luhn.go

package luhn

// Utility functions for generating valid luhn strings and validating against the Luhn algorithm

import (
	"math/rand"
	"strconv"
	"strings"
	"time"
)

// Valid returns a boolean indicating if the argument was valid according to the Luhn algorithm.
func Valid(luhnString string) bool {
	checksumMod := calculateChecksum(luhnString, false) % 10

	return checksumMod == 0
}

// Generate creates and returns a string of the length of the argument targetSize.
// The returned string is valid according to the Luhn algorithm.
func Generate(size int) string {
	random := randomString(size - 1)
	controlDigit := strconv.Itoa(generateControlDigit(random))

	return random + controlDigit
}

// GenerateWithPrefix creates and returns a string of the length of the argument targetSize
// but prefixed with the second argument.
// The returned string is valid according to the Luhn algorithm.
func GenerateWithPrefix(size int, prefix string) string {
	size = size - 1 - len(prefix)

	random := prefix + randomString(size)
	controlDigit := strconv.Itoa(generateControlDigit(random))

	return random + controlDigit
}

func randomString(size int) string {
	rand.Seed(time.Now().UTC().UnixNano())
	source := make([]int, size)

	for i := 0; i < size; i++ {
		source[i] = rand.Intn(9)
	}

	return integersToString(source)
}

func generateControlDigit(luhnString string) int {
	controlDigit := calculateChecksum(luhnString, true) % 10

	if controlDigit != 0 {
		controlDigit = 10 - controlDigit
	}

	return controlDigit
}

func calculateChecksum(luhnString string, double bool) int {
	source := strings.Split(luhnString, "")
	checksum := 0

	for i := len(source) - 1; i > -1; i-- {
		t, _ := strconv.ParseInt(source[i], 10, 8)
		n := int(t)

		if double {
			n = n * 2
		}

		double = !double

		if n >= 10 {
			n = n - 9
		}

		checksum += n
	}

	return checksum
}

func integersToString(integers []int) string {
	result := make([]string, len(integers))

	for i, number := range integers {
		result[i] = strconv.Itoa(number)
	}

	return strings.Join(result, "")
}