Setup basic fields for credit note

We want to be able to correct invoices send into the KSeF API.
To correct the invoices we need to prepare a credit note.
There are a few differences between a regular VAT invoice and
a polish credit note. The first is the InvoiceType that changes
from VAT to KOR. The second is a CorrectionType it is used to
define when the correction should take legal effect, with values
1-3 meaning:
1 - Changes take effect in the date of the invoice. This is mostly used
if you made a mistake in the original invoice.
2 - Changes take effect in the date of the correction. This is used for
example when giving a discount or returning items.
3 - Changes take effect in a different date than any of the invoices or
the date is different for different elements.
There is also a optional field CorrectionReason where you can give a reason
for the correction.
The last element is data of the Invoice you are correcting. You need to give
the SequentialNumber and IssueDate of the original invoice. If the invoice
has a KSeF number you need to provide it and set the KsefNumberPresent flag
to 1. If the original invoice didn't have a KSeF number you need to set the
NoKsefNumberPresent flag to 1.

invoice.go

@@ -3,6 +3,7 @@ package ksef
 /**/
 import (
 	"github.com/invopop/gobl/bill"
+	"github.com/invopop/gobl/head"
 	"github.com/invopop/gobl/regimes/pl"
 	"github.com/invopop/gobl/tax"
 )
@@ -41,10 +42,21 @@ type Inv struct {
 	TotalAmountReceivable              string       `xml:"P_15"`
 	Annotations                        *Annotations `xml:"Adnotacje"`
 	InvoiceType                        string       `xml:"RodzajFaktury"`
+	CorrectionReason                   string       `xml:"PrzyczynaKorekty,omitempty"`
+	CorrectionType                     int          `xml:"TypKorekty,omitempty"`
+	CorrectedFa                        *CorrectedFa `xml:"DaneFaKorygowanej,omitempty"`
 	Lines                              []*Line      `xml:"FaWiersz"`
 	Payment                            *Payment     `xml:"Platnosc"`
 }
 
+type CorrectedFa struct {
+	IssueDate           string `xml:"DataWystFaKorygowanej,omitempty"`
+	SequentialNumber    string `xml:"NrFaKorygowanej,omitempty"`
+	KsefNumberPresent   int    `xml:"NrKSeF,omitempty"`
+	NoKsefNumberPresent int    `xml:"NrKSeFN,omitempty"`
+	KsefNumber          string `xml:"NrKSeFFaKorygowanej,omitempty"`
+}
+
 // Annotations defines the XML structure for KSeF annotations
 type Annotations struct {
 	CashAccounting                      int `xml:"P_16"`
@@ -86,6 +98,13 @@ func NewInv(inv *bill.Invoice) *Inv {
 		TotalAmountReceivable: inv.Totals.Payable.Rescale(cu).String(),
 		Lines:                 NewLines(inv.Lines),
 		Payment:               NewPayment(inv.Payment, inv.Totals),
+		CorrectionReason:      inv.Preceding[0].Reason,
+		// CorrectionType defines when the correction should have legal consequences
+		// 1 - in the date of the original invoice
+		// 2 - in the date of the correction invoice
+		// 3 - other date or the dates are different for different position on the invoice
+		CorrectionType: 2,
+		CorrectedFa:    NewCorrectedFa(inv.Preceding[0]),
 	}
 
 	ss := inv.ScenarioSummary()
@@ -116,3 +135,28 @@ func NewInv(inv *bill.Invoice) *Inv {
 
 	return Inv
 }
+
+func NewCorrectedFa(prc *bill.Preceding) *CorrectedFa {
+	fa := &CorrectedFa{
+		IssueDate:        prc.IssueDate.String(),
+		SequentialNumber: prc.Series + prc.Code,
+	}
+
+	if id := findStamp(prc.Stamps, "KSEF"); id != -1 {
+		fa.KsefNumberPresent = 1
+		fa.KsefNumber = prc.Stamps[id].Value
+	} else {
+		fa.NoKsefNumberPresent = 1
+	}
+
+	return fa
+}
+
+func findStamp(a []*head.Stamp, x string) int {
+	for i, n := range a {
+		if x == string(n.Provider) {
+			return i
+		}
+	}
+	return -1
+}