2 files changed, 103 insertions, 0 deletions
diff --git a/verhoeff/verhoeff.go b/verhoeff/verhoeff.go
new file mode 100644
index 0000000..259a7df
--- /dev/null
+++ b/verhoeff/verhoeff.go
@@ -0,0 +1,69 @@
+package verhoeff
+
+import (
+	"strconv"
+	"strings"
+
+	"github.com/neonxp/checksum"
+)
+
+var d = [10][10]int{
+	{0, 1, 2, 3, 4, 5, 6, 7, 8, 9},
+	{1, 2, 3, 4, 0, 6, 7, 8, 9, 5},
+	{2, 3, 4, 0, 1, 7, 8, 9, 5, 6},
+	{3, 4, 0, 1, 2, 8, 9, 5, 6, 7},
+	{4, 0, 1, 2, 3, 9, 5, 6, 7, 8},
+	{5, 9, 8, 7, 6, 0, 4, 3, 2, 1},
+	{6, 5, 9, 8, 7, 1, 0, 4, 3, 2},
+	{7, 6, 5, 9, 8, 2, 1, 0, 4, 3},
+	{8, 7, 6, 5, 9, 3, 2, 1, 0, 4},
+	{9, 8, 7, 6, 5, 4, 3, 2, 1, 0},
+}
+
+var p = [8][10]int{
+	{0, 1, 2, 3, 4, 5, 6, 7, 8, 9},
+	{1, 5, 7, 6, 2, 8, 3, 0, 9, 4},
+	{5, 8, 0, 3, 7, 9, 6, 1, 4, 2},
+	{8, 9, 1, 6, 0, 4, 3, 5, 2, 7},
+	{9, 4, 5, 3, 1, 2, 6, 8, 7, 0},
+	{4, 2, 8, 6, 5, 7, 3, 9, 0, 1},
+	{2, 7, 9, 3, 8, 0, 6, 4, 1, 5},
+	{7, 0, 4, 6, 9, 1, 3, 2, 5, 8},
+}
+
+var inv = [10]int{0, 4, 3, 2, 1, 5, 6, 7, 8, 9}
+
+// Check number is correct by luhn algorithm
+func Check(number string) error {
+	c := 0
+	numbers := strings.Split(number, "")
+	l := len(numbers)
+	for i := l; i > 0; i-- {
+		ch := numbers[i-1]
+		num, err := strconv.Atoi(string(ch))
+		if err != nil {
+			return checksum.ErrInvalidNumber
+		}
+		c = d[c][p[(l-i)&7][num]]
+	}
+	if c != 0 {
+		return checksum.ErrInvalidChecksum
+	}
+	return nil
+}
+
+// Generate checksum (must be added to number at right)
+func Generate(number string) (string, error) {
+	c := 0
+	numbers := strings.Split(number, "")
+	l := len(numbers)
+	for i := l; i > 0; i-- {
+		ch := numbers[i-1]
+		num, err := strconv.Atoi(string(ch))
+		if err != nil {
+			return "", checksum.ErrInvalidNumber
+		}
+		c = d[c][p[(l-i+1)&7][num]]
+	}
+	return strconv.Itoa(inv[c]), nil
+}
diff --git a/verhoeff/verhoeff_test.go b/verhoeff/verhoeff_test.go
new file mode 100644
index 0000000..7678b1f
--- /dev/null
+++ b/verhoeff/verhoeff_test.go
@@ -0,0 +1,34 @@
+package verhoeff
+
+import (
+	"testing"
+
+	"github.com/neonxp/checksum"
+)
+
+func TestVerhoeff(t *testing.T) {
+	samples := map[string]error{
+		"4561261212345464":        checksum.ErrInvalidChecksum,
+		"A561261212345464":        checksum.ErrInvalidNumber,
+		"758722":                  nil,
+		"123451":                  nil,
+		"1428570":                 nil,
+		"1234567890120":           nil,
+		"84736430954837284567892": nil,
+	}
+	for num, result := range samples {
+		if err := Check(num); err != result {
+			t.Errorf("Expected %+v actual %+v", result, err)
+		}
+	}
+
+	num := "4561261212345467"
+	checksum, err := Generate(num)
+	if err != nil {
+		t.Error(err)
+	}
+	numberWithChecksum := num + checksum
+	if err := Check(numberWithChecksum); err != nil {
+		t.Errorf("Expected no error actual %+v", err)
+	}
+}