Allow to save downloaded metadata as torrent file

src/base/bittorrent/torrentinfo.cpp

@@ -32,6 +32,8 @@
 #include <boost/optional.hpp>
 #endif
 
+#include <libtorrent/bencode.hpp>
+#include <libtorrent/create_torrent.hpp>
 #include <libtorrent/error_code.hpp>
 
 #include <QByteArray>
@@ -42,6 +44,7 @@
 #include <QStringList>
 #include <QUrl>
 
+#include "base/exceptions.h"
 #include "base/global.h"
 #include "base/utils/fs.h"
 #include "base/utils/misc.h"
@@ -151,6 +154,27 @@ TorrentInfo TorrentInfo::loadFromFile(const QString &path, QString *error) noexc
     return load(data, error);
 }
 
+void TorrentInfo::saveToFile(const QString &path) const
+{
+    if (!isValid())
+        throw RuntimeError {tr("Invalid metadata.")};
+
+#if (LIBTORRENT_VERSION_NUM < 10200)
+    const lt::create_torrent torrentCreator = lt::create_torrent(*(nativeInfo()), true);
+#else
+    const lt::create_torrent torrentCreator = lt::create_torrent(*(nativeInfo()));
+#endif
+    const lt::entry torrentEntry = torrentCreator.generate();
+
+    QByteArray out;
+    out.reserve(1024 * 1024);  // most torrent file sizes are under 1 MB
+    lt::bencode(std::back_inserter(out), torrentEntry);
+
+    QFile torrentFile{path};
+    if (!torrentFile.open(QIODevice::WriteOnly) || (torrentFile.write(out) != out.size()))
+        throw RuntimeError {torrentFile.errorString()};
+}
+
 bool TorrentInfo::isValid() const
 {
     return (m_nativeInfo && m_nativeInfo->is_valid() && (m_nativeInfo->num_files() > 0));