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BitTorrent Client Library (Core)
There is a newer version: 1.10
package bt.data.file;

import bt.data.StorageUnit;
import bt.data.Storage;
import bt.metainfo.Torrent;
import bt.metainfo.TorrentFile;

import java.io.File;

/**
 * Provides file-system based storage for torrent files.
 *
 * Information about the collection of files in a torrent is stored as a list of strings in the torrent metainfo.
 * This makes it possible for malicious parties to create torrents with malformed pathnames,
 * e.g. by using relative paths (. and ..), empty and invalid directory and file names, etc.
 * In the worst case this can lead to loss and corruption of user data and execution of arbitrary code.
 *
 * 
This storage implementation performs normalization of file paths, ensuring that:
 * 

 *     all torrent files are stored inside the root directory of this storage (see {@link #FileSystemStorage(File)})
 *     all individual paths are checked for potential issues and fixed in a consistent way (see below)
 * 
 *
 * Algorithm for resolving paths:

 * 1) The following transformations are applied to each individual path element:
 * 

 *     trimming whitespaces,
 *     truncating trailing dots and whitespaces recursively,
 *     substituting empty names with an underscore character
 *         (this also includes names that became empty after the truncation of whitespaces and dots),
 *     in case there is a leading or trailing file separator,
 *         it is assumed that the path starts or ends with an empty name, respectively.
 * 


 * 2) Normalized path elements are concatenated together using {@link File#separator} as the delimiter.
 *
 * Examples:

 * {@code "a/b/c"     => "a/b/c"}

 * {@code " a/  b /c" => "a/b/c"}

 * {@code ".a/.b"     => ".a/.b"}

 * {@code "a./.b."    => "a/.b"}

 * {@code ""          => "_"}

 * {@code "a//b"      => "a/_/b"}

 * {@code "."         => "_"}

 * {@code ".."        => "_"}

 * {@code "/"         => "_/_"}

 * {@code "/a/b/c"    => "_/a/b/c"}

 *
 * @since 1.0
 */
public class FileSystemStorage implements Storage {

    private final File rootDirectory;
    private final PathNormalizer pathNormalizer;

    /**
     * Create a file-system based storage inside a given directory.
     *
     * @param rootDirectory Root directory for this storage. All torrent files will be stored inside this directory.
     * @since 1.0
     */
    public FileSystemStorage(File rootDirectory) {
        this.rootDirectory = rootDirectory;
        this.pathNormalizer = new PathNormalizer();
    }

    @Override
    public StorageUnit getUnit(Torrent torrent, TorrentFile torrentFile) {

        File torrentDirectory;
        if (torrent.getFiles().size() == 1) {
            torrentDirectory = rootDirectory;
        } else {
            String normalizedName = pathNormalizer.normalize(torrent.getName());
            torrentDirectory = new File(rootDirectory, normalizedName);
        }

        String normalizedPath = pathNormalizer.normalize(torrentFile.getPathElements());
        return new FileSystemStorageUnit(torrentDirectory, normalizedPath, torrentFile.getSize());
    }
}