用golang写一个大文件分片下载的代码

这是一个简单的大文件分片下载的代码，使用golang实现：

package main

import (
    "fmt"
    "io"
    "net/http"
    "os"
    "strconv"
    "sync"
)

func main() {
    url := "https://example.com/largefile.iso"
    filename := "largefile.iso"
    numChunks := 10
    wg := &sync.WaitGroup{}
    for i := 0; i < numChunks; i++ {
        start := i * (filesize(url) / numChunks)
        end := (i + 1) * (filesize(url) / numChunks)
        if i == numChunks-1 {
            end = filesize(url)
        }
        wg.Add(1)
        go downloadChunk(url, start, end, filename, wg)
    }
    wg.Wait()
    fmt.Println("Downloaded", filename)
}

func filesize(url string) int64 {
    resp, err := http.Head(url)
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close()
    size, err := strconv.Atoi(resp.Header.Get("Content-Length"))
    if err != nil {
        panic(err)
    }
    return int64(size)
}

func downloadChunk(url string, start int, end int, filename string, wg *sync.WaitGroup) {
    defer wg.Done()
    req, err := http.NewRequest("GET", url, nil)
    if err != nil {
        panic(err)
    }
    req.Header.Set("Range", fmt.Sprintf("bytes=%d-%d", start, end))
    client := &http.Client{}
    resp, err := client.Do(req)
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close()
    file, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0644)
    if err != nil {
        panic(err)
    }
    defer file.Close()
    _, err = file.Seek(int64(start), 0)
    if err != nil {
        panic(err)
    }
    _, err = io.Copy(file, resp.Body)
    if err != nil {
        panic(err)
    }
    fmt.Printf("Downloaded bytes %d-%d\n", start, end)
}

这个代码将文件分成了10个块，每个块使用HTTP Range头请求。然后，每个块都被写入到同一个文件中。最后，在所有块都下载完成后，程序将输出“Downloaded filename”来表示下载完成