1 files changed, 32 insertions, 0 deletions
diff --git a/luhn/luhn.go b/luhn/luhn.go
new file mode 100644
index 0000000..0724320
--- /dev/null
+++ b/luhn/luhn.go
@@ -0,0 +1,32 @@
+package luhn
+
+import (
+	"strconv"
+
+	"github.com/neonxp/checksum"
+)
+
+// Check number is correct by luhn algorithm
+func Check(number string) error {
+	mod := len(number) % 2
+	sum := 0
+	for i, ch := range number {
+		num, err := strconv.Atoi(string(ch))
+		if err != nil {
+			return checksum.ErrInvalidNumber
+		}
+		if i%2 == mod {
+			if num < 5 {
+				sum += num * 2
+			} else {
+				sum += num*2 - 9
+			}
+		} else {
+			sum += num
+		}
+	}
+	if sum%10 != 0 {
+		return checksum.ErrInvalidChecksum
+	}
+	return nil
+}