CODEWITHSUNDEEP
bittorrent
Stav Alfi
Stav Alfi

Reputation: 13923

If torrent contain multiple files, how to know what piece correspond to each file?

I'm building a BitTorrent client application in Java and I have 2 small question:

  1. Can torrent contain folders? recursively?
  2. If a torrent contains n files (not directories - for simplicity), do I need to create n files with their corresponding size? When I receive a piece from a peer, how do I know to which file it (the piece) belong?

For example, here is a torrent which contains 2 files:

TorrentInfo{Created By: ruTorrent (PHP Class - Adrien Gibrat)
Main tracker: http://tracker.hebits.net:35777/tracker.php?do=announce&passkey=5d3ab309eda55c1e7183975099958ab2
Comment: null
Info_hash: c504216ca4a113d26f023a10a1249ca3a6217997
Name: Veronica.2017.1080p.BluRay.DTS-HD.MA.5.1.x264-HDH
Piece Length: 16777216
Pieces: 787
Total Size: null
Is Single File Torrent: false
File List:
TorrentFile{fileLength=13202048630, fileDirs=[Veronica.2017.1080p.BluRay.DTS-HD.MA.5.1.x264-HDH.mkv]}
TorrentFile{fileLength=62543, fileDirs=[Veronica.2017.1080p.BluRay.DTS-HD.MA.5.1.x264-HDH.srt]}

The docs doesn't say much: https://wiki.theory.org/index.php/BitTorrentSpecification

Upvotes: 7

Views: 3347

Answers (2)

the8472
the8472

Reputation: 43042

Can torrent contain folders? recursively?

Yes.
Sortof. In BEP3 Nested directories are mapped into path elements, i.e. /dir1/dir2/dir3/file.ext is represented as path: ["dir1", "dir2", "dir3", "file.ext"] in the file list. BEP52 changes this to a tree-based structure more closely resembling a directory tree.

If a torrent contains n files (not directories - for simplicity), do I need to create n files with their corresponding size? When I receive a piece from a peer, how do I know to which file it (the piece) belong?

The bittorrent wire protocol deals with a contiguous address space of bytes which are grouped into fixed-sized pieces. How a client stores those bytes locally is in principle up to the implementation. But if you want to store it in the file layout described in the .torrent then you have to calculate a mapping between the pieces address space and file offsets. In BEP3 files are not aligned to piece boundaries, so a single piece can straddle multiple files. BEP 47 and BEP 52 aim to simplify this by introducing padding files or implicit alignment gaps respectively.

Upvotes: 1

user9525821
user9525821

Reputation: 116

what you are doing is similar to mine...

The following bold fonts are important to your questions.

1.yes; no

Info in Multiple File Mode

name: the name of the directory in which to store all the files. This is purely advisory. (string)

path: a list containing one or more string elements that together represent the path and filename. Each element in the list corresponds to either a directory name or (in the case of the final element) the filename. For example, a the file "dir1/dir2/file.ext" would consist of three string elements: "dir1", "dir2", and "file.ext". This is encoded as a bencoded list of strings such as l4:*dir*14:*dir*28:file.exte

Info in Single File Mode

name: the filename. This is purely advisory. (string)

Filename includes floder name.

2.maybe;

Whether you need to create n files with their corresponding size depend on whether you need to download n files.

Peer wire protocol (TCP)

piece:

The piece message is variable length, where X is the length of the block. The payload contains the following information:

index: integer specifying the zero-based piece index

begin: integer specifying the zero-based byte offset within the piece

block: block of data, which is a subset of the piece specified by index.

For the purposes of piece boundaries in the multi-file case, consider the file data as one long continuous stream, composed of the concatenation of each file in the order listed in the files list. The number of pieces and their boundaries are then determined in the same manner as the case of a single file. Pieces may overlap file boundaries.

I am sorry for my english, because I am not native speaker...

Upvotes: 10

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