feat: add credit_card predefined mask

README.md

@@ -93,6 +93,7 @@ For more usage and examples see the [Godoc](http://godoc.org/github.com/ln80/str
 ### Predefined masks:
 - `email`
 - `ipv4_addr`
+- `credit_card`
 
 ## Limitations
 1.  Only fields of types convertible to `string` or `*string` are supported, although nesting structs directly or through collections (slices and maps) is also supported.

mask/credit_card.go

@@ -0,0 +1,87 @@
+package mask
+
+import (
+	"errors"
+	"strings"
+
+	"github.com/ln80/struct-sensitive/internal/option"
+)
+
+type CreditCardConfig struct {
+	MaskBankIdentifier bool // default false
+}
+
+func CreditCard(cardNumber string, opts ...func(*Config[CreditCardConfig])) (string, error) {
+	cfg := DefaultConfig(CreditCardConfig{
+		MaskBankIdentifier: false,
+	})
+	option.Apply(&cfg, opts)
+
+	formatCardNumber := func(cardNumber string, groupings []int) string {
+		totalLength := len(cardNumber)
+		formattedLength := totalLength + len(groupings) - 1
+
+		result := make([]rune, formattedLength)
+		pos := 0
+		index := 0
+
+		for _, group := range groupings {
+			if pos >= totalLength {
+				break
+			}
+
+			if index > 0 {
+				result[index] = ' '
+				index++
+			}
+
+			end := pos + group
+			if end > totalLength {
+				end = totalLength
+			}
+
+			for i := pos; i < end; i++ {
+				result[index] = rune(cardNumber[i])
+				index++
+			}
+
+			pos += group
+		}
+
+		return string(result)
+	}
+
+	cardNumber = strings.ReplaceAll(cardNumber, " ", "")
+	length := len(cardNumber)
+
+	if length < 15 || length > 16 {
+		return "", errors.New("unsupported credit card number format")
+	}
+
+	var groupings []int
+	if length == 15 {
+		groupings = []int{4, 6, 5} // Typical for American Express
+	} else {
+		groupings = []int{4, 4, 4, 4} // Default for standard cards
+	}
+
+	if cfg.Kind.MaskBankIdentifier {
+		maskUntil := length - groupings[len(groupings)-1]
+		masked := strings.Repeat(string([]rune{cfg.Symbol}), maskUntil) + cardNumber[maskUntil:]
+		return formatCardNumber(masked, groupings), nil
+	}
+
+	visibleStart := groupings[0]
+	visibleEnd := groupings[len(groupings)-1]
+	if length <= visibleStart+visibleEnd {
+		return cardNumber, nil
+	}
+	masked := cardNumber[:visibleStart] +
+		strings.Repeat(string([]rune{cfg.Symbol}), length-visibleStart-visibleEnd) +
+		cardNumber[length-visibleEnd:]
+	return formatCardNumber(masked, groupings), nil
+}
+
+func init() {
+	Register("credit_card", DefaultMasker(CreditCard))
+}

mask/credit_card_test.go

@@ -0,0 +1,67 @@
+package mask_test
+
+import (
+	"testing"
+
+	"github.com/ln80/struct-sensitive/mask"
+	"github.com/ln80/struct-sensitive/masktest"
+)
+
+func TestCreditCard(t *testing.T) {
+	masktest.Run(t, mask.CreditCard, []masktest.Tc[mask.CreditCardConfig]{
+		{
+			Value: "3012",
+			OK:    false,
+		},
+		{
+			Value: "4111 3145 4001 1111",
+			Want:  "4111 **** **** 1111",
+			OK:    true,
+		},
+		{
+			Value: "4111 3145 4001 1111",
+			Want:  "**** **** **** 1111",
+			OK:    true,
+			Option: func(c *mask.Config[mask.CreditCardConfig]) {
+				c.Kind.MaskBankIdentifier = true
+			},
+		},
+		{
+			Value: "3714 496353 98431",
+			Want:  "3714 ****** 98431",
+			OK:    true,
+		},
+		{
+			Value: "371449635398431",
+			Want:  "3714 ****** 98431",
+			OK:    true,
+		},
+		{
+			Value: "3714 496353 98431",
+			Want:  "**** ****** 98431",
+			OK:    true,
+			Option: func(c *mask.Config[mask.CreditCardConfig]) {
+				c.Kind.MaskBankIdentifier = true
+			},
+		},
+		{
+			Value: "3714 496353 98431",
+			Want:  "$$$$ $$$$$$ 98431",
+			OK:    true,
+			Option: func(c *mask.Config[mask.CreditCardConfig]) {
+				c.Kind.MaskBankIdentifier = true
+				c.Symbol = '$'
+			},
+		},
+	})
+}
+
+func BenchmarkCreditCard(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		if _, err := mask.CreditCard("4111 3145 4001 1111", func(mc *mask.Config[mask.CreditCardConfig]) {
+			mc.Kind.MaskBankIdentifier = true
+		}); err != nil {
+			b.Fatal(err)
+		}
+	}
+}