improve torrent file creator optimizer a bit

docs/make_torrent.rst

@@ -268,7 +268,7 @@ The ``create_torrent`` class has the following synopsis::
 			, calculate_file_hashes = 16
 		};
 		create_torrent(file_storage& fs, int piece_size = 0, int pad_size_limit = -1
-			, int flags = optimize);
+			, int flags = optimize, int alignment = 0x4000);
 		create_torrent(torrent_info const& ti);
 
 		entry generate() const;
@@ -305,7 +305,7 @@ create_torrent()
 			, calculate_file_hashes = 16
 		};
 		create_torrent(file_storage& fs, int piece_size = 0, int pad_size_limit = -1
-			, int flags = optimize);
+			, int flags = optimize, int alignment = 0x4000);
 		create_torrent(torrent_info const& ti);
 
 The ``piece_size`` is the size of each piece in bytes. It must
@@ -362,6 +362,10 @@ calculate_file_hashes
 	with each file. Note that unless you use the `set_piece_hashes()`_ function,
 	this flag will have no effect.
 
+``alignment`` is used when pad files are enabled. This is the size eligible
+files are aligned to. The default is the default bittorrent block size of
+16 kiB. It is common to align to the piece size of the torrent.
+
 generate()
 ----------
 

examples/dump_torrent.cpp

@@ -133,7 +133,8 @@ int main(int argc, char* argv[])
 	{
 		int first = t.map_file(index, 0, 0).piece;
 		int last = t.map_file(index, (std::max)(size_type(i->size)-1, size_type(0)), 0).piece;
-		printf("  %11"PRId64" %c%c%c%c [ %4d, %4d ] %7u %s %s %s%s\n"
+		printf(" %8"PRIx64" %11"PRId64" %c%c%c%c [ %5d, %5d ] %7u %s %s %s%s\n"
+			, i->offset
 			, i->size
 			, (i->pad_file?'p':'-')
 			, (i->executable_attribute?'x':'-')

include/libtorrent/create_torrent.hpp

@@ -74,7 +74,7 @@ namespace libtorrent
 		};
 
 		create_torrent(file_storage& fs, int piece_size = 0
-			, int pad_file_limit = -1, int flags = optimize);
+			, int pad_file_limit = -1, int flags = optimize, int alignment = 0x4000);
 		create_torrent(torrent_info const& ti);
 		~create_torrent();
 		entry generate() const;

include/libtorrent/file_storage.hpp

@@ -244,7 +244,7 @@ namespace libtorrent
 		// if pad_file_limit >= 0, files larger than
 		// that limit will be padded, default is to
 		// not add any padding
-		void optimize(int pad_file_limit = -1);
+		void optimize(int pad_file_limit = -1, int alignment = 0x10000);
 
 		sha1_hash hash(int index) const;
 		std::string const& symlink(int index) const;

src/create_torrent.cpp

@@ -261,7 +261,7 @@ namespace libtorrent
 
 	create_torrent::~create_torrent() {}
 
-	create_torrent::create_torrent(file_storage& fs, int piece_size, int pad_file_limit, int flags)
+	create_torrent::create_torrent(file_storage& fs, int piece_size, int pad_file_limit, int flags, int alignment)
 		: m_files(fs)
 		, m_creation_date(time(0))
 		, m_multifile(fs.num_files() > 1)
@@ -310,7 +310,7 @@ namespace libtorrent
 #endif
 		m_files.set_piece_length(piece_size);
 		if (flags & optimize)
-			m_files.optimize(pad_file_limit);
+			m_files.optimize(pad_file_limit, alignment);
 		m_files.set_num_pieces(static_cast<int>(
 			(m_files.total_size() + m_files.piece_length() - 1) / m_files.piece_length()));
 		m_piece_hash.resize(m_files.num_pieces());

src/file_storage.cpp

@@ -515,14 +515,16 @@ namespace libtorrent
 		}
 	}
 
-	void file_storage::optimize(int pad_file_limit)
+	void file_storage::optimize(int pad_file_limit, int alignment)
 	{
-		// the main purpuse of padding is to optimize disk
-		// I/O. This is a conservative memory page size assumption
-		int alignment = 8*1024;
-
 		// it doesn't make any sense to pad files that
-		// are smaller than one piece
+		// are smaller than one block
+		if (pad_file_limit >= 0 && pad_file_limit < 0x4000)
+			pad_file_limit = 0x4000;
+
+		// also, it doesn't make any sense to pad files
+		// that are smaller than the alignment, since they
+		// won't get aligned anyway; they are used as padding
 		if (pad_file_limit >= 0 && pad_file_limit < alignment)
 			pad_file_limit = alignment;
 
@@ -615,9 +617,8 @@ namespace libtorrent
 
 				reorder_file(index, cur_index);
 
-				// skip the pad file we just added and point
-				// at the current file again
-				++i;
+				TORRENT_ASSERT((off & (alignment-1)) == 0);
+				continue;
 			}
 			i->offset = off;
 			off += i->size;