using LiteDB;
using LehrerApp.Core.Models;

namespace LehrerApp.Data;

/// <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 = 1;

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

    public int SchemaVersion => ReadSchemaVersion();

    private void RunVersionedMigrations()
    {
        var version = ReadSchemaVersion();
        if (version < 1)
        {
            MigrateExistingData();
            version = 1;
        }
        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);
        }
    }

    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);
        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);
    }

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