From 1ef5669a325159c6d902c34e04e20ab24c8e77da Mon Sep 17 00:00:00 2001
From: Ferdy Pruis <ferdypruis@users.noreply.github.com>
Date: Mon, 28 Dec 2020 12:51:11 +0100
Subject: [PATCH] Performance optimizations

---
 go.mod  |  2 +-
 luhn.go | 78 ++++++++++++++++++++++++++++++++++-----------------------
 2 files changed, 48 insertions(+), 32 deletions(-)

diff --git a/go.mod b/go.mod
index fabf3c9..3e38fc2 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
 module github.com/ferdypruis/go-luhn
 
-go 1.12
+go 1.15
diff --git a/luhn.go b/luhn.go
index c0c00b5..a6418b3 100644
--- a/luhn.go
+++ b/luhn.go
@@ -1,9 +1,9 @@
 // Package luhn implements the Luhn algorithm, or Luhn formula, also known as the "modulus 10" or "mod 10" algorithm.
 package luhn
 
-type Error string
-
-func (e Error) Error() string { return "luhn: " + string(e) }
+import (
+	"fmt"
+)
 
 // mod10 is the pre-calculated value of 10 minus the modulos-10 of every digit
 const mod10 = "0987654321"
@@ -12,60 +12,76 @@ const mod10 = "0987654321"
 // When the result of doubling is greater than 9 the digits are added together; 16 => 1 + 6 => 7
 var doubles = [10]int{0, 2, 4, 6, 8, 1, 3, 5, 7, 9}
 
-// Checksum returns the Lühn check digit for number.
+// Checksum returns the check digit for the number.
 func Checksum(number string) (string, error) {
-	if number == "" {
+	l := len(number)
+	if l == 0 {
 		return "", Error("input is too short")
 	}
 
-	// determine even or odd length, so we know when to double
-	double := 1&len(number) == 1
-
 	var sum int
-	for _, c := range []byte(number) {
-		// assert character is numeric
-		if c < '0' || c > '9' {
-			return "", Error("input contains a non-number; " + string(c))
+
+	for i := l - 1; i >= 0; i -= 2 {
+		n := int(number[i] - '0')
+		if n < 0 || n > 9 {
+			return "", Error(fmt.Sprintf("character %q at position %d is not a digit", number[i], i))
 		}
 
-		// numeric value from character
-		n := int(c - '0')
+		sum += doubles[n]
+	}
 
-		// double every other digit
-		if double {
-			n = doubles[n]
+	for i := l - 2; i >= 0; i -= 2 {
+		n := int(number[i] - '0')
+		if n < 0 || n > 9 {
+			return "", Error(fmt.Sprintf("character %q at position %d is not a digit", number[i], i))
 		}
-		double = !double
 
 		sum += n
 	}
 
 	unit := sum % 10
-	return mod10[unit : unit+1], nil
+	return mod10[unit:unit+1], nil
 }
 
-// Valid returns whether number verifies against its appended check digit
+// Valid returns whether number verifies against its appended check digit.
 func Valid(number string) bool {
-	l := len(number) - 1
-	if l < 1 {
+	l := len(number)
+	if l < 2 {
 		return false
 	}
 
-	checkdigit, err := Checksum(number[:l])
-	if err != nil {
-		return false
+	var sum = int(number[l-1] - '0')
+
+	for i := l - 2; i >= 0; i -= 2 {
+		n := int(number[i] - '0')
+		if n < 0 || n > 9 {
+			return false
+		}
+
+		sum += doubles[n]
 	}
 
-	// compare checksum with checkdigit in input
-	return checkdigit == number[l:]
+	for i := l - 3; i >= 0; i -= 2 {
+		n := int(number[i] - '0')
+		if n < 0 || n > 9 {
+			return false
+		}
+
+		sum += n
+	}
+
+	return sum % 10 == 0
 }
 
-// Sign returns number with its Lühn check digit appended
-func Sign(number string) (string, error) {
-	digit, err := Checksum(number)
+// Sign returns the partial number with its Lühn check digit appended, creating a full number.
+func Sign(partial string) (string, error) {
+	checkdigit, err := Checksum(partial)
 	if err != nil {
 		return "", err
 	}
 
-	return number + digit, nil
+	return partial + checkdigit, nil
 }
+
+type Error string
+func (e Error) Error() string { return "luhn: " + string(e) }