using MediatR;
using System.Collections.Concurrent;
using TakeoutSaaS.Application.App.Tenants.Commands;
using TakeoutSaaS.Application.App.Tenants.Dto;
using TakeoutSaaS.Application.App.Tenants;
using TakeoutSaaS.Domain.Tenants.Entities;
using TakeoutSaaS.Domain.Tenants.Enums;
using TakeoutSaaS.Domain.Tenants.Repositories;
using TakeoutSaaS.Shared.Abstractions.Constants;
using TakeoutSaaS.Shared.Abstractions.Exceptions;
using TakeoutSaaS.Shared.Abstractions.Ids;
using TakeoutSaaS.Shared.Abstractions.Tenancy;

namespace TakeoutSaaS.Application.App.Tenants.Handlers;

/// <summary>
/// 租户初次绑定订阅处理器。
/// </summary>
public sealed class BindInitialTenantSubscriptionCommandHandler(
    ITenantRepository tenantRepository,
    IIdGenerator idGenerator,
    ITenantProvider tenantProvider)
    : IRequestHandler<BindInitialTenantSubscriptionCommand, TenantSubscriptionDto>
{
    private static readonly ConcurrentDictionary<long, SemaphoreSlim> TenantLocks = new();

    /// <inheritdoc />
    public async Task<TenantSubscriptionDto> Handle(BindInitialTenantSubscriptionCommand request, CancellationToken cancellationToken)
    {
        // 1. 校验租户上下文
        var currentTenantId = tenantProvider.GetCurrentTenantId();
        if (currentTenantId == 0 || currentTenantId != request.TenantId)
        {
            throw new BusinessException(ErrorCodes.Forbidden, "租户上下文不匹配，请在请求头 X-Tenant-Id 指定目标租户");
        }

        // 1.2 获取租户级幂等锁，避免并发重复创建
        var tenantLock = GetTenantLock(request.TenantId);
        await tenantLock.WaitAsync(cancellationToken);
        try
        {
            // 2. 获取租户
            var tenant = await tenantRepository.FindByIdAsync(request.TenantId, cancellationToken)
                ?? throw new BusinessException(ErrorCodes.NotFound, "租户不存在");

            // 3. 幂等校验：若已存在订阅则直接返回
            var existing = await tenantRepository.GetActiveSubscriptionAsync(request.TenantId, cancellationToken);
            if (existing is not null)
            {
                return existing.ToSubscriptionDto()
                    ?? throw new BusinessException(ErrorCodes.InternalServerError, "订阅读取失败");
            }

            // 4. 创建 0 个月订阅（待支付/待生效）
            var now = DateTime.UtcNow;
            var subscription = new TenantSubscription
            {
                Id = idGenerator.NextId(),
                TenantId = tenant.Id,
                TenantPackageId = request.TenantPackageId,
                EffectiveFrom = now,
                EffectiveTo = now,
                NextBillingDate = now,
                Status = SubscriptionStatus.Pending,
                AutoRenew = request.AutoRenew,
                Notes = "初次绑定订阅"
            };

            // 5. 记录订阅与历史
            await tenantRepository.AddSubscriptionAsync(subscription, cancellationToken);
            await tenantRepository.AddSubscriptionHistoryAsync(new TenantSubscriptionHistory
            {
                Id = idGenerator.NextId(),
                TenantId = tenant.Id,
                TenantSubscriptionId = subscription.Id,
                FromPackageId = request.TenantPackageId,
                ToPackageId = request.TenantPackageId,
                ChangeType = SubscriptionChangeType.New,
                EffectiveFrom = now,
                EffectiveTo = now,
                Amount = null,
                Currency = null,
                Notes = "初次绑定订阅（0 个月）"
            }, cancellationToken);

            // 6. 记录审计日志
            await tenantRepository.AddAuditLogAsync(new TenantAuditLog
            {
                TenantId = tenant.Id,
                Action = TenantAuditAction.SubscriptionUpdated,
                Title = "初次绑定订阅",
                Description = $"套餐 {request.TenantPackageId}，时长 0 月"
            }, cancellationToken);

            // 7. 保存变更
            await tenantRepository.SaveChangesAsync(cancellationToken);

            // 8. 返回 DTO
            return subscription.ToSubscriptionDto()
                ?? throw new BusinessException(ErrorCodes.InternalServerError, "订阅绑定失败");
        }
        finally
        {
            // 9. 释放幂等锁
            tenantLock.Release();
        }
    }

    // 获取或创建租户级幂等锁实例。
    private static SemaphoreSlim GetTenantLock(long tenantId)
        => TenantLocks.GetOrAdd(tenantId, _ => new SemaphoreSlim(1, 1));
}