refactor: extract shared Luhn check digit and party tax ID helpers

CHANGELOG.md

@@ -17,6 +17,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/) and this p
 
 ### Changed
 
+- `pkg/luhn`: Replaced duplicate `computeLuhnCheckDigit` implementations in the `fr` and `it` regimes with a shared `CheckDigit` function.
+- `org`, `bill`, `regimes`, `addons/it/sdi`: Replaced duplicate `hasTaxIDCode` helpers with a shared `Party.HasTaxIDCode()` method.
 - `bill`: `Invoice.Invert()` returns an error if the invoice has the `bypass` tag.
 - `num`: `AmountFromString` now limits precision to 18 significant digits.
 - `tax`: Added `$defs` and `$refs` to the `tax.RegimeCode` JSON schema

addons/it/sdi/bill.go

@@ -160,7 +160,7 @@ func validateCustomer(value interface{}) error {
 		),
 		validation.Field(&customer.Identities,
 			validation.When(
-				isItalianParty(customer) && !hasTaxIDCode(customer),
+				isItalianParty(customer) && !customer.HasTaxIDCode(),
 				org.RequireIdentityKey(it.IdentityKeyFiscalCode),
 			),
 			validation.Skip,
@@ -287,10 +287,6 @@ func validateItalianTelephone(value any) error {
 	)
 }
 
-func hasTaxIDCode(party *org.Party) bool {
-	return party != nil && party.TaxID != nil && party.TaxID.Code != ""
-}
-
 func hasFiscalCode(party *org.Party) bool {
 	if party == nil {
 		return false

bill/invoice.go

@@ -214,17 +214,13 @@ func validateInvoiceCustomer(value any) error {
 	return validation.ValidateStruct(p,
 		validation.Field(&p.Name,
 			validation.When(
-				partyHasTaxIDCode(p),
+				p.HasTaxIDCode(),
 				validation.Required,
 			),
 		),
 	)
 }
 
-func partyHasTaxIDCode(party *org.Party) bool {
-	return party != nil && party.TaxID != nil && party.TaxID.Code != ""
-}
-
 // Invert effectively reverses the invoice by inverting the sign of all quantity
 // or amount values. Caution should be taken when using this method as
 // advances will also be inverted, while payment terms will remain the same,

org/party.go

@@ -50,6 +50,11 @@ type Party struct {
 	Meta cbc.Meta `json:"meta,omitempty" jsonschema:"title=Meta"`
 }
 
+// HasTaxIDCode returns true if the party has a tax identity with a non-empty code.
+func (p *Party) HasTaxIDCode() bool {
+	return p != nil && p.TaxID != nil && p.TaxID.Code != ""
+}
+
 // Calculate will perform basic normalization of the party's data without
 // using any tax regime or addon.
 func (p *Party) Calculate() error {

org/party_test.go

@@ -137,6 +137,20 @@ func TestPartyValidation(t *testing.T) {
 		assert.NoError(t, party.Validate())
 		assert.Equal(t, "DE", party.GetRegime().String())
 	})
+	t.Run("has tax id code", func(t *testing.T) {
+		var nilParty *org.Party
+		assert.False(t, nilParty.HasTaxIDCode())
+
+		assert.False(t, (&org.Party{Name: "Test"}).HasTaxIDCode())
+
+		assert.False(t, (&org.Party{
+			TaxID: &tax.Identity{Country: "ES"},
+		}).HasTaxIDCode())
+
+		assert.True(t, (&org.Party{
+			TaxID: &tax.Identity{Country: "ES", Code: "B85905495"},
+		}).HasTaxIDCode())
+	})
 	t.Run("with regime and bad code", func(t *testing.T) {
 		party := org.Party{
 			Regime: tax.WithRegime("DE"),

pkg/luhn/luhn.go

@@ -4,6 +4,7 @@ package luhn
 
 import (
 	"regexp"
+	"strconv"
 
 	"github.com/invopop/gobl/cbc"
 )
@@ -40,3 +41,23 @@ func Check(code cbc.Code) bool {
 
 	return checksum%10 == 0
 }
+
+// CheckDigit computes the Luhn check digit for the given numeric string.
+// The caller is responsible for ensuring the input contains only ASCII digit
+// characters; no validation is performed on the input.
+func CheckDigit(number string) string {
+	sum := 0
+	pos := 0
+	for i := len(number) - 1; i >= 0; i-- {
+		digit := int(number[i] - '0')
+		if pos%2 == 0 {
+			digit *= 2
+			if digit > 9 {
+				digit -= 9
+			}
+		}
+		sum += digit
+		pos++
+	}
+	return strconv.Itoa((10 - sum%10) % 10)
+}

pkg/luhn/luhn_test.go

@@ -51,3 +51,29 @@ func TestCheck(t *testing.T) {
 		})
 	}
 }
+
+func TestCheckDigit(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name   string
+		number string
+		want   string
+	}{
+		{name: "single digit", number: "0", want: "0"},
+		{name: "credit card base", number: "411111111111111", want: "1"},
+		{name: "luhn example", number: "7992739871", want: "3"},
+		{name: "italian VAT", number: "0271580010", want: "4"},
+		{name: "french SIREN", number: "73282932", want: "0"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got := luhn.CheckDigit(tt.number)
+			assert.Equal(t, tt.want, got)
+			// Verify consistency: number + check digit should pass Check.
+			full := cbc.Code(tt.number + got)
+			assert.True(t, luhn.Check(full), "number+check digit should pass Check: %s", full)
+		})
+	}
+}

regimes/be/invoices.go

@@ -32,18 +32,14 @@ func validateInvoiceSupplier(value any) error {
 		),
 		validation.Field(&p.Identities,
 			validation.When(
-				!hasTaxIDCode(p),
+				!p.HasTaxIDCode(),
 				org.RequireIdentityType(IdentityTypeBCE),
 			),
 			validation.Skip,
 		),
 	)
 }
 
-func hasTaxIDCode(party *org.Party) bool {
-	return party != nil && party.TaxID != nil && party.TaxID.Code != ""
-}
-
 func hasIdentityBCE(party *org.Party) bool {
 	if party == nil || len(party.Identities) == 0 {
 		return false

regimes/de/invoices.go

@@ -37,7 +37,7 @@ func validateInvoiceSupplier(value any) error {
 		),
 		validation.Field(&p.Identities,
 			validation.When(
-				!hasTaxIDCode(p),
+				!p.HasTaxIDCode(),
 				org.RequireIdentityKey(IdentityKeyTaxNumber),
 			),
 			validation.Skip,
@@ -49,10 +49,6 @@ func isSimplified(inv *bill.Invoice) bool {
 	return inv.HasTags(tax.TagSimplified)
 }
 
-func hasTaxIDCode(party *org.Party) bool {
-	return party != nil && party.TaxID != nil && party.TaxID.Code != ""
-}
-
 func hasIdentityTaxNumber(party *org.Party) bool {
 	if party == nil || len(party.Identities) == 0 {
 		return false

regimes/dk/invoices.go

@@ -32,18 +32,14 @@ func validateInvoiceSupplier(value any) error {
 		),
 		validation.Field(&p.Identities,
 			validation.When(
-				!hasTaxIDCode(p),
+				!p.HasTaxIDCode(),
 				org.RequireIdentityType(IdentityTypeCVR),
 			),
 			validation.Skip,
 		),
 	)
 }
 
-func hasTaxIDCode(party *org.Party) bool {
-	return party != nil && party.TaxID != nil && party.TaxID.Code != ""
-}
-
 func hasIdentityCVR(party *org.Party) bool {
 	if party == nil || len(party.Identities) == 0 {
 		return false

regimes/fr/invoices.go

@@ -32,18 +32,14 @@ func validateInvoiceSupplier(value any) error {
 		),
 		validation.Field(&p.Identities,
 			validation.When(
-				!hasTaxIDCode(p),
+				!p.HasTaxIDCode(),
 				org.RequireIdentityType(IdentityTypeSIREN, IdentityTypeSIRET),
 			),
 			validation.Skip,
 		),
 	)
 }
 
-func hasTaxIDCode(party *org.Party) bool {
-	return party != nil && party.TaxID != nil && party.TaxID.Code != ""
-}
-
 func hasSupplierIdentity(party *org.Party) bool {
 	if party == nil || len(party.Identities) == 0 {
 		return false

regimes/fr/tax_identity.go

@@ -7,6 +7,7 @@ import (
 	"strconv"
 
 	"github.com/invopop/gobl/cbc"
+	"github.com/invopop/gobl/pkg/luhn"
 	"github.com/invopop/gobl/tax"
 	"github.com/invopop/validation"
 )
@@ -98,32 +99,9 @@ func validateSIRENTaxCode(value any) error {
 
 	base := str[:8]
 	chk := str[8:]
-	v := computeLuhnCheckDigit(base)
-	if chk != v {
+	if luhn.CheckDigit(base) != chk {
 		return errors.New("checksum mismatch")
 	}
 
 	return nil
 }
-
-// TODO: refactor this into a shareable method.
-func computeLuhnCheckDigit(number string) string {
-	sum := 0
-	pos := 0
-
-	for i := len(number) - 1; i >= 0; i-- {
-		digit := int(number[i] - '0')
-
-		if pos%2 == 0 {
-			digit *= 2
-			if digit > 9 {
-				digit -= 9
-			}
-		}
-
-		sum += digit
-		pos++
-	}
-
-	return strconv.FormatInt(int64((10-(sum%10))%10), 10)
-}

regimes/it/tax_identity.go

@@ -7,9 +7,9 @@ package it
 
 import (
 	"errors"
-	"strconv"
 
 	"github.com/invopop/gobl/cbc"
+	"github.com/invopop/gobl/pkg/luhn"
 	"github.com/invopop/gobl/tax"
 	"github.com/invopop/validation"
 )
@@ -49,32 +49,9 @@ func validateTaxCode(value interface{}) error {
 		return errors.New("invalid length")
 	}
 
-	chk := computeLuhnCheckDigit(str[:10])
-	if chk != str[10:] {
+	if luhn.CheckDigit(str[:10]) != str[10:] {
 		return errors.New("invalid check digit")
 	}
 
 	return nil
 }
-
-// TODO: refactor this into a shareable method.
-func computeLuhnCheckDigit(number string) string {
-	sum := 0
-	pos := 0
-
-	for i := len(number) - 1; i >= 0; i-- {
-		digit := int(number[i] - '0')
-
-		if pos%2 == 0 {
-			digit *= 2
-			if digit > 9 {
-				digit -= 9
-			}
-		}
-
-		sum += digit
-		pos++
-	}
-
-	return strconv.FormatInt(int64((10-(sum%10))%10), 10)
-}

regimes/nl/invoices.go

@@ -32,18 +32,14 @@ func validateInvoiceSupplier(value interface{}) error {
 		),
 		validation.Field(&p.Identities,
 			validation.When(
-				!hasTaxIDCode(p),
+				!p.HasTaxIDCode(),
 				org.RequireIdentityType(IdentityTypeKVK, IdentityTypeOIN),
 			),
 			validation.Skip,
 		),
 	)
 }
 
-func hasTaxIDCode(party *org.Party) bool {
-	return party != nil && party.TaxID != nil && party.TaxID.Code != ""
-}
-
 func hasSupplierIdentity(party *org.Party) bool {
 	if party == nil || len(party.Identities) == 0 {
 		return false