Distribution/Features/Invoice/InvoiceController.cs

@@ -52,6 +52,13 @@ public class InvoiceController(InvoiceService service, ICurrentUserService curre
         catch (InvalidOperationException ex) { return BadRequest(new { error = ex.Message }); }
     }
 
+    [HttpPost("{id:guid}/preview-totals")]
+    public async Task<IActionResult> PreviewTotals(Guid id, [FromBody] PreviewInvoiceTotalsRequest request)
+    {
+        var result = await service.PreviewTotalsAsync(id, request);
+        return result is null ? NotFound() : Ok(result);
+    }
+
     [HttpGet("{id:guid}/versions")]
     public async Task<IActionResult> GetVersions(Guid id)
     {

Distribution/Features/Invoice/InvoiceDTOs.cs

@@ -1,4 +1,5 @@
-using Distribution.Models;
+using Distribution.Features.ShopPO;
+using Distribution.Models;
 
 namespace Distribution.Features.Invoice;
 
@@ -9,7 +10,14 @@ public record PrintInvoiceRequest(Guid? PrintedByUserId = null);
 
 // InvoiceItemId set = edit/remove/restore an existing line; ItemId set (InvoiceItemId null) = add a new line.
 public record InvoiceItemEdit(Guid? InvoiceItemId, Guid? ItemId, int Quantity, decimal? UnitPrice = null, bool IsRemoved = false);
-public record UpdateInvoiceItemsRequest(List<InvoiceItemEdit> Items, Guid? EditedByUserId = null);
+// DiscountSchemeIds: the percentage-discount scheme chosen in the editor (at most one); null = leave as is.
+public record UpdateInvoiceItemsRequest(List<InvoiceItemEdit> Items, Guid? EditedByUserId = null, List<Guid>? DiscountSchemeIds = null);
+
+public record PreviewLine(Guid ItemId, int Quantity, decimal UnitPrice, bool IsFree = false, bool IsRemoved = false, Guid? SchemeId = null);
+public record PreviewInvoiceTotalsRequest(List<PreviewLine> Items, List<Guid>? DiscountSchemeIds = null);
+// Quantity = how many free units the scheme still entitles for the lines as typed.
+public record FreeEntitlement(Guid SchemeId, int Quantity);
+public record InvoiceTotalsPreview(decimal DiscountAmount, decimal TotalAmount, List<FreeEntitlement> FreeEntitlements);
 
 public record InvoiceVersionItemResponse(Guid? ShopPOItemId, Guid ItemId, string ItemName, int Quantity, decimal UnitPrice, bool IsRemoved);
 
@@ -30,7 +38,10 @@ public record InvoiceItemResponse(
     int? OrderedQuantity,
     int Quantity,
     decimal UnitPrice,
-    bool IsRemoved);
+    bool IsRemoved,
+    bool IsFree = false,
+    Guid? SchemeId = null,
+    string? SchemeName = null);
 
 public record InvoiceResponse(
     Guid Id,
@@ -49,6 +60,7 @@ public record InvoiceResponse(
     decimal PaidAmount,
     bool IsActive,
     DateTime CreatedAt,
-    List<InvoiceItemResponse> Items);
+    List<InvoiceItemResponse> Items,
+    List<ShopPODiscountResponse>? Discounts = null);
 
 public record OutstandingPaymentsResponse(int InvoiceCount, decimal OutstandingAmount);

Distribution/Features/Invoice/InvoiceService.cs

@@ -1,4 +1,5 @@
-using Distribution.Features.ShopPO;
+using Distribution.Features.Scheme;
+using Distribution.Features.ShopPO;
 using Distribution.Features.Stock;
 using Distribution.Infra.Authorization;
 using Distribution.Infra.UoW;
@@ -9,6 +10,7 @@ namespace Distribution.Features.Invoice;
 public class InvoiceService(
     IInvoiceRepository repo,
     IShopPORepository shopPoRepo,
+    SchemeService schemeService,
     IUnitOfWork uow,
     ICurrentUserService currentUser)
 {
@@ -25,20 +27,53 @@ public class InvoiceService(
             i.Items.Where(x => x.IsActive).Select(x =>
                 new InvoiceItemResponse(x.Id, x.FK_ShopPOItemId, x.FK_ItemId,
                     x.Item?.Name ?? string.Empty, x.ShopPOItem?.Quantity,
-                    x.Quantity, x.UnitPrice, x.IsRemoved)).ToList());
+                    x.Quantity, x.UnitPrice, x.IsRemoved, x.IsFree, x.FK_SchemeId, x.SchemeName)).ToList());
 
     private static decimal BillableTotal(IEnumerable<InvoiceItem> items) =>
         items.Where(i => i.IsActive && !i.IsRemoved).Sum(i => i.Quantity * i.UnitPrice);
 
-    // TotalAmount = billable lines - scheme discount redeemed on the PO. Re-derived whenever lines change
-    // and at finalize, so a scheme applied after the invoice was drafted is still picked up before it locks.
+    // The lines that count toward a percentage discount: active, not struck out. Free issues carry the flag so the
+    // scheme rules ignore them.
+    private static List<ShopPOItem> DiscountableLines(IEnumerable<InvoiceItem> items) =>
+        items.Where(i => i.IsActive && !i.IsRemoved)
+            .Select(i => new ShopPOItem { FK_ItemId = i.FK_ItemId, Quantity = i.Quantity, UnitPrice = i.UnitPrice, IsFree = i.IsFree })
+            .ToList();
+
+    // TotalAmount = billable lines - scheme discount. The discount is re-evaluated against the current lines
+    // (so adding/removing/re-pricing items moves it, and a slab level can be gained or lost) whenever lines change
+    // and at print/finalize. The redemption amount is updated with it.
     private async Task ApplyTotalsAsync(Models.Invoice invoice, IEnumerable<InvoiceItem> items)
     {
-        var billable = BillableTotal(items);
-        invoice.DiscountAmount = Math.Min(await repo.GetSchemeDiscountAsync(invoice.FK_ShopPOId), billable);
+        var lines = items.ToList();
+        // Lowering/removing the paid item that earned a free issue lowers the free quantity with it.
+        await schemeService.ReconcileInvoiceFreeLinesAsync(invoice.FK_ShopPOId, lines);
+        var billable = BillableTotal(lines);
+        var discount = await schemeService.CalculateDiscountAsync(invoice.FK_ShopPOId, DiscountableLines(lines), persist: true);
+        invoice.DiscountAmount = Math.Min(discount, billable);
         invoice.TotalAmount = billable - invoice.DiscountAmount;
     }
 
+    // Live preview for the draft editor: what the discount and total would be for these unsaved lines. Changes nothing.
+    public async Task<InvoiceTotalsPreview?> PreviewTotalsAsync(Guid id, PreviewInvoiceTotalsRequest request)
+    {
+        var invoice = await repo.GetByIdAsync(id);
+        if (invoice is null) return null;
+
+        var lines = request.Items.Where(l => !l.IsRemoved)
+            .Select(l => new ShopPOItem { FK_ItemId = l.ItemId, Quantity = l.Quantity, UnitPrice = l.UnitPrice, IsFree = l.IsFree, FK_SchemeId = l.SchemeId })
+            .ToList();
+        var billable = lines.Sum(l => l.Quantity * l.UnitPrice);
+        var chosen = request.DiscountSchemeIds is null
+            ? await schemeService.CalculateDiscountAsync(invoice.FK_ShopPOId, lines, persist: false)
+            : await schemeService.CalculateDiscountForSchemesAsync(request.DiscountSchemeIds, lines);
+        var discount = Math.Min(chosen, billable);
+
+        var schemeIds = lines.Where(l => l.IsFree && l.FK_SchemeId.HasValue).Select(l => l.FK_SchemeId!.Value).ToList();
+        var entitlements = (await schemeService.GetFreeEntitlementsAsync(schemeIds, lines))
+            .Select(e => new FreeEntitlement(e.Key, e.Value)).ToList();
+        return new InvoiceTotalsPreview(discount, billable - discount, entitlements);
+    }
+
     // Immutable copy of the invoice lines as they stand right now.
     public static InvoiceVersion BuildVersion(Models.Invoice invoice, IEnumerable<InvoiceItem> items, int versionNumber, InvoiceVersionStage stage, Guid userId) =>
         new()
@@ -83,13 +118,17 @@ public class InvoiceService(
             FK_ShopPOItemId = i.Id,
             FK_ItemId = i.FK_ItemId,
             Quantity = i.Quantity,
-            UnitPrice = i.UnitPrice
+            UnitPrice = i.UnitPrice,
+            IsFree = i.IsFree,
+            FK_SchemeId = i.FK_SchemeId,
+            SchemeName = i.SchemeName
         }).ToList();
 
     public async Task<InvoiceResponse?> GetByIdAsync(Guid id)
     {
         var invoice = await repo.GetByIdAsync(id);
-        return invoice is null ? null : ToResponse(invoice);
+        if (invoice is null) return null;
+        return ToResponse(invoice) with { Discounts = await schemeService.GetDiscountsForShopPOAsync(invoice.FK_ShopPOId) };
     }
 
     public async Task<PagedResponse<InvoiceResponse>> GetAllAsync(Guid? agentId, Guid? routeId, Guid? shopId, InvoiceStatus? status, PaymentStatus? paymentStatus, int page, int pageSize)
@@ -172,7 +211,12 @@ public class InvoiceService(
                     ValidateQuantity(item, edit.Quantity);
                     item.Quantity = edit.Quantity;
                 }
-                if (edit.UnitPrice is decimal price) item.UnitPrice = price;
+                if (edit.UnitPrice is decimal price)
+                {
+                    if (item.IsFree && price != 0m)
+                        throw new InvalidOperationException("A free-issue line must stay at zero price");
+                    item.UnitPrice = price;
+                }
             }
             else if (edit.ItemId is Guid itemId)
             {
@@ -201,7 +245,10 @@ public class InvoiceService(
                 throw new InvalidOperationException("Each edit needs an invoiceItemId (existing line) or an itemId (new line)");
         }
 
-        await ApplyTotalsAsync(invoice, invoice.Items.Concat(newLines));
+        var allLines = invoice.Items.Concat(newLines).ToList();
+        await schemeService.ApplyInvoiceDiscountSelectionAsync(
+            invoice.FK_ShopPOId, invoice.ShopPO?.FK_StoreId ?? Guid.Empty, allLines, request.DiscountSchemeIds);
+        await ApplyTotalsAsync(invoice, allLines);
 
         await uow.BeginAsync(ct);
         try
@@ -213,7 +260,7 @@ public class InvoiceService(
         }
         catch { await uow.RollbackAsync(ct); throw; }
 
-        return ToResponse((await repo.GetByIdAsync(id))!);
+        return (await GetByIdAsync(id))!;
     }
 
     private static void ValidateQuantity(InvoiceItem item, int quantity)