ERP_Distribution/Distribution/Features/SupplierPayment/SupplierPaymentService.cs

using Distribution.Data;
using Distribution.Features.BankAccount;
using Distribution.Features.ChequeBook;
using Distribution.Features.Grn;
using Distribution.Features.Supplier;
using Distribution.Infra.Authorization;
using Distribution.Infra.UoW;
using Distribution.Models;
using Microsoft.EntityFrameworkCore;

namespace Distribution.Features.SupplierPayment;

public class SupplierPaymentService(
    AppDBContext context,
    ISupplierPaymentRepository repo,
    ISupplierRepository supplierRepo,
    IBankAccountRepository bankAccountRepo,
    IChequeBookRepository chequeBookRepo,
    IUnitOfWork uow,
    ICurrentUserService currentUser)
{
    private static SupplierPaymentResponse ToResponse(Models.SupplierPayment p) =>
        new(p.Id, p.FK_SupplierId, p.Amount, p.Method,
            p.FK_BankAccountId, p.FK_ChequePageId, p.ChequePage?.ChequeNo,
            p.PaidAt, p.PaidByUserId, p.Notes,
            p.Allocations
                .Where(a => a.IsActive)
                .Select(a => new SupplierPaymentAllocationResponse(a.FK_GoodsReceiptId, a.AllocatedAmount))
                .ToList(),
            p.CreatedAt);

    /// <summary>GRN total = sum per line of (gross - discount + NBT + VAT). Rejected qty never costs.</summary>
    private static decimal GrnTotal(Models.GoodsReceipt grn) =>
        grn.Items.Where(i => i.IsActive).Sum(i =>
            GrnPricing.Line(i.AcceptedQuantity, i.UnitCost, i.DiscountPercent, i.NbtPercent, i.VatPercent).Total);

    public async Task<PagedSupplierPaymentsResponse?> ListBySupplierAsync(Guid supplierId, int page, int pageSize)
    {
        var supplier = await supplierRepo.GetByIdAsync(supplierId);
        if (supplier is null) return null;

        if (page < 1) page = 1;
        if (pageSize < 1) pageSize = 20;
        if (pageSize > 200) pageSize = 200;

        var payments = await repo.ListBySupplierAsync(supplierId, page, pageSize);
        var total = await repo.CountBySupplierAsync(supplierId);
        return new PagedSupplierPaymentsResponse(payments.Select(ToResponse).ToList(), page, pageSize, total);
    }

    public async Task<SupplierPaymentResponse?> CreateAsync(Guid supplierId, CreateSupplierPaymentRequest request, CancellationToken ct = default)
    {
        var supplier = await supplierRepo.GetByIdAsync(supplierId);
        if (supplier is null) return null;

        if (request.Amount <= 0)
            throw new InvalidOperationException("Amount must be greater than zero");
        if (request.Method == PaymentMethod.Credit)
            throw new InvalidOperationException("Credit is not a valid outgoing payment method");

        if (request.Allocations is null || request.Allocations.Count == 0)
            throw new InvalidOperationException("At least one GRN allocation is required");

        var duplicateGrn = request.Allocations.GroupBy(a => a.GoodsReceiptId).FirstOrDefault(g => g.Count() > 1);
        if (duplicateGrn is not null)
            throw new InvalidOperationException($"GRN {duplicateGrn.Key} is allocated more than once");

        if (request.Allocations.Any(a => a.AllocatedAmount <= 0))
            throw new InvalidOperationException("Each allocation amount must be greater than zero");

        var allocationSum = request.Allocations.Sum(a => a.AllocatedAmount);
        if (allocationSum != request.Amount)
            throw new InvalidOperationException($"Allocations sum ({allocationSum}) must equal Amount ({request.Amount})");

        // Validate each GRN exists, belongs to this supplier, and the new allocation does not push cumulative paid over the GRN total.
        foreach (var alloc in request.Allocations)
        {
            var grn = await repo.GetGrnWithItemsAsync(alloc.GoodsReceiptId)
                ?? throw new InvalidOperationException($"GRN {alloc.GoodsReceiptId} not found");

            var grnSupplierId = grn.FK_SupplierId
                ?? grn.PurchaseOrder?.FK_SupplierId;
            if (grnSupplierId is null)
            {
                // PO-based GRN without the PO loaded — re-query to confirm supplier match.
                var po = await context.PurchaseOrders.FindAsync([grn.FK_PurchaseOrderId], ct);
                grnSupplierId = po?.FK_SupplierId;
            }
            if (grnSupplierId != supplierId)
                throw new InvalidOperationException($"GRN {alloc.GoodsReceiptId} does not belong to the specified supplier");

            var grnTotal = GrnTotal(grn);
            var alreadyAllocated = await repo.GetAllocatedTotalForGrnAsync(grn.Id);
            if (alreadyAllocated + alloc.AllocatedAmount > grnTotal)
                throw new InvalidOperationException(
                    $"Allocation to GRN {grn.Id} ({alloc.AllocatedAmount}) exceeds outstanding balance " +
                    $"(GRN total {grnTotal}, already paid {alreadyAllocated})");
        }

        var payerId = currentUser.UserId ?? request.PaidByUserId;

        var payment = new Models.SupplierPayment
        {
            FK_SupplierId = supplierId,
            Amount = request.Amount,
            Method = request.Method,
            PaidAt = DateTime.Now,
            PaidByUserId = payerId,
            Notes = request.Notes,
            Allocations = request.Allocations.Select(a => new Models.SupplierPaymentGRN
            {
                FK_GoodsReceiptId = a.GoodsReceiptId,
                AllocatedAmount = a.AllocatedAmount
            }).ToList()
        };

        await uow.BeginAsync(ct);
        try
        {
            if (request.Method == PaymentMethod.Cash)
            {
                if (request.BankAccountId is null)
                    throw new InvalidOperationException("BankAccountId is required when Method is Cash");

                var account = await bankAccountRepo.GetByIdAsync(request.BankAccountId.Value);
                if (account is null)
                    throw new InvalidOperationException("BankAccountId does not reference a valid account");
                if (account.IsActive is false)
                    throw new InvalidOperationException("The selected account is inactive");

                payment.FK_BankAccountId = account.Id;
            }
            else // Cheque
            {
                if (request.ChequePageId is null)
                    throw new InvalidOperationException("ChequePageId is required when Method is Cheque");

                var page = await context.ChequePages
                    .Include(p => p.ChequeBook)
                    .FirstOrDefaultAsync(p => p.Id == request.ChequePageId.Value, ct);

                if (page is null)
                    throw new InvalidOperationException("ChequePageId does not reference a valid cheque page");
                if (page.IssueStatus != ChequePageIssueStatus.Unused)
                    throw new InvalidOperationException($"Cannot issue a cheque in status '{page.IssueStatus}'");
                if (page.ChequeBook is null || page.ChequeBook.Status != ChequeBookStatus.Active)
                    throw new InvalidOperationException("The cheque book is not active");

                page.IssueStatus = ChequePageIssueStatus.Issued;
                page.PayeeType = PayeeType.Supplier;
                page.PayeeName = supplier.Name;
                page.IssueDate = DateTime.Now;
                page.Amount = request.Amount;
                page.CurrencyCode = "LKR";
                page.PrintedAt = DateTime.Now;

                chequeBookRepo.UpdatePage(page);
                payment.FK_ChequePageId = page.Id;
                payment.ChequePage = page;
            }

            await repo.AddAsync(payment);

            // TODO ledger: when accounting is wired in, post a journal entry here —
            //   Debit  Accounts Payable (supplier)
            //   Credit Cash/Bank (BankAccount) OR Cheques-outstanding (for Cheque payments)

            await uow.CommitAsync(ct);
        }
        catch { await uow.RollbackAsync(ct); throw; }

        return ToResponse(payment);
    }
}