feat(torrenfile): parsing & encoding torrent file

pkg/torrentfile/bencode.go

@@ -0,0 +1,71 @@
+package torrentfile
+
+import (
+	"bytes"
+	"crypto/sha1"
+	"fmt"
+
+	"github.com/jackpal/bencode-go"
+)
+
+type bencodeInfo struct {
+	Pieces       string `bencode:"pieces"`
+	PiecesLength int    `bencode:"piece length"`
+	Length       int    `bencode:"length"`
+	Name         string `bencode:"name"`
+}
+
+type bencodeTorrent struct {
+	Announce string      `bencode:"announce"`
+	Info     bencodeInfo `bencode:"info"`
+}
+
+func (bto *bencodeTorrent) toTorrentFile() (TorrentFile, error) {
+	infoHash, err := bto.Info.hash()
+	if err != nil {
+		return TorrentFile{}, err
+	}
+
+	piecesHash, err := bto.Info.splitPieacesHash()
+	if err != nil {
+		return TorrentFile{}, err
+	}
+
+	t := TorrentFile{
+		Announce:    bto.Announce,
+		InfoHash:    infoHash,
+		PieceHashes: piecesHash,
+		PieceLength: bto.Info.PiecesLength,
+		Length:      bto.Info.Length,
+		Name:        bto.Info.Name,
+	}
+
+	return t, nil
+}
+
+func (bi *bencodeInfo) hash() ([20]byte, error) {
+	var buf bytes.Buffer
+	err := bencode.Marshal(&buf, *bi)
+	if err != nil {
+		return [20]byte{}, err
+	}
+	h := sha1.Sum(buf.Bytes())
+	return h, nil
+}
+
+func (bi *bencodeInfo) splitPieacesHash() ([][20]byte, error) {
+	// length of SHA-1 hash
+	hashLen := 20
+	buf := []byte(bi.Pieces)
+	if len(buf)%hashLen != 0 {
+		err := fmt.Errorf("Received malformed pieces of length %d", len(buf))
+		return nil, err
+	}
+	numHashes := len(buf) / hashLen
+	hashes := make([][20]byte, numHashes)
+
+	for i := 0; i < numHashes; i++ {
+		copy(hashes[i][:], buf[i*hashLen:(i+1)*hashLen])
+	}
+	return hashes, nil
+}