downloader_hasher.go

package main

import (
	"crypto/sha256"
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
)

type FileInfo struct {
	sourceURL  string
	fileSHA256 string
	fileSize   int64
	filePath   string
}

// Load a remote PDF file from a source URL
// and simultaneously write it locally to disk
// calculates its size and SHA256 checksum
func Load(sourceURL string) (*FileInfo, error) {
	tmpFile, err := ioutil.TempFile("", "*.pdf")
	if err != nil {
		return nil, err
	}
	defer tmpFile.Close()

	info := &FileInfo{
		sourceURL: sourceURL,
		filePath:  tmpFile.Name(),
	}

	resp, err := http.Get(sourceURL)
	if err != nil {
		return info, err
	}
	defer resp.Body.Close()

	hasher := sha256.New()
	n, err := io.Copy(io.MultiWriter(hasher, tmpFile), resp.Body)
	if err != nil {
		return info, err
	}
	info.fileSize = n
	info.fileSHA256 = fmt.Sprintf("%x", hasher.Sum(nil))
	info.filePath = tmpFile.Name()

	return info, nil
}