Concurrent Downloader

downloader.go

package main

import (
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
	"net/url"
	"os"
	"runtime"
	"strconv"
	"strings"
	"sync"

	"github.com/cheggaaa/pb"
)

var wg sync.WaitGroup

func main() {
	if len(os.Args) <= 1 {
		fmt.Println("Usage:", os.Args[0], "<File URL>")
		os.Exit(1)
	}
	baseURL, err := url.Parse(os.Args[1])
	if err != nil {
		panic(err)
	}
	str := baseURL.RequestURI()
	i := strings.LastIndex(str, "/")
	name := str[i+1:]

	res, _ := http.Head(os.Args[1])

	maps := res.Header
	length, _ := strconv.Atoi(maps["Content-Length"][0])
	limit := 250
	lenSub := length / limit
	diff := length % limit
	bar := pb.New(length).SetUnits(pb.U_BYTES)
	bar.ShowSpeed = true
	bar.Start()

	for i := 0; i < limit; i++ {
		wg.Add(1)

		min := lenSub * i       // Min range
		max := lenSub * (i + 1) // Max range

		if i == limit-1 {
			max += diff
		}
		go func(min int, max int, i int) {

			client := &http.Client{}
			req, _ := http.NewRequest("GET", os.Args[1], nil)

			rangeHeader := "bytes=" + strconv.Itoa(min) + "-" + strconv.Itoa(max-1)
			req.Header.Add("Range", rangeHeader)
			resp, _ := client.Do(req)
			defer resp.Body.Close()

			r := io.Reader(resp.Body)
			w, err := os.OpenFile(name+strconv.Itoa(i)+".tmp", os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0666)
			if err != nil {
				fmt.Println("[-] Can't Create File")
			}

			writer := io.MultiWriter(w, bar)
			io.Copy(writer, r)
			wg.Done()
		}(min, max, i)
	}
	wg.Wait()
	concatFiles(limit, name)
}

func concatFiles(counter int, name string) {
	bar := pb.StartNew(counter)
	f, err := os.OpenFile(name, os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0644)
	if err != nil {
		panic(err)
	}
	defer f.Close()

	for i := 0; i < counter; i++ {
		bar.Increment()
		filename := name + strconv.Itoa(i) + ".tmp"
		b, err := ioutil.ReadFile(filename)
		f.Write(b)
		if err != nil {
			panic(err)
		}
		os.Remove(filename)
		runtime.GC()
	}
	bar.FinishPrint(name + " Complete!")
}