using Lagerbestand.Data;
using Lagerbestand.Models;
using Microsoft.EntityFrameworkCore;

namespace Lagerbestand.Services;

/// <summary>
/// Result of merging a local customer into a Lexware customer.
/// </summary>
public class CustomerMergeResult
{
    public Guid TargetCustomerId { get; set; }
    public int VehiclesMoved { get; set; }
    public int TireStacksMoved { get; set; }
    public int TasksMoved { get; set; }
}

public interface ICustomerMergeService
{
    /// <summary>
    /// Local customers are created by this app and use the "K-" number prefix.
    /// Lexware customers carry the numeric Lexware customer number.
    /// </summary>
    bool IsLocalCustomer(Customer customer);

    /// <summary>
    /// Moves all vehicles, tire stacks and task links from a locally created customer
    /// to an existing Lexware customer and permanently deletes the local customer.
    /// </summary>
    Task<CustomerMergeResult> MergeAsync(Guid sourceId, Guid targetId);
}

public class CustomerMergeService : ICustomerMergeService
{
    private readonly AppDbContext _context;
    private readonly ILogger<CustomerMergeService> _logger;

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

    public static bool IsLocalCustomerNumber(string? customerNumber) =>
        string.IsNullOrWhiteSpace(customerNumber) ||
        customerNumber.StartsWith("K-", StringComparison.OrdinalIgnoreCase);

    public bool IsLocalCustomer(Customer customer) => IsLocalCustomerNumber(customer.CustomerNumber);

    public async Task<CustomerMergeResult> MergeAsync(Guid sourceId, Guid targetId)
    {
        if (sourceId == targetId)
            throw new InvalidOperationException("Quell- und Zielkunde dürfen nicht identisch sein.");

        var strategy = _context.Database.CreateExecutionStrategy();

        return await strategy.ExecuteAsync(async () =>
        {
            await using var transaction = await _context.Database.BeginTransactionAsync();

            var source = await _context.Customers.FirstOrDefaultAsync(c => c.Id == sourceId)
                ?? throw new InvalidOperationException("Lokaler Kunde wurde nicht gefunden.");
            var target = await _context.Customers.FirstOrDefaultAsync(c => c.Id == targetId)
                ?? throw new InvalidOperationException("Lexware-Kunde wurde nicht gefunden.");

            if (!IsLocalCustomerNumber(source.CustomerNumber))
                throw new InvalidOperationException("Nur lokal angelegte Kunden (K-…) können zusammengeführt werden.");
            if (IsLocalCustomerNumber(target.CustomerNumber))
                throw new InvalidOperationException("Der Zielkunde muss ein Lexware-Kunde sein.");

            var result = new CustomerMergeResult { TargetCustomerId = targetId };
            var now = DateTime.UtcNow;

            // Vehicles
            var targetHasPrimary = await _context.Vehicles
                .AnyAsync(v => v.CustomerId == targetId && v.IsActive && v.IsPrimary);
            var vehicles = await _context.Vehicles
                .Where(v => v.CustomerId == sourceId)
                .ToListAsync();

            foreach (var v in vehicles)
            {
                v.CustomerId = targetId;
                if (targetHasPrimary && v.IsPrimary)
                    v.IsPrimary = false;
                v.UpdatedAt = now;
            }
            result.VehiclesMoved = vehicles.Count;

            // Tire stacks (including history / picked-up ones)
            var stacks = await _context.TireStacks
                .Where(ts => ts.CustomerId == sourceId)
                .ToListAsync();

            foreach (var ts in stacks)
            {
                ts.CustomerId = targetId;
                ts.UpdatedAt = now;
            }
            result.TireStacksMoved = stacks.Count;

            // Kanban task links (composite key TaskId + CustomerId -> recreate)
            var taskLinks = await _context.KanbanTaskCustomers
                .Where(tc => tc.CustomerId == sourceId)
                .ToListAsync();

            if (taskLinks.Count > 0)
            {
                var taskIds = taskLinks.Select(tc => tc.TaskId).ToList();
                var existingTargetTaskIds = (await _context.KanbanTaskCustomers
                    .Where(tc => tc.CustomerId == targetId && taskIds.Contains(tc.TaskId))
                    .Select(tc => tc.TaskId)
                    .ToListAsync()).ToHashSet();

                foreach (var link in taskLinks)
                {
                    if (!existingTargetTaskIds.Contains(link.TaskId))
                    {
                        _context.KanbanTaskCustomers.Add(new KanbanTaskCustomer
                        {
                            TaskId = link.TaskId,
                            CustomerId = targetId,
                            VehicleId = link.VehicleId
                        });
                        result.TasksMoved++;
                    }
                    _context.KanbanTaskCustomers.Remove(link);
                }
            }

            // Fill empty contact data on target from source
            var targetChanged = false;
            if (string.IsNullOrWhiteSpace(target.Phone) && !string.IsNullOrWhiteSpace(source.Phone)) { target.Phone = source.Phone; targetChanged = true; }
            if (string.IsNullOrWhiteSpace(target.Email) && !string.IsNullOrWhiteSpace(source.Email)) { target.Email = source.Email; targetChanged = true; }
            if (!string.IsNullOrWhiteSpace(source.Notes))
            {
                var mergedNotes = string.IsNullOrWhiteSpace(target.Notes)
                    ? source.Notes
                    : $"{target.Notes}\n[Zusammengeführt aus {source.CustomerNumber}] {source.Notes}";
                if (mergedNotes.Length > 500) mergedNotes = mergedNotes[..500];
                target.Notes = mergedNotes;
                targetChanged = true;
            }
            if (!target.IsActive) { target.IsActive = true; targetChanged = true; }
            if (targetChanged) target.UpdatedAt = now;

            // Persist FK changes first, then hard-delete the source customer
            await _context.SaveChangesAsync();
            _context.Customers.Remove(source);
            await _context.SaveChangesAsync();

            await transaction.CommitAsync();

            _logger.LogInformation(
                "Customer {SourceNumber} merged into {TargetNumber}: {Vehicles} vehicles, {Stacks} tire stacks, {Tasks} task links",
                source.CustomerNumber, target.CustomerNumber, result.VehiclesMoved, result.TireStacksMoved, result.TasksMoved);

            return result;
        });
    }
}