如何用Golang的goroutine和channel构建一个并发下载文件的程序

type DownloadPart struct {
    URL  string
    Start int64
    End   int64
    Index int
}
<p>type Result struct {
Index int
Data  []byte
Error error
}</p>

jobs := make(chan DownloadPart, numParts)
results := make(chan Result, numParts)

for w := 0; w < 4; w++ { // 启动4个并发worker
    go func() {
        for part := range jobs {
            resp, err := http.Get(part.URL)
            if err != nil {
                results <- Result{Index: part.Index, Error: err}
                continue
            }
            defer resp.Body.Close()
<pre class='brush:php;toolbar:false;'>        // 设置Range头下载指定字节范围
        req, _ := http.NewRequest("GET", part.URL, nil)
        req.Header.Set("Range", fmt.Sprintf("bytes=%d-%d", part.Start, part.End))
        client := &http.Client{}
        resp, err = client.Do(req)
        if err != nil {
            results <- Result{Index: part.Index, Error: err}
            continue
        }
        data, _ := io.ReadAll(resp.Body)
        results <- Result{Index: part.Index, Data: data}
    }
}()

for i := 0; i < numParts; i++ {
    start := int64(i) * chunkSize
    end := start + chunkSize - 1
    if end > fileSize-1 {
        end = fileSize - 1
    }
    jobs <- DownloadPart{URL: fileURL, Start: start, End: end, Index: i}
}
close(jobs)

var parts = make([][]byte, numParts)
for i := 0; i < numParts; i++ {
    result := <-results
    if result.Error != nil {
        log.Fatalf("下载失败: %v", result.Error)
    }
    parts[result.Index] = result.Data
}

out, _ := os.Create("output.bin")
defer out.Close()
for _, data := range parts {
    out.Write(data)
}