using System.Text.Json;
using MeterVault.Core.Domain;
using MeterVault.Infrastructure.Persistence;
using Microsoft.EntityFrameworkCore;

namespace MeterVault.Infrastructure.Backup;

/// <summary>
/// Exports/imports the configuration snapshot (SDD §10). Import restores into an empty instance,
/// remapping surrogate ids so foreign keys stay consistent regardless of the original ids.
/// </summary>
public sealed class ExportService(MeterVaultDbContext db)
{
    private static readonly JsonSerializerOptions Json = new()
    {
        WriteIndented = true,
        ReferenceHandler = System.Text.Json.Serialization.ReferenceHandler.IgnoreCycles,
    };

    private readonly MeterVaultDbContext _db = db;

    /// <summary>
    /// Strips encrypted connector secrets from an export. They are ciphertext, not plaintext, so
    /// this is not a §6.4 requirement — but the export exists for portability (§9), and ciphertext
    /// is bound to the originating instance's key ring, so it is useless anywhere it could be
    /// restored and merely widens the blast radius if the key ring also leaks. Env-var references
    /// survive: they name a variable and reveal nothing. Restoring means re-entering the secrets.
    /// </summary>
    private static List<IngestionEndpoint> RedactSecrets(List<IngestionEndpoint> endpoints)
    {
        foreach (var endpoint in endpoints)
        {
            endpoint.Config = endpoint.Type == EndpointType.HomeAssistant
                ? (Ingestion.HaEndpointConfig.Parse(endpoint.Config) with { TokenEnc = null }).ToJson()
                : (Ingestion.EndpointConfig.Parse(endpoint.Config) with { PasswordEnc = null }).ToJson();
        }

        return endpoints;
    }

    public async Task<string> ExportJsonAsync(CancellationToken cancellationToken = default)
    {
        // Load without navigation includes so serialization is a clean, cycle-free tree.
        var document = new ExportDocument
        {
            EnergyTypes = await _db.EnergyTypes.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            CostCategories = await _db.CostCategories.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            Meters = await _db.Meters.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            IngestionEndpoints = RedactSecrets(
                await _db.IngestionEndpoints.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false)),
            MeterSources = await _db.MeterSources.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            Tanks = await _db.Tanks.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            Tariffs = await _db.Tariffs.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            CostCategoryMembers = await _db.CostCategoryMembers.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            ManualCosts = await _db.ManualCosts.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            MeterEvents = await _db.MeterEvents.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
            AppSettings = await _db.AppSettings.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false),
        };

        return JsonSerializer.Serialize(document, Json);
    }

    public async Task ImportJsonAsync(string json, CancellationToken cancellationToken = default)
    {
        var doc = JsonSerializer.Deserialize<ExportDocument>(json, Json)
            ?? throw new InvalidOperationException("Empty or invalid export document.");

        await using var tx = await _db.Database.BeginTransactionAsync(cancellationToken).ConfigureAwait(false);

        var typeMap = new Dictionary<short, short>();
        foreach (var type in doc.EnergyTypes)
        {
            var old = type.Id;
            type.Id = 0;
            _db.EnergyTypes.Add(type);
            await _db.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
            typeMap[old] = type.Id;
        }

        var categoryMap = await InsertMappedAsync(doc.CostCategories, c => c.Id, (c, _) => c.Id = 0, cancellationToken).ConfigureAwait(false);

        var meterMap = new Dictionary<int, int>();
        foreach (var meter in doc.Meters)
        {
            var old = meter.Id;
            meter.Id = 0;
            meter.EnergyTypeId = typeMap.GetValueOrDefault(meter.EnergyTypeId, meter.EnergyTypeId);
            meter.EnergyType = null;
            _db.Meters.Add(meter);
            await _db.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
            meterMap[old] = meter.Id;
        }

        var endpointMap = await InsertMappedAsync(doc.IngestionEndpoints, e => e.Id, (e, _) => e.Id = 0, cancellationToken).ConfigureAwait(false);

        foreach (var source in doc.MeterSources)
        {
            source.Id = 0;
            source.MeterId = meterMap.GetValueOrDefault(source.MeterId, source.MeterId);
            source.EndpointId = source.EndpointId is { } eid ? endpointMap.GetValueOrDefault(eid, eid) : null;
            source.Meter = null;
            source.Endpoint = null;
            _db.MeterSources.Add(source);
        }

        foreach (var tank in doc.Tanks)
        {
            tank.Id = 0;
            tank.MeterId = meterMap.GetValueOrDefault(tank.MeterId, tank.MeterId);
            tank.Meter = null;
            _db.Tanks.Add(tank);
        }

        foreach (var tariff in doc.Tariffs)
        {
            tariff.Id = 0;
            tariff.ScopeId = tariff.ScopeType switch
            {
                TariffScope.Meter when tariff.ScopeId is { } id => meterMap.GetValueOrDefault(id, id),
                TariffScope.EnergyType when tariff.ScopeId is { } id => typeMap.GetValueOrDefault((short)id, (short)id),
                _ => tariff.ScopeId,
            };
            _db.Tariffs.Add(tariff);
        }

        foreach (var member in doc.CostCategoryMembers)
        {
            member.Id = 0;
            member.CategoryId = categoryMap.GetValueOrDefault(member.CategoryId, member.CategoryId);
            member.MeterId = member.MeterId is { } mid ? meterMap.GetValueOrDefault(mid, mid) : null;
            member.EnergyTypeId = member.EnergyTypeId is { } tid ? typeMap.GetValueOrDefault(tid, tid) : null;
            member.Category = null;
            _db.CostCategoryMembers.Add(member);
        }

        foreach (var cost in doc.ManualCosts)
        {
            cost.Id = 0;
            cost.CategoryId = cost.CategoryId is { } cid ? categoryMap.GetValueOrDefault(cid, cid) : null;
            cost.MeterId = cost.MeterId is { } mid ? meterMap.GetValueOrDefault(mid, mid) : null;
            cost.ImportBatchId = null; // the original import batch isn't part of the export
            _db.ManualCosts.Add(cost);
        }

        foreach (var meterEvent in doc.MeterEvents)
        {
            meterEvent.Id = 0;
            meterEvent.MeterId = meterMap.GetValueOrDefault(meterEvent.MeterId, meterEvent.MeterId);
            meterEvent.ImportBatchId = null;
            _db.MeterEvents.Add(meterEvent);
        }

        foreach (var setting in doc.AppSettings)
        {
            if (!await _db.AppSettings.AnyAsync(s => s.Key == setting.Key, cancellationToken).ConfigureAwait(false))
            {
                _db.AppSettings.Add(setting);
            }
        }

        await _db.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
        await tx.CommitAsync(cancellationToken).ConfigureAwait(false);
    }

    private async Task<Dictionary<int, int>> InsertMappedAsync<T>(
        List<T> items, Func<T, int> getId, Action<T, int> clearId, CancellationToken cancellationToken)
        where T : class
    {
        var map = new Dictionary<int, int>();
        foreach (var item in items)
        {
            var old = getId(item);
            clearId(item, 0);
            _db.Add(item);
            await _db.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
            map[old] = getId(item);
        }

        return map;
    }
}