Lookup the location attributed to an IP by IP2Location Lite Country DB (https://lite.ip2location.com/database/ip-country)

ip2location.go

package main

import (
	"bufio"
	"encoding/csv"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
	"sort"
	"strconv"
	"strings"
)

func ipStrToInt(ipStr string) (int, error) {
	ipArr := strings.Split(ipStr, ".")
	if len(ipArr) != 4 {
		err := errors.New("IP not valid: length not 4")
		return -1, err
	}

	var ipInt int
	for idx, elem := range ipArr {
		elemInt, err := strconv.Atoi(elem)
		if err != nil {
			log.Print("IP not valid: couldn't convert to Int")
			return -1, err
		}
		switch idx {
		case 0:
			ipInt += elemInt * 16777216
		case 1:
			ipInt += elemInt * 65536
		case 2:
			ipInt += elemInt * 256
		case 3:
			ipInt += elemInt
		}
	}

	return ipInt, nil
}

func loadIP2LocationLiteDB(path string) (ranges []int, countries []string, err error) {
	f, err := os.Open(path)
	if err != nil {
		log.Print("error opening")
		return nil, nil, err
	}
	defer f.Close()

	r := csv.NewReader(bufio.NewReader(f))
	for {
		record, err := r.Read()
		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, nil, err
		}
		rangeInt, err := strconv.Atoi(record[0])
		if err != nil {
			return nil, nil, err
		}
		rangeInt--
		ranges = append(ranges, rangeInt)
		countries = append(countries, record[3])
	}

	return ranges, countries, nil
}

func ipCountryLookup(ip string, ranges []int, countries []string) (string, error) {
	n, err := ipStrToInt(ip)
	if err != nil {
		log.Print(err)
		return "", err
	}
	return countries[sort.SearchInts(ranges, n)], nil
}

func main() {
	ranges, countries, err := loadIP2LocationLiteDB("./IP2LOCATION-LITE-DB1.CSV")
	if err != nil {
		log.Print(err)
		return
	}
	ipStr := "163.172.164.39"
	country, err := ipCountryLookup(ipStr, ranges, countries)
	if err != nil {
		log.Print(err)
		return
	}
	fmt.Println(country)
}