Added Damm algorithm

1 files changed, 49 insertions, 0 deletions

diff --git a/damm/damm.go b/damm/damm.go
new file mode 100644
index 0000000..388eb00
--- /dev/null
+++ b/damm/damm.go
@@ -0,0 +1,49 @@
+package damm
+
+import (
+	"strconv"
+
+	"github.com/neonxp/checksum"
+)
+
+var d = [10][10]int{
+	{0, 3, 1, 7, 5, 9, 8, 6, 4, 2},
+	{7, 0, 9, 2, 1, 5, 4, 8, 6, 3},
+	{4, 2, 0, 6, 8, 7, 1, 3, 5, 9},
+	{1, 7, 5, 0, 9, 8, 3, 4, 2, 6},
+	{6, 1, 2, 3, 0, 4, 5, 9, 7, 8},
+	{3, 6, 7, 4, 2, 0, 9, 5, 8, 1},
+	{5, 8, 6, 9, 7, 2, 0, 1, 3, 4},
+	{8, 9, 4, 5, 3, 6, 2, 0, 1, 7},
+	{9, 4, 3, 8, 6, 1, 7, 2, 0, 5},
+	{2, 5, 8, 1, 4, 3, 6, 7, 9, 0},
+}
+
+// Check number is correct by damm algorithm
+func Check(number string) error {
+	c := 0
+	for _, ch := range number {
+		num, err := strconv.Atoi(string(ch))
+		if err != nil {
+			return checksum.ErrInvalidNumber
+		}
+		c = d[c][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
+	for _, ch := range number {
+		num, err := strconv.Atoi(string(ch))
+		if err != nil {
+			return "", checksum.ErrInvalidNumber
+		}
+		c = d[c][num]
+	}
+	return strconv.Itoa(c), nil
+}