[#BF7] Got test fail when cast String to get piece hashes

Challenges
1

I’m stuck on Stage #BF7

I’ve first tried to do String cast but I found out that it got wrong

Here are my logs:

remote: [tester::#BF7] Running ./your_program.sh info /tmp/torrents3080061021/test.torrent
remote: [your_program] Tracker URL: http://bttracker.debian.org:6969/announce
remote: [your_program] Length: 786432
remote: [your_program] Info Hash: 4fdbe0051276faacdeeadc97846c29b420afca34
remote: [your_program] Piece Length: 262144
remote: [your_program] Piece Hashes:
remote: [your_program] 3f301d263f52063f3f2d3f3f1e643f3f3f3f013f
remote: [your_program] 155a133f7c3c3f651d4f3f3f3f3f3f3f3f6b703f
remote: [your_program] 3f3f611a3f293f3f673f3f3f5d3f083f3f3f
remote: [tester::#BF7] Expected stdout to contain "8f301d26c6895206ec91de2df5971e64b4d2e1d5", g

And here’s a snippet of my code:

info = (Map<String, Object>) root.get("info");

        announce = (String) root.get("announce");
        length = (Long) info.get("length");
        try{
            MessageDigest messageDigest = MessageDigest.getInstance("SHA-1");
            byte[] encoded = messageDigest.digest(bencode2.encode(
                    (Map<String,Object>) bencode2.decode(bytes,Type.DICTIONARY).get("info")));
            infoHash = getInfoHash(encoded);
        }catch(NoSuchAlgorithmException e) {
            System.out.println("No such algorithm");
            infoHash = null;
        }
        pieceLength = (Long)info.get("piece length");
        System.out.println(length);
        String pieceString = (String) info.get("pieces");
        pieces = pieceString.getBytes();
        printPieceHashes(info);


System.out.print("Piece Hashes: ");
        byte[] piece = torrent.pieces;
        for(int i = 0; i < piece.length; ++i){
            if(i % 20 == 0){
                System.out.println();
            }
            System.out.printf("%02x",piece[i]);
        }

Thank you for helping me

3

i am also stuck, whenever I typecast to String, some characters are converted to ? which result in incorrect hash. what is the solution

4

You need to encode using true in BenCode constructor. this ensures, you encode using Bytes, so that you do not end up with ? in typecasting to String, you can then directly typecast to byte[] array. Here is the code snippet:


 public Torrent(byte[] bytes) throws NoSuchAlgorithmException {
        Bencode bencode1 = new Bencode(false);
        Bencode bencode2 = new Bencode(true);
        MessageDigest digest2 = MessageDigest.getInstance("SHA-1");

        Map<String, ?> root = bencode1.decode(bytes, Type.DICTIONARY);
        Map<String, ?> info = (Map<String, Object>) root.get("info");
        Map<String, ?> infoByteBuffer = (Map<String, ?>) bencode2.decode(bytes, Type.DICTIONARY).get("info");

        announce = (String) root.get("announce");
        length = (long) info.get("length");
        ByteBuffer pieceBuffer = (ByteBuffer) infoByteBuffer.get("pieces");
        pieces = new byte[pieceBuffer.remaining()];
        pieceBuffer.get(pieces);
        pieceLength = (long) info.get("piece length");
        infoHash = digest2.digest(bencode2.encode(infoByteBuffer));
    }

public List<byte[]> getPieces() {
        List<byte[]> list = new ArrayList<>();
        for (int i = 0; i + 20 <= pieces.length; i += 20) {
            list.add(Arrays.copyOfRange(pieces, i, i + 20));
        }
        return list;
    }

Don’t forget to import the ByteBuffer line
import java.nio.ByteBuffer;

2 Likes
6

Thank you so much for helping me. I have tried your implementation and it worked

7

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