piece

type Block struct {
	Index  int    // index in piece
	Begin  uint32 // offset in piece
	Length uint32 // always equal to BlockSize except the last block of a piece.
}

type Piece struct {
	Index   uint32            // index in torrent
	Length  uint32            // always equal to Info.PieceLength except last piece
	Data    filesection.Piece // the place to write downloaded bytes
	Hash    []byte
	Writing bool
	Done    bool
}