mdownloader.go

/**
理论基础: https://en.wikipedia.org/wiki/Byte_serving
使用range header可以得到网络资源的一部分字节
 */
package main
import (
	"net/http"
	"fmt"
	"os"
	"io/ioutil"
	"strconv"
)

func myPanic(err error) {
	fmt.Printf("%s\n", err)
	os.Exit(1)
}

const url = "http://download.cardinfolink.net/android/showmoney.apk"
const threadNum = 3

func getPartOfFile(start, end int) ([]byte, error) {
	client := &http.Client{}
	req, err := http.NewRequest("GET", url, nil)
	if err != nil {
		return nil, err
	}
	rangeVal := fmt.Sprintf("bytes=%d-%d", start, end)
	req.Header.Set("Range", rangeVal)

	response, err := client.Do(req)
	if err != nil {
		return nil, err
	}

	defer response.Body.Close()

	contents, err := ioutil.ReadAll(response.Body)
	if err != nil {
		return nil, err
	}
	fmt.Println("getPartOfFile done")

	c <- FilePiece{start:start, end:end, contents:contents}
	return contents, nil
}

type  FilePiece struct {
	start, end int
	contents   []byte
}
const filename = "/Users/jiahua/Downloads/xxx.apk"
var c = make(chan FilePiece, threadNum)

func main() {
	res, err := http.Head(url)
	if err != nil {
		myPanic(err)
	}
	fileLen, err := strconv.Atoi(res.Header.Get("Content-Length"))
	if err != nil {
		myPanic(err)
	}

	blockSize := fileLen / threadNum
	remainingSize := fileLen % threadNum
	fmt.Printf("fileSize: %d, blockSize for each thread: %d, remainingSize: %d\n", fileLen, blockSize, remainingSize)


	for i := 0; i < threadNum; i++ {
		start := i * blockSize
		end := (i + 1) * blockSize - 1
		if threadNum - 1 == i {
			end += remainingSize
		}
		fmt.Printf("Thread%d, start: %d end: %d\n", i, start, end)
		go getPartOfFile(start, end)
	}


	file, err := os.Create(filename)
	if err != nil {
		myPanic(err)
	}
	defer file.Close()
	for i := 0; i < threadNum; i++ {
		//routine返回的结果
		fp := <-c
		fmt.Printf("Content received, start: %d end: %d\n", fp.start, fp.end)
		// WriteAt会overwrite原有数据,之前一段不存在也能写入..
		_, err := file.WriteAt(fp.contents, int64(fp.start))
		if err != nil {
			myPanic(err)
		}
	}
	file.Sync()
	fmt.Println("File download Done")
}