    ParticipationEntries.Delete(entry.Id);
                ParticipationSessions.Delete(session.Id);
            }

            foreach (var aspect in ParticipationAspects.Find(a => a.GroupId == id).ToList())
                ParticipationAspects.Delete(aspect.Id);

            foreach (var section in ParticipationSections.Find(s => s.GroupId == id).ToList())
                ParticipationSections.Delete(section.Id);

            foreach (var plan in SeatingPlans.Find(p => p.GroupId == id).ToList())
                SeatingPlans.Delete(plan.Id);

            // Dokumentation und Arbeitszeit sind historische Nachweise. Sie bleiben erhalten,
            // werden aber von der nicht mehr existierenden Lerngruppe entkoppelt.
            foreach (var documentation in Documentation.Find(d => d.GroupId == id).ToList())
            {
                documentation.GroupId = null;
                documentation.UpdatedAt = DateTime.UtcNow;
                Documentation.Update(documentation);
            }

            foreach (var task in Tasks.Find(t => t.GroupId == id).ToList())
            {
                task.GroupId = null;
                task.UpdatedAt = DateTime.UtcNow;
                Tasks.Update(task);
            }

            foreach (var timeEntry in TimeEntries.Find(t => t.GroupId == id).ToList())
            {
                timeEntry.GroupId = null;
                TimeEntries.Update(timeEntry);
            }

            Groups.Delete(id);
        });
    }

    /// Führt dieselbe Kaskade wie <c>ExamRepository.Delete</c> aus (siehe <see cref="CascadeDeleteGroup"/>).
    internal void CascadeDeleteExam(Guid id)
    {
        foreach (var result in ExamResults.Find(r => r.ExamId == id).ToList())
            ExamResults.Delete(result.Id);
        Exams.Delete(id);
    }

    /// Führt dieselbe Kaskade wie <c>ParticipationSessionRepository.Delete</c> aus (siehe
    /// <see cref="CascadeDeleteGroup"/>).
    internal void CascadeDeleteParticipationSession(Guid id)
    {
        ParticipationSessions.Delete(id);
        foreach (var e in ParticipationEntries.Find(e => e.SessionId == id).ToList())
            ParticipationEntries.Delete(e.Id);
    }

    /// Führt dieselbe Kaskade wie <c>DocumentationRepository.HardDelete</c> aus (siehe
    /// <see cref="CascadeDeleteGroup"/>).
    internal void CascadeHardDeleteDocumentation(Guid id)
    {
        if (Documentation.FindById(id) is { } doc)
            foreach (var attachment in doc.Attachments) Attachments.Delete(attachment.StorageId);
        Documentation.Delete(id);
    }

    internal void ExecuteInTransaction(Action action)
    {
        _db.BeginTrans();
        try
        {
            action();
            _db.Commit();
        }
        catch
        {
            _db.Rollback();
            throw;
        }
    }

    private void RunVersionedMigrations()
    {
        var version = ReadSchemaVersion();
        if (version < 1)
        {
            MigrateExistingData();
            version = 1;
        }
        if (version < 2)
        {
            MigrateLessonPhases();
            version = 2;
        }
        if (version < 3)
        {
            MigrateLessonShorthand();
            version = 3;
        }
        if (version < 4)
        {
            MigrateLessonAlternativePaths();
            version = 4;
        }
        WriteSchemaVersion(version);
    }

    private int ReadSchemaVersion()
    {
        var meta = _db.GetCollection<BsonDocument>("meta").FindById(1);
        return meta?["SchemaVersion"].AsInt32 ?? 0;
    }

    private void WriteSchemaVersion(int version) =>
        _db.GetCollection<BsonDocument>("meta").Upsert(new BsonDocument
        {
            ["_id"] = 1,
            ["SchemaVersion"] = version,
        });

    private void MigrateExistingData()
    {
        MigrateMemberships();
        RemoveRedundantLegacyFields();

        var groups = _db.GetCollection<BsonDocument>("groups");
        var subjects = _db.GetCollection<Subject>("subjects");
        var missingArchiveState = groups.FindAll()
            .Where(g => !g.ContainsKey(nameof(LearningGroup.IsActive)))
            .ToList();
        foreach (var group in missingArchiveState)
        {
            group[nameof(LearningGroup.IsActive)] = true;
            groups.Update(group);
        }

        // Subject war anfangs nur als Text in der Gruppe gespeichert. Alte
        // Werte werden einmalig an die Fachstammdaten angebunden und danach
        // entfernt, damit es nur noch eine führende Fachbezeichnung gibt.
        foreach (var group in groups.FindAll().ToList())
        {
            var changed = false;
            var legacyName = group.TryGetValue("Subject", out var value) && value.IsString
                ? value.AsString.Trim()
                : "";
            var hasSubjectId = group.TryGetValue(nameof(LearningGroup.SubjectId), out var id)
                               && id.IsGuid && subjects.FindById(id.AsGuid) is not null;

            if (!hasSubjectId && legacyName.Length > 0)
            {
                var subject = subjects.FindAll().FirstOrDefault(s =>
                    string.Equals(s.Name.Trim(), legacyName, StringComparison.OrdinalIgnoreCase));
                if (subject is null)
                {
                    subject = new Subject { Name = legacyName };
                    subjects.Insert(subject);
                }
                group[nameof(LearningGroup.SubjectId)] = subject.Id;
                changed = true;
            }
            else if (!hasSubjectId && group.ContainsKey(nameof(LearningGroup.SubjectId)))
            {
                group[nameof(LearningGroup.SubjectId)] = BsonValue.Null;
                changed = true;
            }

            changed |= group.Remove("Subject");
            if (changed) groups.Update(group);
        }
    }

    private void MigrateMemberships()
    {
        const string oldName = "enrollments";
        const string newName = "group_memberships";
        var names = _db.GetCollectionNames().ToHashSet(StringComparer.OrdinalIgnoreCase);
        if (names.Contains(oldName) && !names.Contains(newName))
            _db.RenameCollection(oldName, newName);

        var memberships = _db.GetCollection<BsonDocument>(newName);
        foreach (var membership in memberships.FindAll().ToList())
        {
            var changed = false;
            if (!membership.ContainsKey(nameof(GroupMembership.AddedOn)) &&
                membership.TryGetValue("EnrolledAt", out var enrolledAt))
            {
                membership[nameof(GroupMembership.AddedOn)] = enrolledAt;
                changed = true;
            }
            changed |= membership.Remove("EnrolledAt");
            changed |= membership.Remove("SchoolYear");
            if (changed) memberships.Update(membership);
        }
    }

    /// Fasst die frühere flache Struktur einer Stunde (einzelnes Phase-Textfeld, Methoden-/
    /// Materialien-Listen) verlustfrei in eine einzige <see cref="LessonPhaseStep"/>-Zeile der
    /// neuen Verlaufsplan-Tabelle zusammen. Nur die alten Felder werden gelesen — die typisierte
    /// <see cref="Lesson"/>-Klasse kennt sie nicht mehr, ein Zugriff über die typisierte
    /// Lessons-Collection würde sie beim Deserialisieren bereits verwerfen.
    private void MigrateLessonPhases()
    {
        var lessons = _db.GetCollection<BsonDocument>("lessons");
        foreach (var lesson in lessons.FindAll().ToList())
        {
            if (lesson.ContainsKey(nameof(Lesson.Phases))) continue;

            var phase = lesson.TryGetValue("Phase", out var p) && p.IsString ? p.AsString.Trim() : "";
            var methods = lesson.TryGetValue("Methods", out var m) && m.IsArray
                ? string.Join("; ", m.AsArray.Select(x => x.AsString)) : "";
            var materials = lesson.TryGetValue("Materials", out var mat) && mat.IsArray
                ? string.Join(", ", mat.AsArray.Select(x => x.AsString)) : "";

            var phases = new BsonArray();
            if (phase.Length > 0 || methods.Length > 0 || materials.Length > 0)
            {
                phases.Add(new BsonDocument
                {
                    [nameof(LessonPhaseStep.Id)]              = Guid.NewGuid(),
                    [nameof(LessonPhaseStep.Name)]             = phase,
                    [nameof(LessonPhaseStep.DurationMinutes)]  = 0,
                    [nameof(LessonPhaseStep.Activity)]         = methods,
                    [nameof(LessonPhaseStep.Material)]         = materials,
                    [nameof(LessonPhaseStep.Shorthand)]        = "",
                });
            }
            lesson[nameof(Lesson.Phases)] = phases;
            lesson.Remove("Phase");
            lesson.Remove("Methods");
            lesson.Remove("Materials");
            lessons.Update(lesson);
        }
    }

    /// Führt die anfangs erzwungene Von/Nach-Struktur des Kurzsymbols (<c>ShorthandFrom</c>/
    /// <c>ShorthandTo</c>) in ein einzelnes Freitextfeld zusammen — der Praxis nach ist ein
    /// Kurzsymbol nicht immer ein Materialfluss-Pfeil, manchmal nur eine Sozialform ("Plenum").
    /// Beide Felder gesetzt ergeben "Von->Nach", nur eines gesetzt bleibt als Einzelwert erhalten.
    private void MigrateLessonShorthand()
    {
        var lessons = _db.GetCollection<BsonDocument>("lessons");
        foreach (var lesson in lessons.FindAll().ToList())
        {
            if (!lesson.TryGetValue(nameof(Lesson.Phases), out var phasesValue) || !phasesValue.IsArray) continue;

            var changed = false;
            foreach (var phaseValue in phasesValue.AsArray)
            {
                if (phaseValue is not BsonDocument phase) continue;
                if (phase.ContainsKey(nameof(LessonPhaseStep.Shorthand))) continue;

                var from = phase.TryGetValue("ShorthandFrom", out var f) && f.IsString ? f.AsString : "";
                var to   = phase.TryGetValue("ShorthandTo", out var t) && t.IsString ? t.AsString : "";
                phase[nameof(LessonPhaseStep.Shorthand)] = (from.Length > 0, to.Length > 0) switch
                {
                    (true, true)  => $"{from}->{to}",
                    (true, false) => from,
                    (false, true) => to,
                    _             => "",
                };
                phase.Remove("ShorthandFrom");
                phase.Remove("ShorthandTo");
                changed = true;
            }
            if (changed) lessons.Update(lesson);
        }
    }

    /// Führt das anfangs freie Textfeld für den alternativen Ablauf einer Phase
    /// (<c>AlternativePath</c>, String) in einen Verweis auf einen Katalogeintrag
    /// (<see cref="LessonPhaseStep.AlternativePathId"/>) über — pro bisher verwendetem, distinktem
    /// Namen wird ein <see cref="AlternativeLessonPath"/> angelegt (bzw. ein gleichnamiger
    /// wiederverwendet) und referenziert. Die neue Collection ist von dieser Migration selbst nicht
    /// betroffen (keine Altstruktur), daher direkter Zugriff über die typisierte Collection.
    private void MigrateLessonAlternativePaths()
    {
        var lessons = _db.GetCollection<BsonDocument>("lessons");
        var nameToId = new Dictionary<string, Guid>(StringComparer.OrdinalIgnoreCase);
        foreach (var existing in AlternativeLessonPaths.FindAll()) nameToId[existing.Name] = existing.Id;

        foreach (var lesson in lessons.FindAll().ToList())
        {
            if (!lesson.TryGetValue(nameof(Lesson.Phases), out var phasesValue) || !phasesValue.IsArray) continue;

            var changed = false;
            foreach (var phaseValue in phasesValue.AsArray)
            {
                if (phaseValue is not BsonDocument phase) continue;
                if (!phase.TryGetValue("AlternativePath", out var oldVal) || !oldVal.IsString) continue;

                var name = oldVal.AsString.Trim();
                phase.Remove("AlternativePath");
                changed = true;
                if (name.Length == 0) continue;

                if (!nameToId.TryGetValue(name, out var id))
                {
                    var entry = new AlternativeLessonPath { Name = name };
                    AlternativeLessonPaths.Insert(entry);
                    id = entry.Id;
                    nameToId[name] = id;
                }
                phase[nameof(LessonPhaseStep.AlternativePathId)] = id;
            }
            if (changed) lessons.Update(lesson);
        }
    }

    private void RemoveRedundantLegacyFields()
    {
        RemoveFields("exams", "Subject");
        RemoveFields("units", "Subject", "SchoolYear");
        RemoveFields("grades", "SchoolYear");
        RemoveFields("participation", "GroupId", "Date");
    }

    private void RemoveFields(string collectionName, params string[] fieldNames)
    {
        var collection = _db.GetCollection<BsonDocument>(collectionName);
        foreach (var document in collection.FindAll().ToList())
        {
            var changed = false;
            foreach (var fieldName in fieldNames) changed |= document.Remove(fieldName);
            if (changed) collection.Update(document);
        }
    }

    private void EnsureIndexes()
    {
        Students.EnsureIndex(x => x.LastName);
        Students.EnsureIndex(x => x.IsActive);
        Groups.EnsureIndex(x => x.SchoolYear);
        Groups.EnsureIndex(x => x.IsActive);
        SeatingPlans.EnsureIndex(x => x.GroupId);
        Memberships.EnsureIndex(x => x.StudentId);
        Memberships.EnsureIndex(x => x.GroupId);
        Memberships.EnsureIndex("ux_student_group",
            BsonExpression.Create("STRING($.StudentId) + ':' + STRING($.GroupId)"), unique: true);
        Exams.EnsureIndex(x => x.GroupId);
        Exams.EnsureIndex(x => x.Status);
        ExamResults.EnsureIndex(x => x.ExamId);
        ExamResults.EnsureIndex(x => x.StudentId);
        ExamResults.EnsureIndex("ux_exam_student",
            BsonExpression.Create("STRING($.ExamId) + ':' + STRING($.StudentId)"), unique: true);
        Grades.EnsureIndex(x => x.StudentId);
        Grades.EnsureIndex(x => x.GroupId);
        GradingSchemes.EnsureIndex(x => x.GroupId);
        GradingSchemes.EnsureIndex(x => x.GroupType);
        ReportGrades.EnsureIndex(x => x.GroupId);
        ReportGrades.EnsureIndex(x => x.StudentId);
        ReportGrades.EnsureIndex("ux_student_group_period",
            BsonExpression.Create("STRING($.StudentId) + ':' + STRING($.GroupId) + ':' + $.Period"), unique: true);
        GradingKeyTemplates.EnsureIndex(x => x.GradingSystem);
        Units.EnsureIndex(x => x.GroupId);
        Lessons.EnsureIndex(x => x.UnitId);
        Lessons.EnsureIndex(x => x.GroupId);
        Lessons.EnsureIndex(x => x.Date);
        Documentation.EnsureIndex(x => x.StudentId);
        Tasks.EnsureIndex(x => x.Status);
        TimeEntries.EnsureIndex(x => x.Date);
        ParticipationSessions.EnsureIndex(x => x.GroupId);
        ParticipationSessions.EnsureIndex(x => x.Date);
        ParticipationEntries.EnsureIndex(x => x.SessionId);
        ParticipationEntries.EnsureIndex(x => x.StudentId);
        ParticipationEntries.EnsureIndex("ux_session_student",
            BsonExpression.Create("STRING($.SessionId) + ':' + STRING($.StudentId)"), unique: true);
        ParticipationAspects.EnsureIndex(x => x.GroupId);
        ParticipationSections.EnsureIndex(x => x.GroupId);
        Subjects.EnsureIndex(x => x.Name);
        Subjects.EnsureIndex("ux_subject_name", BsonExpression.Create("LOWER(TRIM($.Name))"), unique: true);
        CompetencyDomains.EnsureIndex(x => x.SubjectId);
        CompetencyDomains.EnsureIndex(x => x.GradeLevel);
        ShorthandCodes.EnsureIndex("ux_shorthand_code", BsonExpression.Create("LOWER(TRIM($.Code))"), unique: true);
        AlternativeLessonPaths.EnsureIndex("ux_alt_lesson_path_name", BsonExpression.Create("LOWER(TRIM($.Name))"), unique: true);
        TimetableSlots.EnsureIndex(x => x.GroupId);
        TimetableSlots.EnsureIndex("ux_timetable_weekday_period",
            BsonExpression.Create("STRING($.Weekday) + ':' + STRING($.PeriodNumber)"), unique: true);
        SupervisionDuties.EnsureIndex("ux_supervision_weekday_period",
            BsonExpression.Create("STRING($.Weekday) + ':' + STRING($.AfterPeriod)"), unique: true);
        SubstitutionEntries.EnsureIndex(x => x.Date);
    }

    public void Dispose() => _db.Dispose();
}