Ensuring that all invoice discounts and charges are correctly inverted

bill/invoice.go

@@ -184,10 +184,22 @@ func (inv *Invoice) ValidateWithContext(ctx context.Context) error {
 }
 
 // Invert effectively reverses the invoice by inverting the sign of all quantity
-// or amount values.
-func (inv *Invoice) Invert() {
+// or amount values. Caution should be taken when using this method as
+// advances will also be inverted, while payment terms will remain the same,
+// which could be confusing if no further modifications are made.
+// After inverting the invoice is recalculated and any differences will raise
+// an error.
+func (inv *Invoice) Invert() error {
+	payable := inv.Totals.Payable.Invert()
+
 	for _, row := range inv.Lines {
 		row.Quantity = row.Quantity.Invert()
+		for _, d := range row.Discounts {
+			d.Amount = d.Amount.Invert()
+		}
+		for _, c := range row.Charges {
+			c.Amount = c.Amount.Invert()
+		}
 	}
 	for _, row := range inv.Charges {
 		row.Amount = row.Amount.Invert()
@@ -198,7 +210,24 @@ func (inv *Invoice) Invert() {
 	for _, row := range inv.Outlays {
 		row.Amount = row.Amount.Invert()
 	}
+	if inv.Payment != nil {
+		for _, row := range inv.Payment.Advances {
+			row.Amount = row.Amount.Invert()
+		}
+	}
 	inv.Totals = nil
+
+	if err := inv.Calculate(); err != nil {
+		return err
+	}
+
+	// The following check tries to ensure that any future fields do not cause
+	// unexpected results.
+	if !payable.Equals(inv.Totals.Payable) {
+		return fmt.Errorf("inverted invoice totals do not match %s != %s", payable.String(), inv.Totals.Payable.String())
+	}
+
+	return nil
 }
 
 // Empty is a convenience method that will empty all the lines and

bill/invoice_test.go

@@ -836,6 +836,77 @@ func TestCalculate(t *testing.T) {
 	assert.Equal(t, i.Totals.Due.String(), "675.00")
 }
 
+func TestCalculateInverted(t *testing.T) {
+	i := &bill.Invoice{
+		Code: "123TEST",
+		Tax: &bill.Tax{
+			PricesInclude: tax.CategoryVAT,
+		},
+		Supplier: &org.Party{
+			TaxID: &tax.Identity{
+				Country: l10n.ES,
+				Code:    "B98602642",
+			},
+		},
+		Customer: &org.Party{
+			TaxID: &tax.Identity{
+				Country: l10n.ES,
+				Code:    "54387763P",
+			},
+		},
+		IssueDate: cal.MakeDate(2022, 6, 13),
+		Lines: []*bill.Line{
+			{
+				Quantity: num.MakeAmount(10, 0),
+				Item: &org.Item{
+					Name:  "Test Item",
+					Price: num.MakeAmount(10000, 2),
+				},
+				Taxes: tax.Set{
+					{
+						Category: "VAT",
+						Rate:     "standard",
+					},
+				},
+				Discounts: []*bill.LineDiscount{
+					{
+						Reason: "Testing",
+						Amount: num.MakeAmount(10000, 2),
+					},
+				},
+				Charges: []*bill.LineCharge{
+					{
+						Reason: "Testing Charge",
+						Amount: num.MakeAmount(5000, 2),
+					},
+				},
+			},
+		},
+		Outlays: []*bill.Outlay{
+			{
+				Description: "Something paid in advance",
+				Amount:      num.MakeAmount(1000, 2),
+			},
+		},
+		Payment: &bill.Payment{
+			Advances: []*pay.Advance{
+				{
+					Description: "Test Advance",
+					Amount:      num.MakeAmount(25000, 2),
+				},
+			},
+		},
+	}
+
+	require.NoError(t, i.Calculate())
+	assert.Equal(t, i.Totals.Sum.String(), "950.00")
+	assert.Equal(t, i.Totals.Due.String(), "710.00")
+
+	require.NoError(t, i.Invert())
+	assert.Equal(t, i.Totals.Sum.String(), "-950.00")
+	assert.Equal(t, i.Totals.Due.String(), "-710.00")
+}
+
 func TestValidation(t *testing.T) {
 	inv := &bill.Invoice{
 		Currency:  currency.EUR,