Turn any HTTP file into a torrent

main.go

package main

import (
	"crypto/sha1"
	"fmt"
	"io"
	"net/http"
	"os"
	"strings"

	"github.com/marksamman/bencode"
)

const file = "http://link.us1.storjshare.io/s/jwqpp4kdw2nhm4ep2lkrxmmhw6oa/billtest/bunny.mp4?download=1"
const pieceLength = 2621440

func main() {
	size, hashes, err := GenerateHashes(file)
	if err != nil {
		fmt.Printf("error: %+v", err)
		return
	}
	fmt.Printf("%d %d %f\n", size, len(hashes)/40, float64(size)/float64(pieceLength))

	dict := make(map[string]interface{})
	dict["announce"] = "udp://tracker.torrent.eu.org:451"
	dict["comment"] = "Testing torrents"
	dict["creation date"] = 1391870037
	// This is how the original spec HTTP seeding would work (see http://www.bittorrent.org/beps/bep_0019.html)
	// dict["httpseeds"] = []interface{}{file}
	// This is "get-right" style HTTP seeding. Most clients lack HTTPS support. (https://www.getright.com/seedtorrent.html)
	dict["url-list"] = []interface{}{file}
	infoDict := make(map[string]interface{})
	infoDict["length"] = size
	infoDict["name"] = "bunny.mp4"
	infoDict["piece length"] = pieceLength
	infoDict["pieces"] = hashes
	dict["info"] = infoDict

	os.WriteFile("file.torrent", bencode.Encode(dict), 0644)
}

func GenerateHashes(url string) (int, string, error) {
	size := 0
	var sb strings.Builder

	resp, err := http.Get(url)
	if err != nil {
		return 0, "", err
	}
	defer resp.Body.Close()

	for {
		buf := make([]byte, pieceLength)
		n := 0
		for n < pieceLength && err == nil {
			var nn int
			nn, err = resp.Body.Read(buf[n:])
			n += nn
		}
		if n > 0 {
			size += n
			h := sha1.New()
			h.Write(buf[:n])
			sb.WriteString(string(h.Sum(nil)))
		}
		if err != nil {
			if err == io.EOF {
				err = nil
			}
			break
		}
	}
	return size, sb.String(), err
}