using MediatR;
using TakeoutSaaS.Application.App.Billings.Commands;
using TakeoutSaaS.Domain.Tenants.Repositories;
using TakeoutSaaS.Shared.Abstractions.Constants;
using TakeoutSaaS.Shared.Abstractions.Exceptions;

namespace TakeoutSaaS.Application.App.Billings.Handlers;

/// <summary>
/// 批量更新账单状态处理器。
/// </summary>
public sealed class BatchUpdateStatusCommandHandler(
    ITenantBillingRepository billingRepository)
    : IRequestHandler<BatchUpdateStatusCommand, int>
{
    /// <summary>
    /// 处理批量更新账单状态请求。
    /// </summary>
    /// <param name="request">批量更新状态命令。</param>
    /// <param name="cancellationToken">取消标记。</param>
    /// <returns>成功更新的账单数量。</returns>
    public async Task<int> Handle(BatchUpdateStatusCommand request, CancellationToken cancellationToken)
    {
        // 1. 参数验证
        if (request.BillingIds.Length == 0)
        {
            throw new BusinessException(ErrorCodes.BadRequest, "账单 ID 列表不能为空");
        }

        // 2. 查询所有账单
        var billings = await billingRepository.GetByIdsAsync(request.BillingIds, cancellationToken);
        if (billings.Count == 0)
        {
            throw new BusinessException(ErrorCodes.NotFound, "未找到任何匹配的账单");
        }

        // 3. 批量更新状态
        var now = DateTime.UtcNow;
        var updatedCount = 0;
        foreach (var billing in billings)
        {
            // 业务规则检查：某些状态转换可能不允许
            if (CanTransitionStatus(billing.Status, request.NewStatus))
            {
                billing.Status = request.NewStatus;
                billing.UpdatedAt = now;

                if (!string.IsNullOrWhiteSpace(request.Notes))
                {
                    billing.Notes = string.IsNullOrWhiteSpace(billing.Notes)
                        ? $"[批量操作] {request.Notes}"
                        : $"{billing.Notes}\n[批量操作] {request.Notes}";
                }

                await billingRepository.UpdateAsync(billing, cancellationToken);
                updatedCount++;
            }
        }

        // 4. 持久化变更
        await billingRepository.SaveChangesAsync(cancellationToken);

        return updatedCount;
    }

    /// <summary>
    /// 检查状态转换是否允许。
    /// </summary>
    private static bool CanTransitionStatus(
        TakeoutSaaS.Domain.Tenants.Enums.TenantBillingStatus currentStatus,
        TakeoutSaaS.Domain.Tenants.Enums.TenantBillingStatus newStatus)
    {
        // 已支付的账单不能改为其他状态
        if (currentStatus == TakeoutSaaS.Domain.Tenants.Enums.TenantBillingStatus.Paid
            && newStatus != TakeoutSaaS.Domain.Tenants.Enums.TenantBillingStatus.Paid)
        {
            return false;
        }

        // 已取消的账单不能改为其他状态
        if (currentStatus == TakeoutSaaS.Domain.Tenants.Enums.TenantBillingStatus.Cancelled)
        {
            return false;
        }

        return true;
    }
}