gistfile1.go

package main

import (
	"bufio"
	"flag"
	"log"
	"os"
	"sort"
	"strings"
)

func readFile(path string, values map[string]struct{}) error {
	file, err := os.Open(path)
	if err != nil {
		return err
	}
	defer file.Close()
	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		values[strings.ToLower(scanner.Text())] = struct{}{}
	}
	if err := scanner.Err(); err != nil {
		return err
	}
	return nil
}

func writeFile(values map[string]struct{}, path string) error {
	domains := make([]string, 0)
	for d, _ := range values {
        domains = append(domains, d)
	}
	sort.Strings(domains)
	file, err := os.Create(path)
	if err != nil {
		return err
	}
	defer file.Close()
	if _, err := file.WriteString(strings.Join(domains, "\n")); err != nil {
		return err
	}
	return nil
}

func main() {
	flag.Parse()
	values := make(map[string]struct{})
	for _, path := range flag.Args() {
		if err := readFile(path, values); err != nil {
			log.Fatal(err)
		}
	}
	if err := writeFile(values, "whitelist.txt"); err != nil {
		log.Fatal(err)
	}
}