sparse-download.go

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

func Download(url string, filepath string) error {
	offset := int64(0)
	info, err := os.Stat(filepath)
	if err == nil {
		// continue to donwload
		offset = info.Size()
	}

	fs, err := os.OpenFile(filepath, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
	if err != nil {
		// failre to open/create file
		return err
	}
	defer fs.Close()

	// check file size from remote
	request, err := http.NewRequest("HEAD", url, nil)
	if err != nil {
		return err
	}

	response, err := http.DefaultClient.Do(request)
	if err != nil {
		return err
	}

	if response.StatusCode < 200 || response.StatusCode > 299 {
		return fmt.Errorf("request HEAD with status code %d", response.StatusCode)
	}

	totalSize := response.ContentLength
	response.Body.Close()
	if totalSize <= offset {
		return nil
	}

	request, err = http.NewRequest("GET", url, nil)
	if err != nil {
		return err
	}

	request.Header.Set("Range", fmt.Sprintf("bytes=%d-", offset))

	response, err = http.DefaultClient.Do(request)
	if err != nil {
		return err
	}

	if response.StatusCode < 200 || response.StatusCode > 299 {
		return fmt.Errorf("request with status code %d", response.StatusCode)
	}

	defer response.Body.Close()

	_, err = fs.Seek(offset, io.SeekStart)
	if err != nil {
		return err
	}

	_, err = io.Copy(fs, response.Body)
	return err
}