Luhn Generator

main.go

// Given a first 6 digits and last 4 digits.
// It will generate full 16-digit PANs that satisfy Luhn algorithm.
package main

import (
	"flag"
	"fmt"
	"strconv"
	"strings"
)

func main() {
	flag.Parse()

	args := flag.Args()
	lead := args[0]
	tail := args[1]

	for i := 0; i <= 999999; i++ {
		if luhnValid(fmt.Sprintf("%s%06d%s", lead, i, tail)) {
			fmt.Printf("%s%06d%s\n", lead, i, tail)
		}
	}
}

func luhnValid(luhnString string) bool {
	checksumMod := calculateChecksum(luhnString, false) % 10

	return checksumMod == 0
}

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
}