download progress in go

download_progress.go

// PassThru wraps an existing io.Reader.
//
// It simply forwards the Read() call, while displaying
// the results from individual calls to it.
type PassThru struct {
	io.Reader
	total    int64 // Total # of bytes transferred
	length   int64 // Expected length
	progress float64
}

// Read 'overrides' the underlying io.Reader's Read method.
// This is the one that will be called by io.Copy(). We simply
// use it to keep track of byte counts and then forward the call.
func (pt *PassThru) Read(p []byte) (int, error) {
	n, err := pt.Reader.Read(p)
	if n > 0 {
		pt.total += int64(n)
		percentage := float64(pt.total) / float64(pt.length) * float64(100)
		i := int(percentage / float64(10))
		is := fmt.Sprintf("%v", i)
		if percentage-pt.progress > 2 {
			fmt.Fprintf(os.Stderr, is)
			pt.progress = percentage
		}
	}
	return n, err
}

func download(url string, output string) (err error) {
	// create the file
	out, err := os.Create(output)
	if err != nil  {
		return err
	}
	defer out.Close()

	response, err := http.Get(url)
	if err != nil {
		return err
	}

	defer response.Body.Close()

	readerpt := &PassThru{Reader: response.Body, length: response.ContentLength}
  
	// check server response
	if response.StatusCode != http.StatusOK {
		return fmt.Errorf("bad status: %s", response.Status)
	}

	// Writer the body to file
	_, err = io.Copy(out, readerpt)
	if err != nil  {
		return err
	}

	return nil
}