package main

import (
	"bufio"
	"fmt"
	"io"
	"io/fs"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"
	"regexp"
	"strings"
)

type AliasMap map[string]string

const (
	HostsFile string = "/etc/hosts"
)

func FixedSplit(s, sep string, parts int) []string {
	n := make([]string, parts)
	p := strings.SplitN(s, sep, parts)
	copy(n, p)
	return n
}

func IsLegalLine(line string) bool {
	c := line[0]
	return c != '[' && c != '#'
}

func BuildRegionString(regionName string, aliases AliasMap) string {
	b := strings.Builder{}
	b.WriteString("#region ")
	b.WriteString(regionName)
	b.WriteString("\n")

	for ip, alias := range aliases {
		b.WriteString(ip)
		b.WriteString("\t")
		b.WriteString(alias)
		b.WriteString("\n")
	}

	b.WriteString("#endregion")
	return b.String()
}

func BuildRegionRegexp(regionName string) *regexp.Regexp {
	b := strings.Builder{}
	b.WriteString("(?s)\n#region ")
	b.WriteString(regexp.QuoteMeta(regionName))
	b.WriteString(".*#endregion")
	r := regexp.MustCompile(b.String())
	return r
}

func ScanAliases(fileReader io.Reader) (AliasMap, error) {
	aliasMap := AliasMap{}
	scanner := bufio.NewScanner(fileReader)
	for scanner.Scan() {
		line := scanner.Text()
		if IsLegalLine(line) {
			s := FixedSplit(line, "#", 2)
			ip, alias := strings.TrimSpace(s[0]), strings.TrimSpace(s[1])
			if _, ok := aliasMap[ip]; !ok && alias != "" {
				aliasMap[ip] = alias
			}
		}
	}

	if err := scanner.Err(); err != nil {
		return nil, err
	}

	return aliasMap, nil
}

func AddAliases(fileName string) {
	file, err := os.Open(fileName)
	if err != nil {
		log.Fatal(err)
	}

	h, err := ScanAliases(file)
	if err != nil {
		log.Fatal(err)
	}

	file.Close()

	r := BuildRegionRegexp(fileName)
	s := BuildRegionString(fileName, h)

	content, err := ioutil.ReadFile(HostsFile)
	if err != nil {
		log.Fatal(err)
	}

	c := r.ReplaceAllString(string(content), s)
	if !r.MatchString(c) {
		c += ("\n" + s)
	}

	err = os.WriteFile(HostsFile, []byte(c[:]), fs.FileMode(os.O_WRONLY|os.O_TRUNC))
	if err != nil {
		log.Fatal(err)
	}
}

func RemoveAliases(fileName string) {
	regionReg := BuildRegionRegexp(fileName)
	content, err := ioutil.ReadFile(HostsFile)
	if err != nil {
		log.Fatal(err)
	}

	c := regionReg.ReplaceAll(content, []byte(""))
	err = os.WriteFile(HostsFile, c, fs.FileMode(os.O_WRONLY|os.O_TRUNC))
	if err != nil {
		log.Fatal(err)
	}
}

func main() {
	u := fmt.Sprintf("Please use: %s <add|rm> <file:path>\n", os.Args[0])

	if len(os.Args) < 3 {
		fmt.Println(u)
		os.Exit(1)
	}

	p, err := filepath.Abs(os.Args[2])
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}

	switch os.Args[1] {
	case "add":
		AddAliases(p)
	case "rm":
		RemoveAliases(p)
	default:
		fmt.Println(u)
		os.Exit(1)
	}
}